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The Truth about Toxic Mercury in Fish


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From a public health perspective, I think this is one of the most important shows I’ve ever recorded. As it is, most people do not eat enough cold-water, fatty fish, and this is especially true of pregnant women. Concern about mercury toxicity is one of the main reasons for this. But as you’ll learn in this episode, such concerns are unfounded and not supported by the science the majority of the time. In fact, we could go as far as saying it’s much safer to eat fish than it is to not eat it. DHA is a crucial nutrient for the development of the brain and nervous system, and has many other important properties. And as I’ve pointed out elsewhere, eating plant sources of omega-3 like flax or walnuts doesn’t cut it, because only a very small percentage (i.e. about one-half percent) of the short-chain omega-3 fats found in plants get converted into the beneficial long-chain omega-3 fat DHA.

I know not everyone will have time to listen to the show or read the transcript, so here’s the 30,000 foot takeaway: mercury causes harm by damaging selenoenzymes in the body that protect against oxidative damage. As long as a fish contains more selenium than mercury (which the vast majority of both ocean fish do), and as long as background selenium intake is sufficient (which it is in most industrialized nations), then there is no reason to limit consumption of ocean fish. Please do listen to the interview with Dr. Ralston, an expert in mercury in fish and the protective effects of selenium, to learn more about this important subject.

In this episode, we cover:

10:37 How did the idea that “eating fish is unsafe due to mercury” get started?
12:27 Why mercury in fish can cause problems
15:07 How selenium protects against the toxicity of mercury
18:12 Does it matter where selenium comes from in the diet?
20:25 The therapeutic dose of selenium that’s helpful in mercury toxicity
25:10 Why you need to know about the Selenium health benefit value (SE-HBV)
33:07 Is there any limit to how much fish a pregnant woman should eat?
35:15 The safety of freshwater fish
39:10 Where the Suppversity article went wrong
49:10 How to sustainably increase fish consumption

Full Text Transcript:

Steve Wright:  Hey everyone, and welcome back to another episode of the Revolution Health Radio Show brought to you by ChrisKresser.com.  I’m your host, Steve Wright from SCDLifestyle.com, and with me is integrative medical practitioner, licensed acupuncturist, and healthy skeptic, Chris Kresser.  How’s the Bay Area, man?

Chris Kresser:  Haha, it’s a little bit schizophrenic actually, Steve.  We had the biggest heat wave we’ve had all summer about a week ago.  It was like sweating-at-night type of heat, because we don’t have air conditioning out here.  We don’t really need it usually.  But there are a few days a year that I wish we did, and that was it!  And then it’s been raining and sprinkling a little bit the past couple of days, so it’s a little strange.  How are you doing?

Steve Wright:  I gotta tell you, man, I’m a little sleep deprived.  I haven’t been looking after my health because my team, the Tigers, have been on the West Coast battling the Oakland A’s out there in Cali.

Chris Kresser:  Haha.

Steve Wright:  And I haven’t been able to turn it off, so I keep staying up all night long, trying to wait for them to win, and luckily last night the man Verlander pulled it out for us, but it’s been a long week.

Chris Kresser:  Haven’t you heard of TiVo, Steve?  TiVo, video on demand, anything like that?  You gotta watch it live, huh?

Steve Wright:  Dude, with sports, I think you have to watch it live.  Otherwise, you know, my phone, I’ll have to look in the morning to find out if they won.  Yeah, everything else is totally recorded.  I don’t watch commercials anymore.  I don’t do any of that stuff.

Chris Kresser:  Yeah, right.  Well, I’m doing all right with the sleep, but I’ve definitely been busy.  I’m getting ready to speak at the Weston A. Price Wise Traditions Conference in early November, which with everything that’s been going on, I was like, Oh, man, this is not the right time to do this!  But I’m actually really excited about the talk.  It’s on the gut-brain-skin axis, which is really fascinating, so I’ll be exploring the connections between the gut and the brain, which we’ve talked about, of course, on the show before, and then the gut and the skin, which we’ve talked a little bit about on the show before as well.  But then this kind of triangulation between the gut and the brain and the skin and how that plays out, and it turns out that there are bidirectional connections in each cause.  So the gut affects the skin, but inflammation in the skin can actually trigger a stress response that will affect the brain, obviously, but also the gut.  And then the gut and the brain have this bidirectional communication, which we’ve talked about where inflammation in the gut can decrease activity in the frontal cortex, and that in turn reduces the output into the pontomedullary area and the vagus nerve and causes more gut problems in this whole big, vicious cycle.  And then stress can reduce stomach acid secretion and cause SIBO, which in turn causes intestinal permeability, aka leaky gut, which causes local and systemic inflammation, which can cause or exacerbate skin conditions.  So it’s this crazy system of feedback loops, and it’s a really hot topic in the scientific literature, and I think it’s a kind of frontier for treating gut and brain and skin issues as looking at it as this holistic piece instead of completely separate conditions.

Steve Wright:  Well, that sounds like a spiderweb of amazingness for me because I’m a big nerd on that stuff too, so I cannot wait to see that talk.

Chris Kresser:  Are you coming?

Steve Wright:  Hopefully, hopefully.

Chris Kresser:  Cool.

Steve Wright:  We’re doing a lot of changes on our site as well, so it’s been kinda busy and just trying to make sure that we can make it out there and get the stuff that we’re doing done as well.

Chris Kresser:  Yeah.  Well, I hope to see you guys there.  One thing I’m excited about is it’s like a 40-minute drive from my house, so I don’t have to do any big travel at least.

Steve Wright:  Oh, very nice, very nice.  Well, I know that you’ve done a skin series and obviously we’ve talked a lot about gut-brain on the blog, so hopefully after this you can maybe distill some of the best points down and put them on the blog for the people who won’t be able to make it, because I don’t think that’s a free conference online, right?  People have to buy that?

Chris Kresser:  That’s right.  They sell the DVDs afterward.  So yeah, we’ll definitely do a cliffsnotes version or something on the show.  Maybe it’d be good.  We can talk a little bit about it in November or December or something.

Steve Wright:  All right, cool.  Well, without further ado, we have a very special guest on the show today.  Chris, do you want to tell us a little bit about him?

Chris Kresser:  Sure.  Yeah, I’m really excited.  I’m so grateful that he accepted our invitation.  His name is Dr. Nicholas Ralston.  And I first became aware of his work back when I wrote the special report on essential fatty acids and an article that I think a lot of people have read by now called Is it safe to eat fish?  A lot safer than not eating fish!  And the gist of the article was that the concerns that a lot of people have about mercury in fish turn out to be unwarranted in most cases and that, in reality, the benefit of eating fish, all the beneficial nutrients like the long-chain omega-3 fats and selenium and other nutrients, far outweigh any potential risks because selenium can protect against the adverse effects of mercury toxicity.  And in doing research for this, I came across Dr. Ralston’s work because he’s one of the most prominent researchers in the field and has a really extensive background in this area, and he’s really the perfect person to have on the show to discuss this.  And I wanted to do it now because there’s been some chatter in the paleo blogosphere lately about an article that appeared on a blog called SuppVersity.  And I don’t remember the exact title of the article, but we can post it in the show notes.  But the gist of that article was that contrary to what some people like me have said recently, that selenium doesn’t actually protect against the adverse effects of mercury.  So I wanted to have Dr. Ralston on to talk about the general issue of selenium and mercury, and mercury in fish, and then specifically address some of the things in that article and point out where that analysis was lacking.  So I’m really honored to have Dr. Ralston on the show and can’t wait.

Steve Wright:  I’m extremely excited as well because I think this is a topic near and dear to everyone in the real food community, so I can’t wait to hear what he has to say.

Chris Kresser:  Let’s do it!

Steve Wright:  OK, Chris, before we talk to Dr. Ralston, why don’t you grab some water, get your notes prepped, and I’m gonna tell everyone about Beyond Paleo.  If you’re new to our show, or you’re new to the blog, or you’re new to paleo, you’re probably gonna want to check out a free 13-part email series that Chris has put together that includes his top tips and tricks for burning fat, boosting energy, and preventing and reversing disease without drugs.  Now, over 20,000 people have already signed up to get this, so needless to say, if you’re listening to this and you haven’t read it, it’s something that’s in demand and you’re probably gonna learn a lot from it.  So if you’re interested, head over to ChrisKresser.com, look for the big red box, and go ahead and put your name and email in that box, and Chris will start sending you those emails over the next few weeks.

OK, Chris, how are you doing?  You ready?

Chris Kresser:  Let’s do it.  Dr. Nicholas Ralston is a research scientist at the University of North Dakota Energy and Environmental Research Center, EERC, involved in evaluating potential human health effects and risks resulting from environmental exposure to air toxics.  He received his PhD in biomedical research biochemistry from Mayo Medical Center and his bachelor’s of science in biology from Mayville State University.  Dr. Ralston’s principal areas of expertise include the biochemistry and analytical approaches to quantitative assessment of immune research and inflammation at the molecular and cellular level.  His primary interests are in trace element physiology and the pathophysiology of toxic trace element exposures as well as prevention, protection, and remediation strategies.  His current research interests include examinations of the molecular mechanism of methylmercury toxicity, selenium-dependent biochemical processes involved in preventing the neurotoxic effects of mercury, both of which we’ll be discussing on the show, mechanisms of pulmonary particulate pathologies, mercury phytoremediation, and other means of diminishing bioaccumulation of mercury in fish.

Prior to his position at the EERC, Dr. Ralston worked at Grand Forks Human Nutrition Research Center and Bowman Gray Medical School at Wake Forest University.  He has authored or coauthored over 50 professional publications, research articles, and book chapters and given numerous invited presentations on health issues related to methylmercury-dependent inhibition of brain selenoenzymes and beneficial effects of maternal seafood consumption on child development outcomes.

I first became aware of Dr. Ralston’s work back when I was writing a series on the importance of long-chain omega-3 fatty acids and I was doing some research on the idea that we should avoid fish consumption because of mercury toxicity.  And I came across some papers Dr. Ralston had written, and they really changed my view on this issue, and I wrote an article about that.  And since then there’s been a lot of discussion of it and some further articles written in the paleo blogosphere, shall we say, that have been critical of the idea that selenium protects against mercury toxicity.  So I invited Dr. Ralston to come on the show and discuss this.  I couldn’t think of a better person with more expertise to help us shed light on this issue and clear it up once and for all, so welcome to the show, Dr. Ralston.

Dr. Nicholas Ralston:  Well, thank you very much, Chris.  Happy to be with you.

How did the idea that “eating fish is unsafe due to mercury” get started?

Chris Kresser:  So let’s start with how the idea that eating fish is unsafe due to mercury got started in the first place.

Dr. Nicholas Ralston:  Well, it’s kind of been understood since the time of the Romans and even the early Chinese that mercury is a toxin, so in the late ’50s there were findings that a bay in Japan where mercury had been released from a plastic-producing chemical plant.  Large amounts of mercury were being released, and this mercury was being accumulated in the fish.  The fish had huge amounts of mercury in them.  They were extraordinarily toxic and were lethal to the animal life that was eating it such as cats that would eat the fish that were washing up on shore from being killed by the mercury.  People that were catching fish from that small bay were getting huge doses of mercury, and at that time that was first that scientists recognized that unborn children were particularly sensitive to the effects of mercury because women that weren’t showing any bad effects — Many people were showing severe effects, but even women that were not showing bad effects would have children that were just tremendously harmed by the amount of mercury they were exposed to.  So since that time, there have been a lot of human studies, but that was really the start of it all.  It was called Minamata Bay.

Why mercury in fish can cause problems

Chris Kresser:  Right, the Minamata — I’ve seen some pictures from that tragedy and they’re pretty horrific, so I could definitely see how that would scare people, especially mothers who are thinking about whether to eat fish during pregnancy.  So how does mercury in fish and in general cause problems?  I want to ask you this question particularly because I think there’s some misunderstanding out there about how mercury causes problems.  So can you clear that up for us?

Dr. Nicholas Ralston:  Yeah, actually it’s kind of easy to understand why this misconception has gotten around.  It kind of comes down to simple problems that confuse people.  Like, some people don’t really seem to understand that whales and fish are not the same, that they’re totally different as far as their physiology.  And some people don’t recognize that ocean fish and freshwater fish are not the same, and you know, they’re certainly not.  The same thing about mercury exposures:  It seems like many people think that how you get exposed to mercury doesn’t matter and all you need to know is how much mercury you got exposed to, but that’s really not the case.  We’re seeing that exposures to mercury that are a certain level that are known to cause harm in one population, higher levels of exposure are having no effects at all in other populations.  The things that we’re finding out, however, are those early studies where there were harmful effects seen, they all involved consumption of pilot whale meats.   Pilot whales are predatory whales that eat a lot of — They’re the top predators of the ocean, so they get a lot of mercury in their meat.  They can have twice as much mercury in their muscle meat than selenium, which really makes them almost unique.  A second exception is shark meat.  Shark meats are the only type of fish that we’re aware of where there are consistently high levels of mercury relative to selenium.  Almost all other types of fish have more selenium than mercury, so that’s the important crux of the matter.  We see harm whenever people are exposed to a lot of mercury when it’s mercury in excess of selenium, but people that are exposed to even more mercury than was seen in those studies but in the presence of plenty of selenium, like most ocean fish and even most freshwater fish have, we’re not seeing those adverse effects.  So it’s kind of easy for me to understand why people get confused about this, but we hope to clear up the confusion as time goes on.

How selenium protects against the toxicity of mercury

Chris Kresser:  Great.  So let’s kind of zero in on a more molecular level and talk a little bit about how mercury exactly causes problems, because I think there’s maybe some misunderstanding about this as well.  The idea, for example, that mercury has a primarily harmful effect by causing oxidative damage.  Is that really how it works?

Dr. Nicholas Ralston:  Well, it is true that in animals or humans that are exposed to huge amounts of mercury, oxidative damage is the primary thing that we see, but the reason for the oxidative damage has been misunderstood.  Certain elements, like if you have iron toxicity or if you have too much copper exposure, you’ll get oxidative damage because direct reactions between this excess amount of iron or copper in your tissues causes oxidative damage to lipids.  But in the case of mercury, that doesn’t happen.  You can have a lot of mercury in the presence of lipids and no direct damage occurs.  What it’s really all coming down to is that tissues that have a lot of oxygen consumption, like brain or heart, they produce a lot of free radicals or oxidative molecules, and so you need a real potent antioxidant system to keep those oxidative molecules from harming your brain lipids or your heart lipids, and if you don’t have those enzymes doing their job, you will start accumulating damage rapidly.

Now, the thing that’s crucial about the whole issue is that selenium-dependent enzymes are probably the most functionally elite enzymes there are for preventing oxidative damage.  Without selenium-dependent enzymes, vitamin C can’t perform its function.  It would do its job once and then it would be useless.  It takes a selenoenzyme to make vitamin C able to do its job again and again and again so it’s always able to protect.  The same thing with several of the other very important antioxidant molecules that we all recognize are huge in protection against oxidative damage.   So without selenoenzymes, oxidative damage is going to be happening fast and furious.  The only thing that we know of that can kill selenoenzyme activities is mercury.  Mercury specifically binds selenium-dependent enzymes.  Mercury has a million-times higher affinity for selenium than it does for sulfur, its second best binding partner.

Chris Kresser:  Wow.

Dr. Nicholas Ralston:  So you get too much mercury and it’s going to find and bind your selenium, and you’ll pretty quickly lose your selenoenzymes.  But if you’re taking in a lot more selenium than you are mercury, well, you will have mercury taking out enzymes, but as soon as they’re being taken out, more selenium-dependent enzymes are being made.  So we don’t see toxic effects when there’s food that has more selenium than mercury being consumed.

Does it matter where selenium comes from in the diet?

Chris Kresser:  OK, yeah, that’s very clear.  So this is a question I’ve been asked a few times:  Does it matter where the selenium is coming from in the diet?  So, for example, does the selenium have to be in the food that contains the mercury, or is it more just the background selenium intake from all sources in the diet that matters most?

Dr. Nicholas Ralston:  Well, let’s just talk ocean fish for a moment.  They’re very, very rich in selenium, so if you’re wanting protection against mercury exposure, like if you happen to be eating a lot of whale meat, the one study that was done that found severe effects from mercury exposure, the moms were eating pilot whale meat and so 95% of their mercury exposure came from eating the whale meat.  And of course, since that whale meat contained four or five times more mercury than selenium, it they’d only been eating that whale meat, those moms would’ve been in a lot of trouble, and their babies would’ve been severely harmed.

Chris Kresser:  Right.

Dr. Nicholas Ralston:  But fortunately those folks were also eating a lot of ocean fish, so the ocean fish provided the selenium that protected against the mercury from the whale meat.  Most people, when they’re concerned about mercury exposure, it’s actually from eating fish, but very few people in the United States eat whale meat, of course, so in most cases, unless somebody was eating a lot of shark or happened to be eating maybe freshwater fish from a lake that did happen to have more mercury than selenium — That occurs in very few places, but we’re fairly sure it’s occurring in places that need to be looked at more carefully.  If that is happening, selenium from other sources — Fortunately, Americans, everything we eat tends to be rich in selenium, so if you’re eating a healthy diet, you’re getting a good amount of selenium.  But if you do happen to be eating some foods that have more mercury than selenium, such as shark meat, pilot whale meat, or maybe a freshwater fish that has more mercury than selenium, any other source of selenium will be protective, but it’s probably best to simply avoid the foods that have such high mercury relative to selenium.

Chris Kresser:  Right.  So let’s say someone has mercury toxicity, perhaps not from eating pilot whale meat but from another source.  Is there a therapeutic dose of selenium that would be helpful in that circumstance?  My understanding is, of course, that like most nutrients there’s a U-shaped curve for selenium.  There’s a sweet spot.  Too little is not good, but too much is not good either because of selenosis.

The therapeutic dose of selenium that’s helpful in mercury toxicity

Dr. Nicholas Ralston:  Absolutely right.  Generally in nutrition we figure that anything over 1 mg per day, 1000 mcg — that’s the same as 1 mg — is too much.  And really we prefer people to stay below that considerably; 200 mcg per day is kind of a typical supplemental dose.  Not too many people in the United States absolutely need it.  Well, certainly they’re not going to be deficient, but there are some health benefits that are supposedly associated with supplemental levels of selenium such as anticancer effects of it have been reported occasionally.

Chris Kresser:  Right.

Dr. Nicholas Ralston:  So 200 mcg is a plenty safe dose, but really if you’re eating normal foods and especially if you happen to be consuming plenty of seafoods, you’re getting a healthy amount of selenium in your diet anyhow.  Most people get about 50 mcg per day, so an additional 200 mcg is taking it up about another couple hundred percent, so it’s a reasonable thing to do if people were concerned.  For instance, I’m working in a study in Peru where people are working in gold mining operations where they do a lot of mercury release into their environment.  It’s something that really would be good if they would stop doing it, but since they can’t be dissuaded at this time, there’s going to be intervention trying to get their selenium levels up to try and protect them and their children that happen to be living in the area with them protected against the mercury to a greater extent.

Chris Kresser:  Right.

Dr. Nicholas Ralston:  But as always, the best thing is to avoid the mercury exposure, and the fortunate thing is in the United States we generally have such rich selenium intakes that for the most part, unless somebody was exposed to really unique cases of either industrial exposures to mercury or something like that, most of the time the concerns are not going to be too great.  This is not the case in other countries where selenium is poorly available and in some cases mercury exposures might be much greater.  So elsewhere in the world, there’s actually far greater concern than in the US.

Chris Kresser:  Do you have an opinion, Dr. Ralston, on the form of selenium that people should take if they choose to supplement with it?  For example, I have some patients that are allergic to seafood, so the main source of selenium that they would get in the diet isn’t available to them.  And there’s, of course, selenomethionine, selenite, methylselenocysteine, and all these different forms and a lot of controversy over which form is the most beneficial to supplement with.

Dr. Nicholas Ralston:  Well, yeah, the selenomethionine is one of the major forms that is produced in plants.  Actually a lot of people will be taking selenized yeast, and what they do there is they provide selenium to yeast while they’re growing, and the yeast will take up the selenium and form typical biological molecules that are a good source of selenium.  Too much selenomethionine has been associated with some problems, so like you were saying, too much is not good either, but pretty much this is the case with everything now.  If you take five times the healthy dose of water, that’s lethal.

Chris Kresser:  Right.

Dr. Nicholas Ralston:  With selenium it’s the same way; 200 mcg is no problem, but once you get more than five times that, you’re starting to get into trouble.  But most people are down around 50 mcg, so nobody ever gets close to 1000 mcg per day, but there simply is nothing in the world that I’m aware of where there is not gonna be a risk associated with having too much of it, everything from water to oxygen.  Oxygen, everybody on the planet eventually dies from oxygen toxicity.  We need to have the oxygen for supporting life processes, but we all die eventually of oxidative damage, and so oxygen is probably the most lethal element on the periodic table.  And the best protection against oxidative damage is healthy amounts of selenium.  Too much, not a good thing, but that’s true of everything.

Why you need to know about the Selenium health benefit value (SE-HBV)

Chris Kresser:  OK, so we’ve discussed the fact that fish that have more selenium than mercury are beneficial because selenium protects against mercury toxicity, so tell us a little more about this Selenium — Is it the Health Benefit Value?  The Se-HBV?

Dr. Nicholas Ralston:  Yeah, the Health Benefit Value.  Because no consumer and very few regulatory scientists would like to try and have to figure out, OK, it’s got this much mercury, this much selenium, so is it a good fish or is it a bad fish?  We created the Selenium Health Benefit Value as a simple, straightforward way that a consumer could know that the fish that they’re eating is actually beneficial and at least as far as mercury-to-selenium relationships, how beneficial it is.  So if it have more mercury than selenium, such as the pilot whale or the shark species that I was mentioning that have been found to be harmful, it has a negative Selenium Health Benefit Value, because any food that has more mercury than selenium, consuming that is not going to be good for you.  Like, for instance, the pilot whale meats had a negative value of -80, so eating that was a fairly extraordinarily bad food for a mom to be eating.

Chris Kresser:  Right.

Dr. Nicholas Ralston:  But fortunately the ocean fish that the moms were eating plenty of had values that were around 50 to 100 positive, so they more or less balanced out almost entirely.  There was still a negative effect, but without the seafood consumption, without those moms eating those ocean fish that had a small amount of mercury, so you would’ve thought that it would’ve made the mercury exposure effects worse, completely did not.  Instead of making it at all any worse, it completely protected or nearly completely protected, so the children only had subtle neurodevelopmental effects.  It was actually less than an entire IQ point in the worst cases with the children of the moms that were eating the most whale meat and the least ocean fish, so it still was — You know, nobody wants to lose even a part of an IQ point, so it’s definitely advisable to avoid eating whale meat.  And in the Faroe Islands — It’s a protectorate of Denmark that’s between Scotland and Iceland.  It’s an island group where the people are mostly Danish heritage — They were eating lots of whale meat during pregnancy, and now they’re being advised during pregnancy especially don’t eat whale meat, but certainly go ahead and eat as much fish as you can because we’re finding that eating ocean fish, we’re getting IQ benefits instead of harms.

Chris Kresser:  So, was that the only study that ever showed harm from eating mercury in fish, Dr. Ralston?

Dr. Nicholas Ralston:  No.  The Faroe study is probably the one that’s most often cited because the statistics on it were done well and it was a good-sized study.  The drawbacks are that the exposure was coming from pilot whale meat instead of ocean fish.  But before that study, there was a study done in New Zealand, where mothers that were eating fish and chips were getting exposed to fair amounts of mercury, but it’s important to note that the fish that they were using to make fish and chips, for some reason they were using large shark meats, which nobody would do that now, but back in that time, apparently they were catching shark and it seemed like it was usable for that purpose.  So in New Zealand, they were getting exposed to shark meat that had high mercury relative to selenium, and the New Zealanders, unlike Americans, New Zealand was one of the most selenium-deficient nations on earth at that time.  So exposure to mercury in New Zealand, they had very little chance to be able to have other dietary sources of selenium to make up for the losses from eating those shark meat filets, so they were in a lot of trouble.  So those are two of the big studies that have shown adverse effects, but now we’ve had a lot more studies, huge studies with many thousands of women that are eating ocean fish, and we’ve been following the children since the time prior to birth to the time — I think now those children in some of the bigger studies are almost 20 years old.

Chris Kresser:  Wow.

Dr. Nicholas Ralston:  And the more fish that the mothers ate in those studies, these very appropriately designed studies — The one I’m referring to was done in England, so you know, very much like what the case is in the United States, the type of foods they eat are generally seafoods.  Maybe they eat a little bit more fish and chips than we do in the US but still eat a lot of fish.  They find that the benefits of eating ocean fish during pregnancy amount to as much as three to five IQ points of benefit.  So moms that are avoiding eating ocean fish because of the harms that are associated with eating whale meat are missing out on the benefits that actually are accompanying eating ocean fish.

Chris Kresser:  So we really need to turn this idea around where not eating fish during pregnancy is what might be considered unsafe rather than eating fish, provided the fish has less mercury than selenium, or course.

Dr. Nicholas Ralston:  Yeah, exactly.  And just about all varieties that are typically sold as commercial types of fish are fine.  Even some sharks are fine.  It’s just certain of the very, very predatory types like mako shark is one that we included in one of our big studies recently.  Mako shark consistently has more mercury than selenium, so a pregnant woman or a woman that might become pregnant should avoid eating mako shark.  Many swordfish can have almost as much mercury as selenium, so perhaps swordfish should be avoided, too.  And of course, shark and swordfish are two of the types of fish that women are already advised by FDA and EPA to stay away from.  And the other two types are also high in mercury relative to selenium; tilefish and king mackerel are foods that definitely need to be avoided during pregnancy.  Men that are not likely to become pregnant don’t have to worry about eating shark or swordfish because really it is that sensitivity of the unborn fetus.  When you’re developing a new brain, you cannot have a shortfall of selenium at any time.  A newborn brain is making 50,000 new brain cells per second, so you don’t want to have even a few seconds of too much mercury relative to selenium.

Chris Kresser:  Right.

Dr. Nicholas Ralston:  So the more ocean fish moms eat during pregnancy, though, the more selenium, omega-3 fatty acids, vitamin D, and other nutrients that are known to be hugely important for developing healthy brains as well as contributing to the health of the mom during pregnancy and the post pregnancy.  Eating more fish is actually good advice for women, but unfortunately still too many people have the misconceptions that are kind of the outdated ideas, and we’re trying hard to get everybody on board so that they understand this and have a unified message so more and more of the people that formerly were arguing, Oh no, fish has to be avoided, are coming around and saying, Oh, no, actually ocean fish need to be eaten in greater amounts.

Is there any limit to how much fish a pregnant woman should eat?

Chris Kresser:  Right.  So is there any limit then to how much fish a pregnant woman should eat, presuming she’s eating the forms that are positive on the Selenium Health Benefit Value Scale?

Dr. Nicholas Ralston:  Well, in nutrition, it’s usually a good idea to not focus too much of your diet in any one area.  Variety is good in many ways, and twice a week would certainly be a — I think that’s what the level was that was associated with the benefits in the British study.  Certainly in Japan they probably eat ocean fish more often than that, and certainly from the effects we see among the Japanese, it looks like they’re beneficial.  In Japan, however, they do have to be cautious because in some parts of Japan they do eat porpoise meat, whale meat, and some of the types that are not safe.  But in the US, commercially available, the store-shelf-type fish almost always are going to be the beneficial ones rather than the adverse ones.

Chris Kresser:  Right.

Dr. Nicholas Ralston:  So, a couple meals a week would be a good idea.  I’m not sure if I’d be in favor of women eating fish three meals a day, seven days a week.

Chris Kresser:  Right.  But something above 12 ounces and not going crazy with it.  A moderate amount there, yeah.  And we’re kinda far from that now.  I think the last statistics I saw, according to the National Fisheries Institute, Americans consume only 5 ounces a week of fish high in long-chain omega-3, which is less than half the recommended amount.  And I think they also estimated that almost 15% of women of childbearing age eat no fish at all, which was somewhat alarming.

Dr. Nicholas Ralston:  And a lot of that’s because they’re concerned.  They’re heard that eating some types of seafood is bad for you, so they just want to avoid seafood entirely.  But they didn’t realize the seafood that was being mentioned was whale meat.

Chris Kresser:  Right.

Dr. Nicholas Ralston:  And while, yeah, that is a seafood, when I think of seafood I’m never thinking whale meat.

Chris Kresser:  Haha, yeah.

Dr. Nicholas Ralston:  Most Americans are not thinking whale meat.

The safety of freshwater fish

Chris Kresser:  That’s definitely accurate.  So what about freshwater fish?  I get a lot of questions about this.  We’ve seen a lot of information about the Selenium Health Benefit Value of ocean fish, but are freshwater fish safe, too?

Dr. Nicholas Ralston:  Yeah, in general, more than 98% of the lakes that we have included in our study — We’re doing an EPA-funded study of lakes from all across North America, and in more than 98% of the places that we have data for, we’re seeing that the selenium is in excess of the mercury, so those fish are fine.  In some of the places where the mercury is approaching equal amounts with the mercury, there we’re actually thinking that further study is needed.  We need to perhaps rather than warning about the 98% that don’t appear to be problems, we should focus our efforts on the 2% where problems might occur.  And the problem is that right now people see, well, it’s got this amount of mercury and it’s the same as mercury everywhere else, while the problems is everywhere else it’s not causing problems because there is plenty of selenium there.

Chris Kresser:  Right.

Dr. Nicholas Ralston:  If you’re happening to eat fish from the 2% of the lakes where the mercury is in excess of the selenium, well, then there could be potential problems.  And it’s completed because wherever selenium is poorly available, and although in general the United States and most of North America is rich in selenium, there are places where there is too little.  And wherever there’s too little selenium, the thing we’re finding that’s happening very consistently across North America is where there’s less selenium available in the environment, there’s more mercury accumulating in the fish.  So that’s one of the reasons why I think this is an area of special concern.  We don’t want to have mercury accumulation in any of the fish but especially not happening in fish where there’s too little selenium to begin with.

Chris Kresser:  Is that lower selenium due to depletion in the soil or different dietary patterns in those regions or both, or do we know?

Dr. Nicholas Ralston:  Well, fortunately in the US we have mostly a centralized food distribution system, so even if you’re living on the West or East Coast, you’re probably eating bread products that may have come from, like, the heartland of the nation where selenium tends to be rich in the soils.  But on both coasts there’s a greater abundance of igneous soil, soil from breakdown of igneous rock.  And around volcanoes there’s a very bad smell.  That’s because sulfur and selenium both volatilize out of rock, and both of them smell very bad.  But that rock that’s left behind from volcanic activity is very low in selenium.  So sedimentary rocks in general are the opposite; they’re rich in selenium.  So if you happen to live in a part of the nation where there’s a lot of igneous rock or if you’re in Florida where the rock material is from coral that has been washed so any selenium that was there has been washed so much so that there’s no selenium left in the soil, that’s two conditions that are just primarily geological.  The other situation is if there’s low pH, and of course, with acid rain deposition that we used to have very severe in many parts of the nation, having low pH has a very bad effect on selenium.  Even if the selenium is in the soil, if acid rain is lowering the pH of the soil and water, the selenium will not be available, and so mercury accumulation in the fish will be greater.

Where the Suppversity article went wrong

Chris Kresser:  So, Dr. Ralston, a couple weeks ago, I sent you an article on the SuppVersity website that claimed that mercury in fish is harmful regardless of selenium content, and they cited an animal study, a mouse study, I believe, and then also a human study that I think took place in Finland.  And you were kind enough to look into that and get back to me on some shortcomings of that analysis.  Can you talk a little bit about that?

Dr. Nicholas Ralston:  Yeah.  In the first study, one of the misconceptions that people have about the mercury-selenium interaction is that it’s all about selenium somehow or another binding the mercury and keeping mercury from causing harm.  So they have heard that mercury and selenium have this high binding affinity, but they’re imagining that mercury is causing harm in the body until selenium binds it.  And, well, that’s not at all the case, that’s kind of the flipside of what’s really going on.  It’s not that selenium binds mercury and keeps it from causing harm.  It’s that when there’s too much mercury, that binds all your body’s selenium and keeps your body’s selenium from doing good.  And so if too much mercury is present, knocks out your enzymes, you have no more protection against oxidative damage, losing all of those enzymes that nothing else can wipe out, damage will definitely happen if you have a lot of mercury coming in and not enough selenium to keep those enzymes going.  And so the first study, they said they were comparing the effects of selenium, but remember the negative Selenium Health Benefit Value of pilot whale meat that I mentioned?

Chris Kresser:  Yes.

Dr. Nicholas Ralston:  That was a value of -80.  The two diets that they compared in the animal study, unfortunately they didn’t understand really the molar relationship.  Both of the diets were below 100, a -100 Selenium Health Benefit Value.

Chris Kresser:  Oh, wow.  I see.

Dr. Nicholas Ralston:  So they were comparing a -100 to a -160, and they found out, Oh, my goodness!  Both of them are bad for you!  And it’s like, well, yeah, if you’d calculated out the Selenium Health Benefit Value, you would’ve said, yeah, both of these should be bad for the animals.  So their study actually proves that when you have more mercury than selenium it’s bad for you, and we all kind of agree on that completely.

Chris Kresser:  Haha, right.

Dr. Nicholas Ralston:  So it’s not that they were wrong.  They actually did a good study, but they didn’t have a control that included a diet that had a positive Selenium Health Benefit Value.  Because you certainly can have even a lower selenium — It could still be negative and not as bad.  So, like a -10 might not be anywhere near as severe as a -100.  But once you’re severe, a -100 is pretty severe, and -160, well, you’re comparing severe to slightly more severe.  So it’s a study that’s a good start, and you know, what I really should do is contact the authors and suggest in their next study that they take into account the molar relationship a little bit more carefully and compare Selenium Health Benefit Value diets that include positive Selenium Health Benefit Values such as ocean fish provide.  Most ocean fish have values of 200 to the good or more.

Chris Kresser:  Right.

Dr. Nicholas Ralston:  The types of fish that were consumed in the Faroes, I think they were between 50 and 100.  But many of the types of fish that people think are worrisome, like various types of tuna.  For some reason, tuna gets focused in on a lot, but almost all of the tuna that I have looked at have had values of around 200 positive.  So rather than being adverse, the more tuna that’s consumed, yes, you are getting exposed to some mercury, but you’re getting 20 seleniums for every mercury, so the worst damage you can have is one mercury takes one selenium, so you’re still 19 to the good.  So it’s just a small matter of just keeping track more carefully of how much mercury relative to selenium.

The second study that was mentioned in the website was from Finland, and eastern Finland is another nation just like New Zealand that has extraordinarily selenium-deficient soils.  In fact, Finland is the only nation on earth that has every place in Finland the fertilizers that farmers put on their fields have to have selenium added, so that’s helping the people in Finland get a little bit less selenium deficient.  But the study is being done in eastern Finland, which is sort of a woodland and lake district.  It’s kind of like the Minnesota of Finland.  So not as many farm fields in this lake country, so there’s not selenium getting into those lakes, so what they have is very high levels of mercury in the fish, very low levels of selenium, so they’re taking in a worst case scenario-type of fish.  And just like I mentioned, brain and heart both have high rates of oxygen consumption, so they need a lot of protection against oxidative damage or you’re going to start showing all of these damages throughout the body, but particularly the brain and heart.

So in that study done in eastern Finland, they were eating freshwater fish, and like I mentioned before, wherever selenium is poor, mercury accumulation is going to be accentuated, and that was definitely the case there.  They weren’t eating ocean fish.  That’s the thing I was pointing out at the start.  Too many people think freshwater, ocean fish, no difference.  That’s not at all true.  Freshwater fish can vary in selenium a lot, and they can have a lot of mercury relative to selenium or a lot of selenium relative to mercury, so a lot of variability in freshwater fish.  Ocean fish are very consistent.  If it’s a type like a shark, it’ll consistently — if it’s like a mako shark, it’ll consistently have a lot more mercury than selenium.  And of course, it gets worse as the fish gets older.  An older shark will have more, but even the younger sharks have a lot of mercury relative to selenium.  Freshwater fish, if the people in Finland that were eating all of the freshwater fish had instead been getting some ocean fish along with their freshwater fish, we would probably see some beneficial protective effects, but as that study showed, the high mercury exposure that those people were getting from eating these high-mercury freshwater fish was accentuating their cardiac disease risk — I think it was their risk of sudden cardiac death — by 50%, which is a fairly significant increase.  But that’s in a population where freshwater fish with high mercury relative to selenium are being consumed.

And another thing that study found was that omega-3 was not as important in protection as the mercury was in causing harm.  So one of the things that we’re looking into more and more right now is that maybe the benefits of eating ocean fish, certainly the omega-3’s are important, but to protect the omega-3’s from oxidative damage — They’re very vulnerable to oxidative damage.  In fact, when we’re talking oxidative damage, that’s one of the types of molecules that is an easy target for an oxidative molecule to harm.  Destroying lipid layers on a nerve cell is going to be very harmful for signal transduction.  Terrible things happen when too much oxidative damage occurs.  But if you have selenium-dependent enzymes that are able to protect them against oxidative damage and there’s another type of selenium-dependent enzyme — There’s actually 30 different types of selenoenzymes in humans.  Another type of selenoenzyme takes fatty acids that have already been damaged and fixes them.

Chris Kresser:  Wow.

Dr. Nicholas Ralston:  Undoes the oxidative damage to them.  And selenium-dependent enzymes are also responsible for undoing a certain type of oxidative damage that occurs to proteins.  So without selenium-dependent enzymes that prevent and reverse oxidative damage, certainly seeing the adverse effects in the Faroes and in New Zealand and in Finland, in each of those cases, those adverse effects agree completely with what we would expect to see.  And conversely, in the areas where selenium-rich ocean fish are being consumed, the beneficial effects that are being seen completely agree with what we’d expect.  So the Selenium Health Benefit Value does predict the adverse effects where they’re being seen and the positive effects where they’re being seen.  And so far as we’re currently aware, this is the only seafood safety criterion that is able to predict both benefits and risks.

Chris Kresser:  That’s fascinating.  I wasn’t aware of that.  So in addition to helping prevent mercury toxicity, it prevents oxidative damage of the long-chain omega-3 fats, and that’s yet another reason to choose fish that have more selenium than mercury.

Dr. Nicholas Ralston:  Yeah.  The body is actually pretty capable, though.  If you’re taking in a low-selenium diet, the body will hang onto all the selenium it’s got, and it’ll take a long time for you to get deficient.  But taking in more is always good.  The body doesn’t mind getting rid of extra selenium, but if you’re taking in too little, well, it’s hard to find extra selenium if there’s not any in your diet.

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How to sustainably increase fish consumption

Chris Kresser:  Right.  So one last question, and then I want to pick your brain a little bit about some additional resources for folks about this stuff.  I think you’ve done a great job of making it clear why we should be eating more fish in general and particularly pregnant women and perhaps kids than the rates suggest that people are eating them now.  What about ecological considerations of increasing fish intake on a population-wide basis?  This is a concern of mine or at least something that I pay attention to, and I’m wondering if you have any thoughts on how we can safely increase fish consumption without destroying fish stocks even further.

Dr. Nicholas Ralston:  Yeah, sustainability is a major issue, and so if we did start having all of the women in this country start eating more fish, it could be a bit of a challenge because it’s certainly not an unlimited supply of ocean fish out there.  Other nations are taking quite a bit more ocean fish than perhaps the US fleets are, and the US fleets are very cautious and careful for various things that we mandate must not be caught, for instance, protecting sea turtles, dolphins, things of that nature, so American fishing fleets are very careful not to, or try as hard as they can not to catch either sea turtles or porpoises or any of the other species that we do want to protect, but not all fleets from around the world are being as cautious as American fleets.  So the fishing haul-in from the oceans, many nations are taking in a lot more and causing harm to turtles and porpoises, etc., around the world.  So we’ve got a problem that American fleets are not taking in as many relative to our population as perhaps some of the other nations.  It has to be sort of a worldwide agreement that certain types of species have to be protected, the sustainability of the fish that are present.  I mean, if you look at the world, they’re all connected, so it’s essentially almost one ocean.  Having too many fish collected in one area means less that can be collected in the other areas, so I think there’s got to be increased international agreement about how to properly take care of the world’s oceans.

Chris Kresser:  Um-hum.  I’ve read a few interesting things about some improvements in aquaculture and certain species like barramundi that have a favorable fatty acid profile and seem to do pretty well in aquaculture environments, so I wonder if that can make a contribution as well.

Dr. Nicholas Ralston:  Yeah, aquaculture is catching on pretty much worldwide.  In the US, there is increasing aquaculture, but we’re actually kind of slow off the starting block with aquaculture.  We have the opportunity to diminish the transportation costs.  Obviously catching a fish out in the middle of the ocean and getting it to the middle of the United States, there are transportation costs that are present that if we went to aquaculture we could probably diminish.  There are also some very nice balances that can be done using excess heat from certain processes.  One of the major costs for aquaculture is keeping the water the right temperature.  So some of the things that need to be done, a lot of good work is already going on, and I think the good news is that there are a lot of very good scientists doing a lot of very responsible work to get high-omega-3, selenium-rich fish developing as an aquaculture product for American consumers.

Chris Kresser:  Well, great.  So let’s talk a little bit about how people can find some more information on this.  On my website, the original article that I wrote about this where I first cited your research, Dr. Ralston, is called Is eating fish safe?  A lot safer than not eating fish!  And I will link to that in the show notes.  You, Dr. Ralston, sent me a link to a great documentary.  It’s about 25 minutes long.  It covers these issues in a way that’s really accessible to people even without a science background.  So can you tell us where people can find that?

Dr. Nicholas Ralston:  OK, that one is by PBS, and so most of the public broadcasters across the country are starting to pick it up.  That one actually has won a series of awards.  I should have my colleague, Laurie Raymond, here.  She was advisory on the making of the documentary.  But we do have an online version that can be watched at our website, and our website is UNDEERC.org, for University of North Dakota Energy and Environmental Research Center.  That’s the department that we’re a part of.  And then UNDEERC.org/Fish, and that’ll take them right to the information about it, and one of the headers is Documentary, and that’ll take them right into it.

Chris Kresser:  Great.  Yeah, I watched that earlier this week.  I think it’s a great resource maybe to send to your friends, family, maybe people who won’t listen to this full podcast or read an article but would watch a 20-minute documentary.  There was a picture I posted in my original article with the Selenium Health Benefit Value of ocean fish with some really easy-to-understand graphic bars.  Is that a good resource, or is there another listing somewhere of the Selenium Health Benefit Value that people can access?

Dr. Nicholas Ralston:  I think that a few other places are starting to post Selenium Health Benefit Values.  I’m pretty sure that’s available in some of the seafood sites on the West Coast and certainly in Hawaii.  We did our study of Pacific Ocean fish collaborating with a group in Hawaii, so I know that they have it on one of the seafood or ocean fish study group sites in Hawaii, and perhaps for your listeners — I don’t know if this is a podcast only or if they are able to go to your site to get this information.  We’ll make sure and get the links to those sites sent to you, too, so that you can have them available.

Chris Kresser:  Great.  Yeah, we can put them in the show notes.  We do a full transcript for all the podcasts and post that along with the show, so that would be helpful.  I appreciate that.

Dr. Nicholas Ralston:  Yeah.

Chris Kresser:  So last question:  What questions do you have as a researcher that you hope to answer in the future on this topic?  Or where do you think the future direction lies in this research?

Dr. Nicholas Ralston:  OK, well, we’re currently initiating studies where we’re going to look at the Gulf of Mexico.  We’re fairly confident that for the most part, Gulf of Mexico fish are going to be fine.  We’re a little bit concerned about some of the estuaries, so we want to look at mercury:selenium ratios in those fish.  Along the Alaska coast, there are some places where some types of fish in the estuaries do have reasonably high mercury:selenium ratios, and we want to identify those and let fishermen know that these fish collected from that certain estuary might be cause for concern.  But nationwide, we’re intending to develop the continuing EPA work that we’ve initiated and look more and more carefully at mercury:selenium molar ratios in freshwater bodies across the United States because as I mentioned, 98% of the lakes, rivers, and streams seem to be OK, but the 2% that don’t look like they have good mercury:selenium molar ratios in their fish, we need to look at them even more carefully and lakes from that entire region.  Maybe there are even cases that worse than the ones that we found.  So we want to spend a lot of time focusing on where are the fish that are cause for concern, focus in on them, and one of the interesting things that’s already been demonstrated in a series of studies across the world is that if you add safe forms of selenium to a water body, you can retire the mercury from the fish very rapidly and very safely.  It has to be done carefully because as we talked at the start of the show, too much selenium is not a good thing, but we’re very cognizant of that.  I serve on boards that are also reviewing places where there is too much selenium, so we’re completely aware of those risks, but so long as we use the low levels of addition that we intend to use, the safe forms that we intend to use, studies in Sweden have shown that you can diminish the amount of mercury in freshwater fish more than 80% in just three years.

Chris Kresser:  Wow.  That’s great news.

Dr. Nicholas Ralston:  Yeah, so we’re hoping to start doing some preliminary studies of that nature with special focus on these areas where there are high-mercury, low-selenium fish.

Chris Kresser:  Thank you so much, Dr. Ralston, for coming on the show.  I really appreciate it, and I look forward to seeing the outcome of some of this new research you’re doing.

Dr. Nicholas Ralston:  Well, I tremendously appreciate the chance to share this information, and I really appreciate having folks like you that are anxious to get the right information out to the public.

Steve Wright:  All right, thanks for listening to the podcast today.  We covered a lot of topics.  I learned a lot, and I’m sure you did, too.  If you have any questions for any upcoming Q&A shows we’re gonna do, please head over to ChrisKresser.com and use the podcast submission link.  If you enjoyed listening to the show today, we’d really appreciate it if you’d head over to iTunes and leave us a review.  It helps us get our message out to more people like yourself who might be in need of this kind of information.  Thanks and we’ll talk to you soon.

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Join the conversation

  1. With all due respect, these studies that you rely on Chris are funded and backed by the EPA! Really bad science, you are basically saying this ‘True there is a whole lot of mercury in the fish but miraculously it gets bonded by the Selenium (another mineral that is scarce!) and released!’- Really really bad science! Please see this medical report for some real science: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3395437/
    The reality is that we have destroyed the eco-systems in the water, globally and mercury is but one of the many problems with consuming fish!

  2. A toxicologist at my university talked about the topic of mercury in fish too. After one of his lectures he said, there was a doctor coming to him and told him that he eats a tin of tuna every day. He tested the doc and he discovered the highest mercury load he had ever seen. How this can be true if the mercury in tuna wouldnt be a problem?

    • I would say always trust your own judgment. Researchers contradict each other every time. Also mind the moderation. Fish is great. We advise everyone to eat fish, but in moderation. Especially large fish like Tuna.

  3. Also wondering about radiation’s effect on fish? Does the radiation exposure after the Japan reactor problem with the tsunami change anything about recommendations for fish from the pacific ocean? Thanks Chris!

  4. Table 2 of Bourdineaud, J. P., Dietary Mercury Exposure Resulted in Behavioral Differences in Mice Contaminated with Fish-Associated Methylmercury Compared to Methylmercury Chloride Added to Diet.

    has the following values in mcg/g for the 3 diets fed to mice

    Hg control MeHg Fish
    8.10-3 253.10-3 237.10-3
    equals 0.008 0.253 0.237
    Se 0.30 0,30 0.48

    To me it seems like the fish diet has a Se/Hg ratio 2:1, MeHg diet 1.2:2.
    Shouldn’t both these ratios be protective? or am I missing something?

  5. LOVED this podcast, thanks Chris and Dr Ralston!
    Just one point (I’m writing from Australia):

    Dr Ralston commented on shark-meat in fish & chips in NZ.
    “…shark meats, which nobody would do that now, but back in that time, apparently they were catching shark and it seemed like it was usable for that purpose…”

    It’s not an ‘oldie-timey’ practice at all. In Australia and New Zealand, shark is often the MOST common fish in our local take-away fish & chips. We call it ‘flake’ here in Oz. It’s usually gummy shark though it may also be Whiskery Shark, School Shark, Saw Shark, Elephant Fish, any of the Dogfish family or any one of a variety of other sharks and even rays. It’s never (as far as I can tell) large shark though.
    Just thought you’d like to know.