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Important Update on Selenium Supplementation


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selenium dosage, best form of selenium
Standard selenium dosage when you're deficient, is important for overall health.

A while back I wrote an article on the role of selenium in autoimmune thyroid disease.

I summarized several studies, which found that selenium supplementation reduced inflammation and damage to thyroid tissue, lowered thyroid antibody levels in the blood, and improved the conversion of T4 to T3.

However, I also sounded a note of caution when it comes to selenium supplementation:

“These preliminary studies show the positive effects of selenium supplementation on inflammatory activity in autoimmune thyroid conditions, but the long-term effects of supplementation on thyroid health are still unknown. And we know that selenium is an essential component of the enzymes that convert T4 to T3, but whether supplementation will increase serum T3 levels is unclear.

While it seems that selenium supplementation would be an obvious solution to poor thyroid function, long-term consumption of high doses of selenium can lead to complications such as gastrointestinal upsets, hair loss, white blotchy nails, garlic breath odor, fatigue, irritability, and mild nerve damage. (1) Additionally, supplementing with selenium in the context of low iodine status may actually aggravate hypothyroidism. Mario Renato Iwakura discusses this particular topic extensively on Paul Jaminet’s Perfect Health Diet blog.

For now, the best option for most people may be to include selenium-rich foods in the context of a healthy Paleo diet.”

A study published last year—which I just came across a few weeks ago—appears to validate this caution, at least for middle-aged men. (2) It was a large clinical trial (with over 35,000 participants) that examined the relationship between baseline selenium status, supplementation with selenium and vitamin E (either together or separately), and future rates of prostate cancer in men over 55 years of age.

Here’s what the researchers found:

  • Baseline selenium status alone was not associated with prostate cancer risk.
  • In men with high baseline toenail selenium (>60th percentile), selenium supplementation of 200 mcg/d increased the risk of high-grade prostate cancer by 62% when taken alone, and by 224% when taken with vitamin E.
  • In men with normal or low baseline toenail selenium (<60th percentile), selenium supplementation of 200 mcg/d (either alone or with vitamin E) did not substantially increase or decrease the risk of prostate cancer.

It’s worth pointing out that these findings contradict two previous (and considerably smaller) observational studies in the United States, which found that low toenail selenium status increases the risk of prostate cancer. (3, 4) In addition, a large Dutch study including over 58,000 participants found that those with the highest toenail selenium status had a 37% lower risk of prostate cancer than those with the lowest selenium status. (5)

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Should you be supplementing with selenium? Find out what @ChrisKresser has to say.

However, in the United States, selenium levels are typically much higher than they are in Holland, where deficiency is widespread. While it’s entirely possible that the very low selenium levels seen in Holland are associated with increased prostate cancer risk, there does not appear to be a correlation between selenium and prostate cancer in the United States—at least within the ranges of toenail selenium found in U.S. men over 55 years of age.

So what are we to make of this?

A growing body of research suggests that the effects of micronutrient supplementation are dependent upon the status of that nutrient in the target population. (6) If baseline levels of a particular nutrient are low, supplementation may lead to improved outcomes.

But if baseline levels are normal or high, supplementation may cause harm—as it appears to have done in this study. This shouldn’t be too much of a surprise; most nutrients have a “U-shaped curve”, which means that both too little and too much can cause problems.

On the other hand, several studies indicate that supplementing with selenium has several benefits for people with autoimmune thyroid disease, including reduced inflammation and damage to thyroid tissue. At least one study found that selenium supplementation produced these benefits even when selenium levels were normal at baseline. (7) Since both Hashimoto’s and Graves’ disease are associated with increased morbidity and mortality, an intervention that reduces the autoimmune/inflammatory burden in these conditions might be expected to reduce morbidity and mortality.

There’s little doubt that maintaining adequate selenium levels is important to immune and thyroid function. But given the potential long-term risk of supplementation, I think the best option for most people is to meet their need for selenium by eating selenium-rich foods. Great sources include: brazil nuts, crimini mushrooms, cod, shrimp, tuna, halibut, salmon, scallops, chicken, eggs, shiitake mushrooms, lamb, and turkey. Brazil nuts are a particularly rich source; just 2–3 a day will provide roughly 200 mcg of selenium, which is the amount used in many of the autoimmune thyroid studies.

Finally, while this study was focused on prostate cancer risk in men over 55, it’s probably prudent to assume that long-term selenium supplementation in both younger men and women of any age may also have undesirable effects.

I will, of course, continue to keep an eye on the research in this area and report back if any new findings come to light that would change my recommendation.

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Join the conversation

  1. Chris, can you recommend a thyroid specialist? I have ultrasound and bloodwork from my girlfriend in a country that has seen an explosion of thyroid cancer diagnosis in recent years. Her doc of course insists thyroidectomy and then systemic radioactive iodine.

    I want a better plan… Preferably less than a full thyroidectomy. And certainly no radiation. Indeed this might be a false diagnosis – papillary carcinoma diagnosis is usually based on visual judgement of ultrasound and biopsy samples under microscope.

  2. I have very low T3 though my T4 is nearly okay. I have been cold nearly all of my life. This is very interesting. I refuse to take synthetic lirothyronine for it and I try to manage it with food.

  3. As a patient with Hashimoto I would like to begin supplementing with Iiodine and selenium together, how long should I continue to supplement selenium before it could become harmful and if I stop supplementing with selenium should I also stop taking iodine?

  4. so many fake users here probably from big pharma with their own agenda.. spreading confusion and encouraging to use synthetic selenium by giving dumb reasons or personal experiences. there are 2 posts for example that were posted in the same day, same month, same year and within the same hour, that bashes the brazil nuts. the way of writing is same with plenty other clues and patterns of writting. do not BELIEVE that all comments are from normal people. i know someone who used to work for such a job, until he left it because he felt very wrong about it.

    big pharma, companies , have hundreds of thousands of websites and comments posted with fake information. don’t believe everything, think for yourself!

  5. Is there something wrong with taking Armor Thyroid? I have read a lot here about a bunch of different strategies (even naltrexone), but no one is talking about the natural hormones that are apparently almost identical to our own.

  6. Flawed Selenium Study
    The overwhelming scientific evidence supports selenium’s anticancer activity, and favors prudent dietary supplementation with selenium. Confusion arose, however, in 2009 with the publication of a single negative study, one that has contributed to substantial misinformation about the value of selenium in cancer prevention. Notably, those with false negative results may teach us more about selenium’s role in preventing malignancies than more positive reports.

    Known as SELECT (for Selenium and Vitamin E Cancer Prevention Trial), the study appeared to show that selenium, alone or in combination with vitamin E, had no detectable effect on preventing cancers.

    Many experts have since questioned the SELECT trial’s methodology and conclusions.One problem with that study was that it used only a SINGLE form of selenium. This selenium compound is just one of several different forms in which selenium is available for nutritional supplementation. The major flaw in this study is that it used synthetic alpha tocopherol, which displaced critical gamma tocopherol from cells, thereby increasing cancer risk.

  7. Danish researchers from Copenhagen University have discovered that methylated selenium compounds found in garlic, broccoli and selenium yeast can better enable the immune system to fight cancer.
    I take a 100mcg tablet of organic high-selenium yeast every 1-2 days, especially because I am worried about the mercury from seafood.

  8. I ask myself is soaking of brazil nuts will just also wash out the selenocysteine out of the nuts. because this is water soluble. In that case the brazil nuts are worthless as selenium delivering food.

    I guess there is no evidence about that?! We only think about soaking helps with phytates but why should it only lead to decreases of phytates? All water soluble minerals and vitamins could be lost by soaking?!

    I bet there was no research done so how do we know?

    • How about drinking the water you soak the nuts in? Also, I’m not too sure that eating 2 to 3 Brazil Nuts a day will have enough phytates to be problematic.

      Just a thought.

    • I have wasted countless hours searching the internet for the answer to this. Very frustrating.