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RHR: How to Make Intermittent Fasting Work For You, with Cynthia Thurlow


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Cynthia Thurlow, the author of Intermittent Fasting Transformation, joins Chris Kresser to discuss the unique challenges women face with intermittent fasting. Cynthia explains how to customize an approach to intermittent fasting that recognizes menstrual cycles and life circumstances, and also how to tell if intermittent fasting isn’t working for you.

In this episode, we discuss:

  • Reasons why women struggle with intermittent fasting
  • How to fast around menstrual cycles
  • Hormetic stressors and the essentialness of intrinsic awareness of your body
  • Signs that intermittent fasting is not working for you
  • What an intermittent fasting routine looks like, including chronobiology, and then to be cautious about intermittent fasting
  • Macro ratios and meal composition

Show notes:

  • Cynthia Thurlow’s website
  • Join us in person at Snowbird Resort this Labor Day weekend. Early Bird registration is now open. Go to kresser.co/adaptlive to learn more and secure your spot.

Hey, everybody, Chris Kresser here. Welcome to another episode of Revolution Health Radio. Intermittent fasting is all the rage. It’s a hot topic in the media. There [are] lots of books that have been written about it, entire podcasts [have been] dedicated to it, [and] lots of research studies have been published on it over the past few years. I’ve been doing it myself and recommending it to patients and writing and speaking about it for many, many years. I think it’s a powerful tool when it’s used appropriately and in the right circumstances. There are also situations where one might need to be cautious about intermittent fasting, and one group that I think needs to be particularly careful is women. And that’s the topic of this show.

I invited Cynthia Thurlow to join me. She’s a nurse practitioner and founder of the Everyday Wellness Project and is an expert on intermittent fasting, particularly for women and how to individualize and customize an approach to intermittent fasting that recognizes the life stage differences with cycling, non-cycling, perimenopause, and menopause, and adapts the practice of intermittent fasting to those different life stage cycles so that women can be more successful with it and have fewer issues. In my clinical practice, I definitely have noticed that women in those various stages tend to have a harder time with intermittent fasting, especially if they’re not careful and they don’t pay attention to the different needs of these different stages. We talk about in the show how women can cultivate more awareness of these stages and the different needs they have in each of them, and how that translates into differences in what they might do in terms of intermittent fasting and food intake and macronutrient ratios, and things like that. [We talk about] the benefits of intermittent fasting] for women in particular, with a focus on metabolic flexibility as a desired outcome. [We talk about] what the signs are that intermittent fasting may not be working and what to do about it. [We talk about] how women can eat, train, and fast around their menstrual cycles to improve their quality of life. This is something that I haven’t seen very many people focus on in their discussions of intermittent fasting, and yet I think it’s a really key factor, as you’ll learn as you listen to this show.

I really enjoyed this discussion. I think there aren’t that many people who are approaching intermittent fasting with as much nuance and consideration of all the important variables as Cynthia does. I really hope [that] you get a lot out of this conversation and that it helps you to, first of all, determine whether intermittent fasting might be right for you. And then second of all, figure out how to make it most effective and fit best into your particular life. Even though the focus of this show was on women, I think there’s still a lot that men will get out of it, as well, especially in the discussion around the importance of individualization.

All right, let’s dive in.

Chris Kresser:  Cynthia, welcome to the show. It’s a pleasure to have you on.

Cynthia Thurlow:  Thanks for having me. I’ve been looking forward to our conversation.

Chris Kresser:  So intermittent fasting is all the rage these days, right? It’s been a hot topic, certainly in scientific literature, for a while. Folks like you and I have been writing about it for many years, and now you can look in pretty much any mainstream media outlet and see articles on intermittent fasting. Even [on] major TV news shows, radio, etc. I think it’s an incredibly powerful tool, and one of the populations [that] needs to pay particular attention to how they do it is women. So I’m really excited about this conversation because I know that [it’s] a particular focus for you, and something that you’ve experimented with a lot [in] the people you’ve worked with, and written about a lot. So let’s start with, since I’ve talked a lot about intermittent fasting in general on the show, what are some of the pitfalls, particularly for women, with intermittent fasting? Where do you see things go off the rails? And then we can talk about what to do about that, and what the potential is when you really get it dialed.

Reasons Women Struggle with Intermittent Fasting

Cynthia Thurlow:  That’s a really great way to position the conversation because, on a lot of levels, what makes women unique in terms of our physiology is also what should cause pause in terms of our approaches. For me, personally, I think that a lot of the mistakes, if you will, or the pitfalls that I see women doing is not fasting for your menstrual cycle. If you’re still in your peak fertile years, you have to fast differently than a woman in perimenopause, the five to 10 years preceding menopause, and you have to fast differently than a woman in menopause. So not really honoring [their] own unique biorhythms of the body. And the way that this typically will manifest is, number one, women will fast the same way they do from day one of their menstrual cycle to the very end, [and number] two, women will, especially [in] that latter part of the luteal phase as they’re transitioning into getting their menstrual cycle, they’ll be [hungrier], they will struggle more with extending that fast, and they’ll just push through it. They’ll make themselves miserable.

I think the last piece of that is, I like to think of a woman’s menstrual cycle as a barometer of her health. So if your period goes away and you’re not pregnant, that is absolutely a sign that your body’s perception of your food frequency or your food window or the lack of nutrients throughout the 24-hour cycle of the day or throughout a week or a month, your body may perceive that you’re in a famine situation. Those are the typical mistakes that I see women making, and then the dovetail to that is women in perimenopause and menopause [who] think [that] if a little bit of fasting is good, then more fasting is better. So they will over fast, over exercise, [and] not get enough sleep, and those things can really disrupt the biorhythms of the body. I always say that when you think about intermittent fasting, it is designed to be a hormetic stressor. The right amount of stress at the right time in the right amount. When we overdo it as women, we can really set ourselves up for adrenal, thyroid, [and] sex hormone dysregulation, not to mention the fact [that] it can inflame our bodies. So [men and] postmenopausal women  often have an easier time with approaching intermittent fasting. But those are definitely some of the common mistakes/pitfalls that I will see.

Chris Kresser:  There’s so much to unpack there. Working backward, and I’ll just put a pin in this so we can come back to it, the concept of a hormetic stressor is so important to understand. And it’s where a lot of people go wrong. Not just women, but [also] men. With all kinds of different interventions, including fasting, but also exercise, sauna, cold therapy, etc.

Let’s rewind to the first few things that you were talking about, because that’s really what I think is such an important concept to understand. And frankly, I’ve been a bit surprised by the lack of attention to this because it is fairly obvious that a woman who’s still menstruating has clear cycles on a regular basis and that that affects all aspects of physiology. Yet, aside from you, I’ve never really heard anybody focus on this in depth, and it rarely gets mentioned in conversations about how women should be approaching intermittent fasting. So let’s start there, because you threw out a couple of cautions or warnings about how women should be adjusting their approach to fasting based on where they are in their menstrual cycle.

Maybe you could break down, just for those listeners who are less aware of the terminology around the phases of the cycle—follicular, luteal, ovulation—what changes are happening physiologically during those parts of the cycle that are specifically related to or influenced by fasting and food intake.

Impact of the Menstrual Cycle

Cynthia Thurlow:  It’s really interesting to me [that] I know more about the menstrual cycle now than I ever did as a nurse or when I was practicing in [a] clinic as a nurse practitioner. We want to think about our menstrual cycle as this infradian rhythm, this 28- to 30-day intrinsic biorhythm that’s unique to us as women. From the day you start bleeding until right before ovulation, that is the follicular phase. I’m going to oversimplify things, but this is when estrogen predominates. And estrogen is our super hormone. We can push our workouts, we can do longer fasts, we can go more ketogenic or low-carb and our body can handle that additional stressor. [Then] we go to the point of ovulation, when we have these surges in testosterone and estrogen and progesterone. After ovulation, we set the stage for the luteal phase. It’s when progesterone predominates.

Progesterone is this lovely mellow hormone, and we have to eat differently and fast differently and exercise differently in the second phase of the menstrual cycle. This is when women sometimes can’t push the envelope. Maybe they can’t do intense exercise. They lean toward yoga or pilates. Maybe they’re doing more walking. This is a time when we become a little bit more insulin resistant, physiologically, as we’re getting closer to our menstrual cycle. And for a lot of women [who] have glucometers or continuous glucose monitors, they can actually monitor these trends. As we have fluctuations in progesterone, that can impact anxiety, that can impact depression, [and] that can disrupt sleep. I never realized why my sleep was so bad the last few days before my menstrual cycle. It’s because we [have] these fluctuating drops in progesterone. As we get closer to menstruation, that five to seven days [beforehand], we want to take our foot off the gas pedal. We want to do less fasting. We can [still] do 12 hours of digestive rest, and there are very few people that would not benefit from 12 hours of not eating. In fact, most people would benefit from eating in a 12-hour feeding window. But we really have to take our foot off the gas when it comes to fasting.

Then when we start bleeding, on day one [of] bleeding, we can go back to a fasting schedule. I find, for a lot of women, in the last five to seven days, they need more high-quality carbohydrates. It might be more sweet potato or root vegetables. This is not the time to pig out on pasta and bread and all the pizza that you want and all the ice cream that you want. But I find for many women [who] are still at their peak fertile years, so under the age of 35, our bodies are really primed for procreation. Even if we’re choosing not to have children, that is still a time when our brain and our ovaries are so interconnected with one another, in [the] best case scenario, that it’s taking in information around us all the time trying to decide, “Am I in a position where I could actually maintain and grow a human?” I think we start to understand that there’s probably three weeks out of the month when women can fast, to a certain extent. I think women at peak childbearing age [who] are very active [and] lean should not be fasting every day. If you’re an obese woman with [polycystic ovary syndrome], or you’ve got some insulin resistance, you probably have some degree of fluctuation that you [are] able to do in those first three weeks. But really leaning into our physiology and saying, “I’m going to honor where I am in time and space, and I’m going to back off on the fasting the five to seven days preceding [my] menstrual cycle,” then women can do really well.

I agree with you that there’s not enough focus on this, and there’s also a lot of fear mongering. It’s something that makes me frustrated and angry as a clinician because I don’t want women [for whom it] would be appropriate to fast at a portion of their menstrual cycle to not do it at all out of fear that somehow they’re going to have some profound detriment to their bodies or to their health.

Chris Kresser:  That’s such an important point, and a lot of the time comes down to nuance. And as we both know very well, nuance doesn’t tend to make it into mainstream media or popular approaches to healthcare. It’s either black [or] white, it’s good [or] bad, and that’s the end of the story. But of course, it’s not the end of the story. It’s usually just the beginning of the story, as we’re seeing here.

External Stressors and Leaning into Your Personal Balance

Let’s talk about the concept of a hormetic stressor. We’ve alluded to that a few times, and you just made reference to other things that might be happening outside of the choice[s] of how much we’re eating and how frequently we’re eating that [have] influence, as well, [on] the decision of when to fast and how much to fast. I’ve talked a lot about this on the show, so we won’t go into too much detail, but a hormetic stressor is something that, when it’s done, promotes a positive adaptation. Exercise is an example of that. Fasting is another example, sauna, cold therapy, etc. But if you overdo a hormetic stressor, [then] it just becomes a plain old stressor. Something that no longer is inducing a positive adaptation, but that’s [instead] causing a stress response in the body. It sounds like you are advocating for not only looking at what’s happening in the menstrual cycle, which affects whether fasting will be perceived as a hormetic stressor or just a stressor, [but] you’re also asking people to look at what else is going on.

If a woman is working full time outside of the home and raising three kids and doing four CrossFit workouts a week and staying up late sending emails and getting four or five hours of sleep, even if she’s in the follicular phase of her cycle, it might not be a good idea to really hit intermittent fasting hard in that circumstance.

Cynthia Thurlow:  No, and I so agree with you. I speak from personal experience that I did all the wrong things in perimenopause without realizing it. I think it’s important for us to understand, as a concept, that these hormetic stressors are designed to make us stronger [and] more resilient. But women have a tendency to overdo it because we’re serving everyone in our lives and we probably think about ourselves less. As an example, I just had a book published. And I remember saying to my husband, “I could stay up all night long to do emails and catch up on things, but what I actually need to do is go to bed.” There’s all this balance that we have to do. Getting high-quality sleep is probably the one thing that every single human, irrespective of gender, needs to be focused on. But it’s something we don’t value as a society. And yet, we’re very hedonistic. We like that dopamine release, we want to ensure that we’re staying connected all the time, we’re making ourselves available, [and] we’re doing all the workouts.

As it pertains to women in particular, and certainly women north of late 30s, early 40s, all of a sudden, these hormetic stressors can be, it’s like a balancing act, and each one of us might have a different lever that can be pushed farther than someone else. I know for a lot of women, they think that if a little bit of fasting is good, then it means more fasting is better. I had a discussion with someone online the other day talking about OMAD—one meal a day. I’m sure your listeners are probably very familiar with this term. I kept saying, “I don’t mind OMAD [if] you’ve eaten too much the day before, [if] you’re on vacation, you overdid it, [or] you overindulged. I get it.” But as a rule, OMAD is not going to give you enough macros. You’re not going to get enough protein, you’re not going to be able to support muscle protein synthesis, [and] you’re not going to ward off sarcopenia, which is muscle loss with aging. It becomes this domino effect when we’re talking about sleep and stress management and anti-inflammatory nutrition and exercise, and then adding in fasting. All these pieces of hormesis, along with cold therapies, heat therapies, exercise, [and] different types of foods that can boost the benefits of fasting. It’s a very delicate balance. And I think we really do ourselves a disservice if we are not leaning into what makes the most sense.

I think a lot of people don’t realize until they get to that point and then all of a sudden, they’re like, “What do I have to do?” So I think, on a lot of levels, we have to start those conversations earlier so we know what to anticipate. And this applies to men and women. It’s not just unique to women. But I think it’s important for us to find that right balance for each one of us.

Chris Kresser:  I couldn’t agree more. In my experience as a clinician, I’ve found, and [this is] just my assessment of the culture at large, we have a tendency to want to create rigid rules and then follow them. I think that’s easier for a lot of people than to pay attention to [the] natural cycle, for example. Or tune in to what’s actually happening in my body and make decisions based on that. Because, understandably, that requires number one, paying attention, which is not always easy, [and] number two, it requires some discernment and thinking about what I’m noticing when I’m paying attention. And number three, it places the responsibility for decision-making squarely on my own shoulders as an individual, and I’m not relying on some guru or internet forum where everybody believes the same thing, or whatever it is. And that’s unfortunate because I think it has led a lot of people in the wrong direction with a lot of different approaches, whether it’s being too extreme with intermittent fasting, or following a carnivore diet when there’s no reason for them to do that objectively, and could potentially cause harm. Or someone adopting a ketogenic diet just because they’ve read about it and it sounds cool, but there’s really no reason for them to do it and they don’t do it properly and it ends up going in the wrong direction.

I think, and I imagine you’d agree, it’s so important [that] all of what we’re talking about starts with some level of awareness of what’s going on in your own body. Being willing to pay attention to that and being willing to listen to what’s happening there. I’ll be the first to admit, that’s not always easy. Especially [for] those of us who are driven and have a tendency to override whatever we’re noticing and that voice. But it seems to me like you’re really trying to encourage women and everybody else to cultivate that awareness and really pay attention to what’s happening.

Cynthia Thurlow:  Well, it’s so contrary to how I was trained. It’s so different from the newbie approach I took as a nurse practitioner. My whole background is in clinical cardiology. I’ve managed very sick patients in ICUs and ERs, and flown people from one hospital to another, and then managed very complex patients in the clinic. And that was not the way I viewed things 20 years ago. But as I became more seasoned, I would encourage my patients to lean into what their bodies were telling them. We have a culture that is very uncomfortable with feeling uncomfortable. They don’t like it. So what do we do? We have vices. We drown ourselves in binge watching on Netflix, or Hulu, or we drink too much, or we use food as a crutch, or we engage in recreational drugs or behaviors that are not as socially acceptable. And there’s no judgment on my part. It’s just something I’ve watched. So a lot of the work that I do now, I really encourage women to lean in. What is your body telling you? I know for myself, if I wake up a few mornings [and] I’m very tired, which is unusual, usually, I’m up without an alarm clock, I’m like, “Okay, what’s my body telling me?”

I think it makes us a little uncomfortable when we’ve conditioned our patients to just be told what needs to happen instead of saying, “Okay, let’s try this. Let me know how it works for you.” I like a much more collegial relationship when I’m working with a patient or a client, but I know that wasn’t always the case. There were many years where my patients would come [in, and] I’d tell [them], “This is the prescription you need. This is how long you take it for. You come back in a month. If you have any problems in between, you talk to the nurse.” Whereas now, I think having a much more collaborative relationship allows people to feel comfortable with that degree of uncertainty. They’re like, “This is different. I don’t know how I feel about this. I like someone else making the decision for me.” And it’s like, “No, no, intrinsically, your body will let you know what it needs. You just have to lean into it.” And that’s a very different way of thinking about health and wellness.

Certainly, you and I both know that we don’t have a wellness system here in the United States. It’s overwhelmingly reactionary. It’s focused on addressing disease, not preventing disease. These are broad, overreaching themes that we’ve been a part of. I always say to people that it’s going to take time to process looking at this differently than where you wait until you get sick and then all of a sudden, everyone’s like, “Oh, now we have to be reactionary. You need this medication; you need to go to this place.” As opposed to being preventive-focused or forward-thinking in terms of methodologies and approaches.

Chris Kresser:  I agree with all that. I think at a fundamental level, and this is something that took me years to realize as a clinician because I had my own experience with chronic illness that forced me to pay attention to what was going on in my body and listen and respond appropriately. And that was not a voluntary process in my case, and it took years for [it] to be. But I think I forgot how that’s not the default mode for most of us in this country. We’re taught to essentially outsource our healthcare to a doctor or nurse or whoever it is that we’re seeing and to leave the decision-making to them. And I realized when I was having these conversations with patients and asking them to make choices based on what was happening in their body, that was a really threatening thing to say. Even when people were game for trying that, it took some time for them to actually cultivate that ability to listen and then to trust their own judgment more than any book they’ve read, or any guru they follow or whatever, and actually respond in the present moment to what’s happening.

I think we’ve done enough on that. But it is important to cover because any conversation about, “Hey, just follow your rhythms,” [is] not going to go anywhere unless somebody’s actually willing to do that and cultivates that awareness and ability. So along those lines, I would love to talk a little bit about what are the signs that someone should be watching out for? Let’s say a woman decides to start intermittent fasting. A few days into it, what signs would you be counseling them to look out for as indicators that maybe this is too much or maybe you need to back off a little bit?

Signs That Intermittent Fasting Is Not Working for You

Cynthia Thurlow:  Well, I think it all starts with something as basic as energy. Now, you should intrinsically have more energy if you are in a fasted state. Should is the operative word. If suddenly, you go from having reasonable energy to feeling very tired, very fatigued, obviously, that’s a concern. I also think about if you suddenly have sleep disturbances, if you’re sleeping really well, and all of a sudden, your sleep is terrible. Again, as I said, sleep is foundational to our health. So I take that really seriously. Another thing that I will sometimes see [is that] it’s not uncommon for women when they start fasting [to] have a cycle or two that might be wonky, a little heavier, a little lighter. I don’t worry about that. But if your menstrual cycle goes away and you’re not pregnant, that is obviously a cause for concern because I think of our cycle as a barometer of our health. So if your cycle goes away, especially if you’re at peak fertility or [are] even in perimenopause, I would definitely want to lean into that.

I think about other things like not having enough energy to get through a workout, not having enough energy to get through your day, not feeling like you’re clear cognitively. Largely because one of the first benefits that people will articulate to me is, “Oh my gosh, I’m so much more mentally clear. I can power through my morning.” If you have exactly the opposite [happen], it’s probably speaking to a variety of things, but it could also be that your body is just so metabolically inflexible, it’s not yet at a point where it can fuel going longer without eating. But those are typically the more common things that I will see. Sometimes people will complain about constipation, which can be a byproduct of changes in their macros. If suddenly, someone concurrently start[s] fasting and then they’re like, “I’m going to go low-carb and ketogenic,” and their body doesn’t process fats all that effectively or they’ve got a sluggish gallbladder, you can sometimes see [constipation and bloating]. But a lot of what I’ve just [gone] over can be a reflection of a lack of hydration; it can be a reflection of not enough electrolyte repletion, [and] that’s critical. I always say these are the things you must do while in your fasted state. Hydration and electrolytes are without question very, very important.

A lot of those headaches, nausea, things like that can be related to dehydration. Those are the more common things. But the most significant thing that I see, the thing I get the most concerned about is the changes in the menstrual cycle. If you all of a sudden don’t have a period and you’ve gone a cycle or two, then that’s a cause for concern because that’s the hormetic stress. The right amount of stress in the right amount at the right time.

Chris Kresser:  I agree. And I’ve, for many, many years, treated women who are having difficulty conceiving. This has always been a major focus in my practice, the menstrual cycle and [it] as an indicator of health. And in Chinese medicine, which I studied as well, that’s always [been] seen as a key reflection of health and well-being. And I became alarmed at one point at the number of young women I was seeing [who] had amenorrhea, or had really disrupted menstrual cycles. Often, in those cases, it was a situation that we’ve been talking about so far, where the number of demands that are being made on the system are far exceeding that woman’s ability to meet those demands. In some cases, those are not changeable; [they’re] just the demands of life. But in other cases, they came down to inappropriate choices of exercise and too much fasting, too much carbohydrate restriction. All those things put together in a perfect storm that leads to those changes in the menstrual cycle. So I have had that experience, as well.

And I think energy, like you started with, is a pretty good old-fashioned barometer. How do you actually feel when you’re doing this? Do you feel better? Or do you feel worse? Which goes back to our conversation about paying attention and seeing what’s going on.

On this episode of Revolution Health Radio, I’m joined by Cynthia Thurlow to learn about how women can properly eat, train, and fast around their menstrual cycles to improve their quality of life. #chriskresser #intermittentfasting #womenshealth

We haven’t even really defined what you mean by intermittent fasting yet. Because there are a lot of different definitions out there. Time-restricted eating, and you alluded to a 12-hour window of not eating as being a good idea for most people. According to statistics, not many people are doing that. I would consider that to be pretty standard and not intermittent fasting. But that gets defined as intermittent fasting in some studies, just a 12-hour period without food. So what do you mean when you say intermittent fasting? Do you have a typical recommendation as a starting place for the window of food intake for women?

What Does It Mean to “Intermittent Fast”?

Cynthia Thurlow:  I think that’s a really good point. I affectionately refer to a 12-hour window as digestive rest, something that all of us should embrace. But when I define intermittent fasting, I define it as eating less often. That we have a prescribed time period during the day in which we eat, and then we have a prescribed time period in which we do not. I like for women to focus on a goal [of] 16 hours fast with an eight-hour feeding window. You may not go from eating 10 times a day to then going 16 hours fasted. There [are] very specific steps that we take, methodology-wise, to make that come to fruition. But I agree with you and I echo everything you just said [in] that there are a lot of definitions, [and] there’s a lot of confusing terms. But when I think about fasting, I think about a minimum [of] 14 hours of not eating.

Now, that still gives people 10 hours in which to eat. The beauty is that, once you are fat-adapted, once your body is able to utilize both carbohydrates and fats as a fuel source, then you can get very flexible with those fasting and feeding windows. But as a good rule of thumb, once you are fat-adapted, you can be more flexible. Maybe you’re going to eat in a six-hour window; maybe you’re going to eat in an eight-hour window. [When] you go on vacation, you open [up] the window. You have a day where you’ve overindulged, and the next day, you just have one big meal. I think that’s the beauty, and it doesn’t have to be rigid. That’s the one thing that I think is very important for people to understand, when I look at the lay press and I look at a lot of what’s on social media, it’s that you do the same thing day in and day out. I remind people, just like we don’t eat the same foods every day, we don’t do the same exact exercise, [and] we don’t do the same fasting regimen every day. From a very basic perspective, fasting is eating less often. Here in the United States, I think the average that I read most recently is [that] we eat anywhere from six to 10 times a day, if you include sugar-sweetened beverages and food. So is it any wonder that we’re a metabolically unhealthy population? When we talk about digestive rest, that’s a really good starting point for people who are eating throughout the day, all day long, all night long.

Chris Kresser:  Absolutely. I saw a fascinating talk, it’s got to be almost 10 years ago, at one of the first Ancestral Health Symposiums and it was Nassim Taleb, who you wouldn’t expect to be speaking at the Ancestral Health Symposium. He’s not in the health field at all. This was the one that was at Harvard, and he did a talk about chaos theory as it might apply to health. It was one of the best talks of any health conference I’ve ever seen, actually. The gist of it was, it’s not natural or normal for humans or any animal to follow a really consistent, rigid routine when it comes to food or exercise, or pretty much any other variable. And that really stuck with me. We’ve touched on it a few times in this conversation where just because you do a little of something and it’s good, more of it isn’t necessarily better. Just because you do something for a few days in a row and it benefits you, it doesn’t mean that doing it for 100 days in a row is going to be better. Just because you’ve had success with something in the past in a certain way doesn’t mean that it’s going to be successful in the future in that same way. Because all these conditions and variables are changing all the time.

I love that you really emphasize, and I always have as well, when it comes to intermittent fasting, not making it a chore or a rigid routine that you always have to follow. If you wake up one morning and you’ve got a huge demanding day, and it feels like you would do better if you ate that morning, then eat that morning. Don’t not eat because you marked on your calendar that it was an intermittent fasting day. I think that’s where so many people tend to go wrong with this is they become a slave to it rather than approaching it as something that can benefit them.

Cynthia Thurlow:  I think it’s really important for people to understand that you may have seasons of intermittent fasting in your lifestyle. As an example, my family and I went to Costa Rica for Christmas and we had a very, very, very active vacation. I have teenage boys, and everything was really focused on being outside and being very active. And I actually sat down and ate breakfast three days in a row. It was very strange for me because I don’t usually eat until 10:00 or 11:00 a.m. But I did it because I was on vacation and I said, “I’m going to [eat] breakfast and then I’m going to eat dinner. I’ll just change things up.” And the beauty of that is you’re not beholden to this rigid dogma that I think is so pervasive in our society right now that if you fall into one bucket, whether it’s carnivore, plant-based, Paleo, keto, fasting, that you can’t ever break out of that bucket. I always encourage the women that I’m working with to be open-minded. Change things up, [and] see what works best.

I can tell you, I still don’t like eating breakfast. But it was an interesting way to approach my vacation because I knew I was going to go mostly all day without eating. So I figured if I’m going to eat, I might as well eat now when I have an opportunity. But I think it’s important for us to lean into, “Did you have a heavy workout? Are you much hungrier today?” Typically, for me, on leg days , I will break my fast earlier. I don’t force myself to make myself miserable until a certain time. I just say, if it’s 9:30 in the morning and I’m starving, and I can recognize true intrinsic hunger, I will lean into that and I will break my fast because fasting should not be torturous. Fasting should be, in my mind, once you’re fat-adapted, it should be effortless. I think the longer I fast, the more it becomes a spiritual practice for me. It really becomes more than just the physiologic things that are going on behind the scenes. And that seems to be the case with a lot of people that I work with. They find that it makes them feel good. Maybe they get a little bit euphoric. Maybe there’s a little bit of endogenous secretion of dopamine and this norepinephrine and epinephrine that’s secreted and these counterregulatory hormones that can really make you feel good in a fasted state.

Chris Kresser:  Absolutely. Before we move on, I just want to also mention, in my experience, because I’ve worked with a lot of patients who are pretty sick over the years, and if someone is in a significant catabolic state where they’ve been chronically ill for a long time, they’ve lost a lot of weight, their energy level [is] really low, and they just haven’t been able to feel their body for various reasons, they have a digestive illness or any number of other chronic conditions, that’s another area where I would suggest caution with intermittent fasting. Because, again, if you think of the concept of hormetic stress, that works well against a background of relative robustness and not a lot of other stressors. But if you have a serious chronic illness that’s already taken you below baseline in terms of stress and adaptation and you’re in that catabolic or breakdown state, if you add another stressor that would be ordinarily hormetic for most people, it doesn’t necessarily mean it will be hormetic for you. So, that’s another situation where I’ve seen people go wrong with fasting.

Cynthia Thurlow:  Yeah, and it’s interesting because I’m always very transparent about this. In 2019, I spent 13 days in the hospital with a ruptured appendix and I lost 15 pounds. I mean, my body literally catabolized all of my muscle[s] to keep me [alive] because I didn’t eat for 13 days. And when I left the hospital, I remember it was the first time I recall as an adult feeling reactive hypoglycemia. I felt terrible. Because I had so many complications, the only thing I could eat for nine months was meat, which is a whole separate conversation. But you better believe until I started gaining some weight back, I wasn’t fasting for months. And so, much to your point that, if you’re in a position where you’re frail, [or] you’ve recently been hospitalized, [or] you had a recent illness, that’s not the time to push the gas pedal down. I think that’s an important distinction.

I always say [that] you could have a chronic disease state, but if you’re stable on medication, you feel good, you’re sleeping well, [and] you have good energy, then you certainly can entertain that. My other comment [that] I would make about people [for whom] fasting is really not appropriate, and I always get hate mail for this, [is] people [who] have a disordered relationship with food. If you are a binge eater [or have a] history of anorexia or bulimia, unless you are concurrently working with a therapist and you and they have decided you are healthy enough to potentially utilize this strategy, I just have consistently found it’s been with very few exceptions that most people [who have] that history really don’t benefit from being restrictive. For them, it can trigger some of those maladaptive processes. [It’s] really important to mention that. There are always exceptions, but it’s been my experience, especially if someone has a history of anorexia, that the cure rate is so low that we definitely want to be sensitive to the fact that we don’t want to put them in it to compromise their rehabilitation and recovery.

Chris Kresser:  I agree with that 100 percent. Let’s go back a little bit to the “how.” We said [an] eight-hour window food intake: do you care when that is? Do you care whether that’s in the morning and through lunch, and you miss dinner? Which in some ways, if you consider physiology and circadian rhythms, [is] the most optimal way to do it. But what I see, which is more common and more optimal socially, perhaps, and what fits into our lifestyle better is people skipping breakfast and eating lunch and dinner. Do you have a preference there? Or is it just up to the individual’s lifestyle and choice?

Cynthia Thurlow:  The more I understand about chronobiology and circadian biology, the more fascinated I am with it. If anyone that’s listening [is] not familiar with that term, we actually, based on research, do better eating earlier in the day. That doesn’t mean within an hour of waking up. It just means you may do better breaking your fast [at] 9:00 [or] 10:00 a.m. and then eating a meal in the middle of the afternoon. Now, not everyone can necessarily do that. But you’re going to do better eating earlier in the day than later. We also know that insulin sensitivity is better in the morning and early afternoon and that the longer we go into the day, late afternoon, early evening, we become less insulin sensitive. This really gets ramped up for men and women, as women transition into perimenopause and menopause and men transition into andropause, as we have these hormonal fluctuations in sex hormones. I have learned during the last two years that I do much better ending my feeding window earlier. And I find for a lot of women, that’s the same. We can talk about the fact that we’re designed to eat when it’s light outside, and we’re really not designed to eat when it’s dark outside. We have melatonin clocks.

Melatonin is a hormone that’s secreted primarily in the pineal gland. But we also have melatonin clocks throughout our body, including our gut. So if you’re eating at 9:00 or 10:00 at night within two to three hours of going to bed, your body will suppress melatonin, [and] it’ll increase cortisol in an effort to process this food bolus that you have consumed. For anyone [who] wears an Oura ring or a Whoop band, you can look at the data. To me, it’s fascinating. My body does not like when I eat within two to three hours before bedtime, and my Oura ring will squawk at me. I was just in Utah last week, and even though I was still eating relatively early for Utah time, it was late for East Coast time. I think bio-individuality rules when we’re looking at meal timing. I think it depends a lot on your lifestyle. When I was working in the hospital, there was no way I could have eaten at 10:00 and 2:00. That would have been woefully unrealistic. But now that I’m an entrepreneur, I can facilitate that. I think it has to be whatever is sanity-inducing for you. But we do know that eating later in the day, eating more, drinking more alcohol, [and] eating more processed carbs [is] definitely going to have a detrimental impact on blood sugar, cortisol, [and] melatonin. It’s really making decisions for yourself for what is in your best interest. I think that’s the easiest way to put it. I’m not rigidly dogmatic about it, but I think everyone should experiment a little bit to see what makes them feel good.

I have a child, as an example. He’s a teenager, [and] he’s an athlete. He doesn’t like eating breakfast. And he eats a massive amount of food in what I will affectionately refer to as [his] feeding window because I don’t recommend that teenagers and children fast at all. The amount of calories he consumes in his feeding window is unbelievable. But he has found a system that works for him. He likes to eat when he’s done with swim team at 9:00 at night. He likes to eat a really late lunch in the middle of the afternoon. And that works for him. I think each one of us has to decide what works for us physiologically, emotionally, etc. But [there are] two things to consider. Number one [is] we become physiologically more insulin resistant as the day goes on. Number two, the later you eat in the day, the more likely it is to disrupt secretion of melatonin. I talked about sleep being foundational to our health, and I think that’s a really important point to really emphasize.

Chris Kresser:  I’ve done a ton of experimentation with this personally and with patients over the years, and also research [on it]. And I would say those all converged to, forgetting about any other factors like social factors, work, etc., the ideal window is probably 10:00 to 6:00, [or] 9:00 to 5:00, or something like that, with the major meals being right [at] 9:00 [or] 10:00, and then maybe [at] 3:00 [or] 4:00, or something, [at] 2:00 [or] 3:00, something like that. And very few people are actually able to pull that off for all the reasons we talked about. Unless you’re working from home [or] you run your own business, that can be difficult. And then there’s the social aspect of people, and I put great value in this, actually. People being able to sit down and eat dinner with their family or socially go out and eat with other people. So I’m definitely not diminishing the importance of those relationships and the social contract around eating. That has to be taken into consideration. But all those variables differ from person to person. Ultimately, it’s what works best for each individual.

Macro Ratios and Meal Composition When Intermittent Fasting

Let’s talk a little bit about what is actually being consumed during that eight-hour intake. And not so much the concept generally of a healthy nutrient-dense diet; we don’t need to go into that. Everyone who listens to this show has [a] pretty good understanding of that. I’m more thinking in terms of macro ratios and [the] concept of carbohydrate backloading, where you eat more of your carbohydrates later in the food intake window, which might be later in the day for most people. Personally, I find that I do better when I eat [fewer] carbohydrates in the morning and whatever carbohydrates I am eating come later in the day. Is that something that you focus on in your approach?

Cynthia Thurlow:  Yeah, carb cycling is something that I find really interesting. For full disclosure, I tend to be lower-carb. I’m not ketogenic. I very much focus and prioritize protein for a number of reasons. A lot of it is the fact that protein is the most satiating macronutrient, and that is important. Because when people are still looking to continue eating after eating a meal, it’s a sign they didn’t put their macros together. So protein is always the focus of every meal. I tend to lean heavily toward animal-based protein because it is more satiating [and] it is not so detrimental when we’re looking at macros. I like to moderate and be aware of how much carbohydrate intake a female patient, in particular, is looking at. Protein is the consistent piece, whether you’re breaking a fast or making a meal. Protein first. If you have a ribeye or a piece of salmon, you already have your healthy fats in it. If you’re having a leaner protein, obviously, [we’re] thinking about, “Do you have avocado in a salad? Do you have some nuts, olive oil, [medium-chain triglyceride] oil, etc.?” And then lots of non-starchy carbohydrates. I always say I’m not anti-carb at all, but [I promote] being smart about your carbohydrates and thinking strategically.

All the recipes in my book are gluten- and dairy-free because I find that [gluten and dairy] tend to be highly inflammatory, especially in middle-aged people. So a lot of what I focus on is if we’re having a heavier-carbohydrate day, it may be sweet potato or root vegetables or squash. The more close to or akin to nature it can be, the better. I think it requires a little bit of experimentation. If I’m going to have a higher-carbohydrate meal, I do better with it earlier in the day versus later. But that’s me. I’m also at a stage in my life that if I wear a continuous glucose monitor, I know the net impact of my carbohydrate intake. I find that the way that I look at meal preparation is protein first, and [then] it’s either a higher-fat day or a higher-carbohydrate day. You never have fat and carbs together. They don’t occur together in nature. The protein is going to help with satiety, and then deciding where you are in your week. Are you having a higher-carb day or a lower-carb day? Higher-carb days are generally associated with more intense exercise, leg days , etc. But I think that this is another piece of that bio-individuality. I have done very well for the last six or seven years cycling my carbs. I would never describe myself as ketogenic. I tend to be lower-carb because that’s just what makes my body happy. But each one of us [has] to do a bit of experimentation. So protein is always the focus, and I aim for no less than 100 grams of protein a day. I find most women eat too little protein, too many of the wrong types of fats like seed oils, and they eat too many carbohydrates. And when I say the wrong types of carbs, we’re talking about the processed carbs, things that have been milled into flour, like pasta and things like that. When they start eating more whole food carbohydrates, more nutrient density, then they will start seeing more benefits from fasting and other types of ways to optimize metabolism.

Chris Kresser:  And it’s great; right here, we have two different approaches that work for two different people who’ve thought a lot about this and done a lot of experimentation. That’s a good reminder that there is no “one size fits all” approach, and that it really does pay off if you do your own experimentation to try a few days of eating more carbohydrates in the morning, and then try a few days of eating more carbohydrates in the afternoon, and then try some days where you’re really not eating very many carbohydrates at all. Nothing’s going to break, especially if you’re doing it for a few days. And almost always, you’re going to learn a ton about what works for you.

Cynthia, this has been a really fascinating conversation. I really enjoyed it. I’m excited about your new book, which is called Intermittent Fasting Transformation, because I rarely see anybody addressing the importance of a cyclical approach for women and really encouraging people to pay attention to their bodies and what’s happening, not just internally, but also what’s happening in their life. How much sleep you’re getting, how much exercise you’re doing, how much stress [you’re] under. Are you traveling? All those factors go into the decision[s] that we make on a day-to-day basis about how much to fast or whether we should be fasting at all. I really appreciate how your book takes all that into account and really empowers women to be their own scientists, so to speak, and do those experiments, and then ultimately create an approach that is individualized based on their life circumstances.

Cynthia Thurlow:  Thank you. It’s been an honor and a privilege to be able to connect with you and your community. And if anyone is interested in connecting outside of the podcast, the book is being sold in all the major places. Amazon, Barnes & Noble, Target, or your local bookstore. I think our local brick-and-mortar businesses have really taken a hit the last two years, so if you’re able to, patronize your local bookstore. I have a great podcast called Everyday Wellness, which I hope to be able to convince Chris to come on at some point this year, and I’m active on Instagram. I’m a little snarky on Twitter, and I have a free Facebook group called Intermittent Fasting Lifestyle/Cynthia Thurlow, which is a community of men and women. But [it’s] a very supportive community learning more about fasting that is anti-drama. I always say, that’s one thing that I don’t ascribe or align [with] at all is lots of drama. So you’re welcome to join that, as well.

Chris Kresser:  I’m all for that. And who isn’t snarky on Twitter? It just seems like that’s what it’s designed for, right? And I’m happy to come on your show at some point when we can figure out the dates. That would be a pleasure.

Intermittent Fasting Transformation [is an] excellent book and corrects a lot of what I consider to be the misinformation and misunderstandings around intermittent fasting. Just as a side note, I think this is also what plagues the research on intermittent fasting. I’m sure you just saw the recent headlines, “intermittent fasting, one year study, doesn’t work.” And then you look at the actual study protocol and what was going on, and you’re like, “Of course this didn’t work. You can see very clearly why this didn’t work.” So for those who are listening to this and who just happened to see those headlines, don’t freak out. It doesn’t mean that your intermittent fasting that’s making you feel way better and helping you lose weight and having all these positive effects isn’t worth doing. It just means that right now, there’s [a] lack of nuance in the way that studies are designed and, of course, in the way that they’re reported on.

It’s a very powerful tool when it’s used appropriately and when all your individual factors are taken into consideration. Check out Cynthia’s book, Intermittent Fasting Transformation, [and her] fantastic podcast, as well. Thanks, everybody, for listening. Keep sending your questions in to ChrisKresser.com/podcastquestion, and we’ll talk to you next time.

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