Cynthia Thurlow on The Best Ways To Use Intermittent Fasting For Weight Loss and Anti-Aging

Content By: Ari Whitten & Cynthia Thurlow

In this episode, I am speaking with Cynthia Thurlow – a nurse, intermittent fasting expert, and author of the book Intermittent Fasting Transformation, the 45-Day Program for Women to Lose Stubborn Weight, improve Hormonal Health, and Slow Aging.

Table of Contents

In this podcast, Cynthia and I discuss: 

  • The main benefits of intermittent fasting
  • The link between hormesis and autophagy
  • How to fast for weight loss
  • The best way to fast based on age and gender
  • The difference between intermittent fasting and time-restricted feeding

Listen or download on iTunes

Listen outside iTunes


Ari: In this episode, I am speaking with Cynthia Thurlow all about intermittent fasting. She’s a brilliant expert who knows a ton about the research in this area, and she is the author of the book, Intermittent Fasting Transformation, the 45-Day Program for Women to Lose Stubborn Weight, improve Hormonal Health, and Slow Aging.

One quick thing, if you are a man, don’t be scared off by the fact that her book subtitle is directed at women. This whole conversation is very, very much relevant to men as well. Though we do touch on women’s specific needs at a certain point in the podcast, and there’s lots of great nuggets here. There’s many areas that I ask her some challenging questions and some areas where we don’t really have research to speak to the answers, and I was interested to get her opinion on those topics.

I think you’ll get a lot of value from this if you are interested in fasting, if you’re interested in weight loss or improving your metabolic health, and if you’re also interested in learning about some of the potential drawbacks or trade-offs or caveats around fasting and intermittent fasting approaches. So enjoy. Welcome to the show, Cynthia. Such a pleasure to have you.

Cynthia Thurlow: Thank you. So nice to be here.

The benefits of intermittent fasting

Ari: First of all, you have written a book all about the benefits of fasting, intermittent fasting. Let’s start with a high-level overview. Obviously, everybody who’s listening to this has heard of fasting and intermittent fasting. These topics have some degree of controversy in the scientific community with different sort of, I would say, groups of people in the scientific community having different opinions on it and I think we’ll touch on that.

But to begin, maybe just give us a 30,000-foot view of what fasting is, what intermittent fasting is, because the definitions are a bit tricky, and different people define it differently with the difference between that and time-restricted feeding, so I’m curious how you define it and sort of a high-level overview of the benefits from your perspective.

Cynthia: Thank you. It’s interesting to me, being a traditional allopathic-trained provider that I used to think about fasting representing itself as starvation. I am happy to say that I have learned better since my original thought process around that.

When I think about intermittent fasting, it’s really eating less frequently. In our harried modern-day lifestyles, more often than not, people are eating between sugar-sweetened beverages and food they’re eating anywhere from six to 10 to 15 times a day. So intermittent fasting is providing a structure around which people are eating within a particular eating window. It could be eight hours, could be 10 hours, could be six hours, could be four.

Although I will explain why I prefer that people have a wider feeding window as opposed to a very compressed one.

In terms of the largest benefits, the ones that I think a lot of women in particular come to intermittent fasting out of an innate curiosity is they want to change body composition, they want to lose weight, but there’s so much more to fasting than that.

For me, I see so much improvement in inflammation. A lot of chronic inflammatory conditions, reduction in oxidative stress, a reduction in, improvement in baseline body metrics, blood pressure, cholesterol, insulin, glucose, improvement in brain health and cognition.

We know that for individuals, once they are transitioning to being able to use fuel more efficiently, they are effectively utilizing both carbohydrates and fats as a fuel source. They will have diffusion of ketones across the blood-brain barrier. So they’ll say, I have so much mental clarity, I have so much energy.

A lot of what I think about is just the process of not eating so frequently so this degree of digestive rest. Fast long enough, you’ll get different benefits than when you fast for shorter periods of time. But we know things like autophagy, which is this waste and recycling process in the body that, based on whomever you’re speaking to, whatever paper you’re looking at, how that can be upregulated around 24 hours. But I don’t ever think about it switching on and off entirely. People are still getting benefits, but those are typically some of the ones that maybe some are better known, and some are less known.

I also think about improvement in neurocognitive disorders, lowering our risk for Alzheimer’s, Parkinson’s, dementia or reduction in specific types of cancers, especially those reproductive cancers. As I always say, people come to it to change body composition and they stay for all the other benefits very humbly.

The difference between time-restricted feeding and intermittent fasting

Ari: How would you distinguish between time-restricted feeding versus intermittent fasting?

Cynthia: You know, it’s interesting. In the metabolic health space, there are some people that don’t make tremendous distinctions. Even if you’re looking at Satcha Panda’s work, he’s one of these amazing researchers that is very focused on chronobiology or the body rhythms and how they respond to insulin sensitivity and eating versus non-eating.

I think when I look at it, it is time-restricted feeding and intermittent fasting. There’s a lot of similarities and then perhaps some differences. However, for the purposes of this conversation, I would say that when I think about time-restricted feeding it’s really having this compressed timeframe in which people are consuming all of their food, which seems very similar to intermittent fasting.

I know when you look at some of the researchers, there are some that believe that it’s very aligned, they’re very synonymous, and others that feel very differently. To me, I don’t see a lot of distinction between either of them. It’s really defining a time period in which you choose not to eat, and defining a time period in which you choose to eat and then determining based on gender, life stage, et cetera, which one of those works best for you and your lifestyle and your hormones, to be completely frank.

Ari: There’s a weird gray area here because we should, obviously we as humans, we need to sleep for part of the day as diurnal animals. We sleep during the nighttime period and during that nighttime period, we should not be consuming any food. So there’s a window of time built into our schedule where we shouldn’t have food coming into the system.

Then based on looking at modern-day hunter-gatherer tribes, and traditional living people, we can– and combining that with modern research into the topic of time-restricted feeding and circadian rhythm, and how nutrition interfaces with that and all the animal experiments that have been done, we have good reason to believe that at least 12 hours, if not more of our day should be fasting.

But that should just be the normal state of eating. We shouldn’t call that if someone has 12 hours of the 24-hour period of the day where they’re not consuming food or 14 hours, to me it doesn’t make sense to call that intermittent fasting. It makes sense to call that just a normal healthy way of consuming food each day.

Then the question becomes, well, how do we define intermittent fasting? Because some people do define intermittent fasting as consuming food within a 10- or an eight-hour window of the day, whereas I would just say that is better term time-restricted eating. Really that is just the normal thing that we should all be doing on a daily basis.

So there has to be semantically some difference of definition between that versus what we’re referring to as intermittent fasting. Do you agree with that or like how would you respond to my position on that?

Cynthia: No, I think that’s certainly reasonable. I mean, when someone goes 12 hours without eating, I just call that digestive rest. My standard, and I talk about this publicly a lot is that should be the gold standard. Unfortunately, that’s not the gold standard. I think your definition in that context is completely reasonable and feasible.

I think for a lot of us that are intermittent fasting, so fasting for longer periods of time, 16, 18 hours, we can acknowledge there’s a pretty significant distinction. You’re still getting benefits even if you fast for 13 hours or 14 hours and certainly, someone that’s new to fasting or just time-restricted eating in the context of this definition, there’s still a lot of benefits that come through that or come with that.

From my perspective, when I’m thinking about longer episodes of fasting, that’s when you start seeing some significant improvement with metrics. But in our current society, we are just encouraged to eat from the moment we get up to the moment we go to bed. That’s completely not aligned with our circadian biology. That’s not aligned with our metabolic physiology.

I think for anyone that’s listening, you may not per se be interested in intermittent fasting, but the time-restricted eating or feeding of a 12-hour time period during a day is certainly feasible. I have all teenagers and even they go at least 12 hours a day without eating, and they eat a lot of food in their quote “time period” in which they eat before they fall asleep.

But for the rest of us that are adults and are fully grown and evolved, I think it should be the mainstay and it should be what all clinicians are talking about with their patients, not just those of us in the functional integrative medicine space.

How fasting duration affect your health

Ari: I want to delve into some of the specific benefits that you mentioned earlier. Let’s talk about autophagy first. Autophagy is often framed as a benefit of fasting and, of course, we have good data to support that during the fasting period, we go into autophagy. I have a few questions around this. Some are difficult to answer because I think the research doesn’t necessarily exist, but I’m curious to get your opinion on it.

One is, first of all, do you think that there is a significant difference between short duration fast, let’s say an 18, 24-hour fast, or even a 36-hour fast versus a three-day or five-day fast? if so, what differences do you think there are and are there any studies to speak to that?

Cynthia: Well, you bring up a really good point. When I reflect on the benefit of autophagy, this waste and recycling process, and look at the research, it really appears to be that greater than that 24-hour mark we get a significant upregulation in autophagy.

That has to be balanced with the individual that is practicing intermittent fasting because if you are of the mindset that I am, I want to preserve lean muscle mass so I’m very careful and conscientious about long fast. When you start looking at 48- 72-hour or longer fast, yes, there’ll be increases in growth hormone, there’ll be increases in autophagy. You fast long enough and you’ll get some stem cell activation, you can impact telomere length.

I think for me clinically, it has to really be balanced with, are you a very lean, 45, 50-year-old woman that you don’t want to sacrifice losing lean muscle mass at the expense of doing longer fast. Now, if someone were being treated for cancer, if someone were dealing with some significant neurological issue you might be able to find a healthy balance in between the different metrics that you’re trying to work on. But from my perspective, the average person doesn’t need to be doing, if you’re thin and lean, you do not need to be doing these really long fast on a regular basis.

Unfortunately, this is something I’m starting to talk more about. Hormesis is a dual edged sword, so beneficial stress in the right amount at the right time but I’m starting to see what’s evolving is if more is better than I need to do more fasting, more exercise, more restriction.

I always counsel patients about being conscientious about what are their goals, where are they in terms of a baseline, are you a man that’s 45 that that is insulin-resistant and not metabolically healthy versus a 45-year-old perimenopausal woman who’s already on the thin side, is struggling to build healthy lean muscle and trying to find where that sweet spot is.

There is a place for fasts that are under 24 hours. There’s also a place for a longer fast But it really, to me, depends on a lot of what’s going on clinically and someone’s metabolic health. Also, again, that hormetic sword, understanding that you want to be conscientious about how much hormesis that gas pedal of pushing things forward and determining what’s going to be most advantageous.

I’m very protective of metabolic health. I’m very protective of muscle protein synthesis. I could go on and on for why I feel that way. in terms of research, everything that I’ve read really leans into more than 24 hours is going to up-regulate autophagy in particular. for a lot of people, that’s what they’re most interested, this waste and recycling process, getting rid of all mitochondria, organelles, et cetera at the expense of going a little longer with a fast. But I think it always has to be taken in the context of what are your goals, what life stage are you in, what’s your metabolic health parameters.

How hormetic stress may stimulate autophagy

Ari: I want to come back to that. I totally agree with you on hormetic stress. This is a subject of my next book. It’s all about hormesis more broadly and different forms of it And there’s absolutely a biphasic dose response and we’ve known this for a long time when it comes to exercise.

If you take athletes, you do too much exercise, you get overtraining syndrome, there’s absolutely a negative consequence to doing too much of even beneficial stressors because at the end of the day, this is a form of metabolic stress to your system. We are deriving benefits by stressing the system transiently in order to stimulate adaptations that ultimately confer benefits. But if you are chronically pushing the stress switch or for too long of a duration or too intensely, you absolutely will get negative consequences, just the same as with any type of stress.

With regards to autophagy, one thing that a lot of people I find don’t know in the general public is that most other types of hormetic stressors also stimulate autophagy. Many people have talked about this as a unique benefit of fasting, but in fact there’s lots of research on sauna stress, heat stress, as well as exercise being powerful stimulators of autophagy.

I’m curious if you have done a deep dive into the research on that to– and I think this is an area where the research doesn’t really exist, but to compare perhaps the degree to which the autophagy switch is turned on, how long it’s turned on for, or if there are distinctions that are important around which specific tissues and organ systems of the body experience a lot of autophagy when that autophagy button is pushed via, let’s say fasting versus exercise versus sauna.

Cynthia: That’s a great question, and I don’t honestly know– I’ll be completely transparent, if there is research that exists that compares these different strategies all at once. I do agree with you that a lot of the social media content I see, or people just reaching out and asking questions on social media, they don’t understand that there’s other ways to evoke or upregulate autophagy and I love that you brought that up.

I think that whether it’s high-intensity interval training, which again is not 30, 40, 60 minutes of hit, it is if you do it right, it’s really short, intense bursts of physical activity to gas yourself or lifting weights or cold exposure or heat exposure and I think there’s this continuum of people that want to take things– they want to take everything to an extreme.

I’ll give you an example. You see a lot of people doing cold plunges, which I think is a fantastic idea if you can tolerate it and you like cold things and there are actually genetic polymorphisms that will lean into people that are less stress tolerant around cold or hormesis. I think that there are those of us that would do very well having 30 seconds of a cold shower or doing some cryotherapy that’s three minutes, which is pretty tolerable versus sitting in a plunge pool for an hour outside in negative 20-degree weather.

I think that hormesis bucket is one that is broad and certainly one that is not synonymous with just fasting or pushing the autophagy envelope. I do think it’s helpful though, I like variety. I think it’s important to practice variety. Whether or not it’s someone’s fasting one day and the next day they’re doing some hit and the following day they’re doing cold exposure, I do think that our bodies are designed to be challenged and not just the same old things every single day.

I think that makes us stronger, more resilient organisms but no, I’m actually not. I think that would be an interesting study to look at different studies on that to find out which one is most efficacious. I’m a fan of being efficient so if there’s a most efficient way to up-regulate autophagy over any other, I’d love to know what those are.

Potential harms on fasting too long

Ari: Absolutely. There’s also one other layer to this story is what trade-offs exist. With that in mind, I’ll mention– to come clean, I rarely do extended fasting anymore, I fast on a daily basis for somewhere between 14 to 18 hours typically but that’s just normal for me. I have an eating window of typically somewhere between six to 10 hours a day, and that’s just my rhythm. I don’t think anything of it, it’s not difficult to do.

But the extended fast, even a full day and beyond or certainly in the past I’ve done two and three and four and five day fast or partial fast, fasting-mimicking diet for five days and that sort of thing and I don’t do that anymore. The main reason is not because I think it’s bad universally, but in my specific context, I am an extremely physically active person. I surf, I lift weights, I play tennis, I rock climb I do jiu-jitsu, I do capoeira, I do a lot of physical activity. Every day there’s stuff going on.

If I don’t feed myself adequately, my physical activity routines really suffer, particularly with the three, five-day, or the fasting-mimicking diet approaches. By day three, four, and five, I have probably a quarter of the workout capacity that I normally do. I’m fatigued very easily. I’m much weaker than normal. I feel fine if I’m just resting, but when I go do intense exercise, it really shows up in a big way. So for me, those trade-offs aren’t necessarily worth it until someone can show me research saying, “Hey, the autophagy benefits of these extended fasts as far as disease prevention or extending lifespan are so profound that, and so far beyond the autophagy benefits that you’re going to get from doing exercise, and cold, and sauna, and all those types of things that you just have to go do those extended periods of fasting, once a month or once every few months.

Until that data exists, for me, it’s not worth the decrements in the quality of life, and my ability to do physical activity. Do you know what I mean?

Cynthia: I do. I do. I think it’s important for people to understand that the longer I have been fasting– Four years ago, I was hospitalized for 13 days with a ruptured appendix and I wasn’t able to eat for 13 days. The joke/not joke is that since that time, I’ve had zero interest in ever doing a long extended period of fasting since then. I tell people that openly, I haven’t done anything longer than a 24-hour fast since then, and that’s by choice.

I also recognize I lost so much lean muscle mass during that hospitalization because I lost 15 pounds, I think within the first week, that I still feel like I’m still going to take the next five years trying to rebuild all of this muscle that I lost. So for me, it’s not something I’m interested in. When I travel, a lot of times, I’ll fast, and sometimes it ends up being a 24-hour fast unintentionally, but I agree with you that we can always reevaluate our circumstances and what’s working or not working for us.

In fact, a lot of the fasting advocates, I think about Dr. Peter Attia as an example. He talks very openly about, he’s really backed off on fasting for long periods of time. I think it really speaks to the fact that we can always have opportunities to reevaluate our own programs. For me, I’m just not willing to lose muscle to have a scooch more in terms of long– maybe it’s going to give me six more months if I’m doing these longer fasts. I’m still young, so I don’t anticipate going anywhere anytime soon. But the muscle piece and preserving lean muscle is far more important to me than continuing with longer fast.

I love that you’ve found what works well for you and certainly, you’re so physically active that it makes sense that you’re doing these shorter intervals of fasting are probably much more beneficial.

Why you need to individualize fasting to your own personal situation

Ari: Actually, with that in mind, with what I was just describing about my own personal situation and linking up with some of what you were describing earlier, different contexts of individuals and how your recommendations would differ, one of the things that I typically recommend is for people who are lean, and fit, and very physically active, that they have a bit longer of a daily feeding window versus someone who’s more sedentary and more overweight, they might pull that lever a bit harder and restrict their time window of food intake harder.

Do you agree with that and what’s your approach to individualizing the daily feeding window according to different personal contexts?

Cynthia: Yes, I think it’s important. If we’re looking at the average American, there’s about 7% of Americans are considered to be metabolically healthy, meaning that we’re not insulin resistant or diabetic. Someone like that has the ability to adjust macros. I’m always very pro-protein. I speak very openly about that. I always say I’m carnivore-ish so I do eat fruits and vegetables but I’m also metabolically healthy.

When I’m looking at someone that’s in their 40s, 50s, or 60s, they’re physically active, they’re insulin sensitive, then I’m going to encourage them to have at least two– and I hate to use the word bolus, but two large meals a day, really focused on 50 to 60 grams of protein, and then adjusting their carbohydrates based on how physically active they are. Today was a leg day for me, so this is a day I’m going to have more discretionary, healthy carbohydrates than on a less active day.

When I’m talking to someone, and again, this is most of the patient population I interact with that are no longer insulin sensitive, that are insulin resistant/pre-diabetic or have diabetes, I do like them to understand that having a more compressed feeding window, again, with two meals and being much more diligent about their macros, measuring their carbohydrates, being conscientious about healthy fats because healthy fats, as an example, nuts and cheese, get women into trouble all the time. They’re very easy to overeat.

But understanding that, really being conscientious, so yes, those fatty coffees, yes, the piece of candy, all those things can incrementally add up, and so I do like them to have a much more regimented, tighter feeding window and eating earlier in the day, which is much more aligned with our chronobiology. I always say eat when it’s light outside and don’t eat when it’s dark outside.

When we’re doing that, people feel a whole lot better going into the evening, understand there’s more insulin sensitivity earlier in the day, don’t sit down and have a bowl of ice cream at nine o’clock at night and think that’s going to do your health any favors, but really understanding and helping women understand in particular that depending on whether they’re insulin sensitive or not governs a lot of the recommendations, both in terms of macros but also in terms of physical activity and winding their bodies down for the day.

The best fasting window

Ari: You mentioned something there in passing about morning versus evening time-restricted feeding. You mentioned what your impression is, I agree with your take on that. However, there is a practical aspect of this that a lot of people, I would say, the majority of people find difficulty with.

What I’m referring to for people listening to this is that there’s a body of scientific literature that has shown that all things being equal as far as the time window that one is consuming food and even the total daily calories consumed and the specific foods one eats during that window, let’s say a 1,200-calorie diet eaten during an 10-hour feeding window with these specific foods, but the difference being whether most of the daily calories are stacked towards the end of the day, in the evening meal, or the beginning of the day, in the breakfast, those studies generally, the body of literature, generally favors increased benefits by stacking more calories earlier in the day.

Now, the practical objection to this that most people struggle with is that most people don’t have time to sit around and eat a big breakfast and most people find, just culturally, we live in a society where it’s go, go, go first thing in the morning, and then in the evening, I get to sit and rest and have a big dinner. What are your recommendations to people as far as navigating those practical issues that I’m sure you’ve encountered with many people many times?

Cynthia: I think it’s important to acknowledge that for those of us that are entrepreneurs, we are certainly very privileged that we probably have the flexibility to be able to eat when we want to. When I was still seeing patients clinically in the hospital and also in clinic, I was lucky if I got to eat lunch, so let’s talk about the fact that a lot of people don’t have the opportunity to eat because they’re caring for other individuals or their schedules just certainly don’t allow for that.

I abide by the concept of good, better, best. Best is what we talked about, eat early in the day, first meal of the day should really be your largest meal, really shutting things down by five or six o’clock at night. That’s not realistic for everyone. I think a lot of what I try to focus on is making sure that people are making good choices when they do break their fast, being prepared doing meal prep, which I know is not a sexy topic.

I have teenagers. If we didn’t meal prep, we would have no food because they eat everything. I’m not kidding, pounds and pounds of meat. So I think it’s working with what is feasible. For some people, it may be, “Cynthia, I have to be in my car. I can’t eat in the car. What are my options?” Finding a clean properly-sourced protein powder so they have something. I have asking them to set aside some time that if you can’t do anything else, eat some protein, or finding alternative options for them and making sure that they’re prepared for success.

I’ll give you an example. I was in Denver at a low-carb event this past weekend and explaining to followers on social media, this is what I do, these are my standard. I go to a restaurant, this is what I ask for. I can generally get it just about anywhere. A lot of my colleagues were doing the same. You can go to McDonald’s and you can get two or three burger patties just all by themselves wrapped up if you just need to do nothing else.

It’s all about reality, but also helping people prepare for– When they’re traveling, if they’re in their car, which I hate having to eat in the car, I’ll be completely transparent, but that’s reality, a lot of people who, by the time they get home from work, they’re exhausted, their kids are screaming, they have to feed their kids, they’ve got to bathe their kids, they have to put their kids to bed, so it could be seven, eight, nine o’clock at night before they really have that wind down time.

Really helping people understand there are going to be those good, better, best food choices, so protein first. Always trying to aim for that protein. Lessening your likelihood that you come home in a cyclone and you eat everything in your pantry because you’re so ravished and so hungry. I think it really comes down to some degree of being deliberate.

Definitely, when I was rounding in the hospital, I would carry, and I’m not suggesting everyone do this, but again, being a little bit of a realist, clean protein source or beef jerky, something so that when you have the ability to sit down for five minutes, you have something healthy to eat, you will make better food choices.

I think it’s also important just to be honest and say every once in a while, you are more than likely going to not make a stellar decision and you might end up eating something that doesn’t agree with you, impacts your sleep quality, spikes your blood sugar. You have to let it go and take it as a learning opportunity.

When I’m working with women in particular who are struggling with the evening, they don’t get their meal done before seven, eight o’clock at night, helping them make the best choice under those circumstances. Have the protein, have the veggies, try to get in bed, understand tomorrow’s a new day, try to stay really well hydrated during the day because dehydration can mimic a lot of symptoms that people are trying to avoid, the headaches, the brain fog, et cetera.

I think it comes down to having a plan, meal prepping as much as you can. Having a plan if you’re traveling and you don’t have consistent control over every parameter under which you can abide by in a restaurant. I am dedicated gluten-free and dairy-free, but when I go to a restaurant, I probably am getting some seed oil exposure, but I don’t allow myself to not enjoy the meal understanding that I may get some transient exposure to things I don’t normally eat because then it would just be obsessive.

The point of what I’m sharing is finding a situation and a solution that works well for each individual. If it’s the weekends when you have more flexibility, that’s really the time to lean into those good habits. We know that even people that fast two days a week are going to derive benefits. Maybe you have a longer fast on a workday and on the weekend, maybe you have a wider window and you really are diligent about hitting your protein metrics and those healthy fats and the right types of carbohydrates.

I think the point of what I’m trying to say to you and your listeners is that give yourself some grace, plan ahead, when you have opportunities to eat early in the day, try to lean into it, but don’t become obsessive about it. I find for a lot of people just eating those protein, trying to hit those protein macros at least 30, 40, 50 grams of protein allows them to make better decisions when they are able to eat.

Metabolic flexibility

Ari: Let’s talk briefly about metabolic flexibility. This is something that you alluded to in passing earlier as far as ketones entering the brain and increased energy and that thing.

First of all, for people listening, can you define what metabolic flexibility is? Then can you talk about how this approach around intermittent fasting, time-restricted feeding is interfacing with that and how you navigate this practically for someone who is very overweight, who has very long feeding windows, who’s used to eating very frequently as far as how they should go about systematically working towards increasing their metabolic flexibility?

Cynthia: I think that’s a really important distinction to explain, how our body can utilize different types of fuel substrates. There’s carbohydrates that can be stored in muscle, they can be stored in liver. If you have too much stored carbohydrate, you can develop non-alcoholic fatty liver disease or NAFLDs as we like to call it. We can also store fats in our body and most of us have quite a bit of fat. I say this like even thin people have plenty of body fat to be able to utilize.

If we are eating too frequently and every time we’re eating, we’re increasing our blood sugar and depending on what macronutrient we’re consuming, whether it’s protein, fat or carbs, has a different net impact on how much blood sugar is fluctuating and correspondingly also insulin. If we’re eating too frequently and we’re eating too many carbohydrates, especially processed carbs, hyper-palatable carbs and not enough protein and the wrong types of fats, we put ourselves in the position where we’re no longer efficient. Our body can’t officially utilize those stored carbs or will preferentially use the stored carbs because there’s so many around.

If insulin remains elevated because we’re eating too frequently and eating the wrong types of foods, our body can’t effectively go in and use the stored fat or fatty acids as a fuel source. So when we go from eating six to 10 times a day to eating two to three times a day, our body and our blood sugar and our insulin levels, they go up appropriately in response to eating a meal. Again, dependent on the type of macronutrients we are consuming, it’ll allow our bodies in an unfed state to be able to go in and utilize the stored fat as a fuel source.

Fat is a– if you want to think about a fire, carbohydrates are the kindling. They’re quick to be readily utilized. For a long duration of sustained energy and improvement in cognition, we want those fats to be used as a fuel source. The kindling are the carbohydrates, the bigger logs are the fats. For anyone that’s ever done a race or done an event and maybe sprinted and then felt like they bonked or did a race and felt like they just couldn’t sustain the energy, it’s because they’re not actually metabolically efficient.

When I start to talk to my patient population about metabolic flexibility, it’s helping them understand that optimally we want to be able to use stored carbohydrates or stored fats as a fuel source. Most of us are stuck in the carbohydrate utilization mode, which means we have significant changes in our energy levels throughout the day. We get hungry, we struggle with weight loss resistance, we really will struggle with longer periods of going without eating. That’s why these people are oftentimes dependent on processed carbs because it’s a quick source of energy. Their blood sugar will go up and come back down and then they start this vicious cycle over and over again versus when you’re able to use both fuel substrates.

You could argue, I’ve had guests on my podcast that have talked about ketones as being a completely separate form of fuel substrate, helping them understand that when you are going from dinner to breakfast, and maybe that is 12 hours, 13 hours for some of these individuals, that you’re helping your body become more optimally efficient.

It’s all about efficiency and helping them understand that over time their body will respond to the lack of food and it’ll go looking for a fuel source to maintain their body, maintain homeostasis. So these fatty acids, if they’re broken down can be very effective in keeping and helping with sustained energy, having people have very clear cognition, allowing people to forget about having to eat.

I think on a lot of different levels, because we have so many metabolically unhealthy individuals, they have gotten so accustomed to taking food with them everywhere they go because they’re so worried that their body’s not going to effectively be able to mentate or move around without these SOS meals, as a lot of my patients call them.

When I think about metabolic flexibility, it’s having the ability to utilize both stored fats or glycogen. Sorry, glycogen as stored carbohydrates, but also being able to break down fat as fatty acids to use as a fuel source and understanding that optimally we want to be able to use both. They’re not one is good and one is bad. Optimally, this is the way that our bodies are designed to thrive and remain in optimal health by using both substrates.

Keys to start fasting

Ari: On a practical level, if somebody’s currently eating, let’s say in a 15 or 16-hour-a-day window, how do you get them down into the range that they should be? Should they just do it from one day to the next, try to start eating two to three meals in a 10-hour window? Or how long of a timeframe do you work them into that and what are the steps in that progression?

Cynthia: That’s a great question. I think if we’re talking about someone that’s eating a standard American diet and is very sedentary, we start with stopping snacking. You got to rip that band aid off because if you understand that you’re only going to have three meals a day, you’re going to have to structure your macronutrient profile very differently and have more protein and ratchet back on carbohydrates.

That’s very scary for a lot of people. When they’ve been eating a certain way, if they’ve been eating the standard American diet, which is more processed carbs, wrong types of seed oils and high fructose corn syrup, and all of a sudden you’re telling them, I want you to eat more protein, that can be a little bit overwhelming. We stop snacking because that will then force people to restructure each one of their meals leaning into 30, 40, 50 grams of protein. This is when I really encourage patients to start tracking their macro so they have a good sense of portions and understanding that carbohydrates are not all bad.

Having a quarter cup of blueberries is very different than having half a pint of ice cream, has a very different nutritional profile. So stop snacking, restructure the macros. Then I actually encourage them to go, the next step is don’t eat from the time that you finish dinner until breakfast the following day and try not to eat within the first hour or two of waking up.

That’s another common misnomer that we have to eat to stoke our metabolism. I remind people one of the worst things you can do is to get up and just eat immediately.

Ari: Why? Just for people listening, why do you say that?

Cynthia: I think that I like people to get moving in the morning and so I remind them, if we’re looking again at this circadian biology, I want you to get up and get moving. Maybe you have some bitter tea, maybe you’re having a plain coffee but get up and get moving. Get some light exposure which can definitely be helpful for insulin sensitivity. Get up good moving, even if it’s you go from your bed to the shower, to your car, and then you walk a little bit, get moving first.

The other thing is, I think people, it’s the routine of a lot of these habits that get them derailed. So we go from no snacking, changing your macros, nothing from dinner to breakfast, and then just encouraging them to maybe go in 30-minute increments. Maybe they go to 13 hours of digestive rest and they go to 13 and a half hours, then they go 14.

I find most people within two to three weeks, the average person in two to three weeks will feel so much better they’re not willing to go back but I do have people who are incredibly metabolically deranged and unhealthy, and it could take them 4, 6, 8 weeks. It’s really that, for some of them, it is really going an extra 15 minutes, an extra 30 minutes every day to get them up to 16 hours fasted with an eight-hour feeding window.

I find for a lot of people, if we do it low and slow, that’s very effective. There’s a lot of coaching involved, obviously, because we’re retraining– all this neuroplasticity, we’re really having to really do a lot of retraining. You know dispelling a lot of conventional wisdom and dogma. I say wisdom with air quotes, things that we’ve been passing along to our patients that are just blatantly wrong. I do find it usually is two to three weeks, but in some instances, it can take four to six weeks. Just small incremental changes.

Then depending on if you’ve got a woman that’s still at peak fertile years or a perimenopausal woman, helping them understand don’t start trying to fast for the first time the week before your menstrual cycle, because you’re going to set yourself up for some degree of inability to be able to do it effectively, and so helping people understand like when in your cycle should you fast, when should you back off on fasting? There’s a whole methodology based on our infradian rhythms, which is the menstrual cycle and where we are in our cycle and fluctuations of sex hormones. That’s kind of the usual approach that I take with most patients.

The other thing that I would just add to that is fasting doesn’t per se work for everybody. There are some people just frankly, they could be metabolically healthy and it just doesn’t work for them. It’s understanding no one should be white-knuckling fasting. Unfortunately, I think there is this prevailing mindset on social media that you just white-knuckle everything that’s tough in life. I just don’t believe in that.

I think we have to acknowledge our own innate bio-individuality to determine what works best for us. It could be maybe it doesn’t work for you, right then, it could be that it’ll work better for you in six months. Maybe you have too much stress in your life and this added stress, it’s like adding gasoline to a fire, can be problematic. That’s my typical approach.

How to set up fasting based on your gender and age

Ari: Got it. I have two more topics on my list that I want to ask you about. One is what you were just talking about with the female menstrual cycle and the infradian rhythm. Can you take me through briefly how intermittent fasting should be applied differently in terms of the principles of dealing with women versus men and women pre-menopause versus post-menopause?

Cynthia: Yes, I think it’s really important to acknowledge this, that women that are peak fertile years, 35 and under, I’ll use that as the starting point, irrespective of whether or not they are choosing to become pregnant, our bodies at that stage in life are just exquisitely attuned to the ability to expose to stress and danger and nutrition. In those women in particular, if they’re a polycystic ovarian syndrome or PCOS patient, very different than my lean athletic, and I say athletic, they’re not an athlete, I don’t recommend fasting for athletes, especially women of that age group, but helping them understand that there is this hormonal regulation that goes on in the body, and so we have–

I’m going to oversimplify things, but we have the follicular phase, which is from the time we menstruate up until right before ovulation when estrogen predominates, and this is an amazing hormone. This is our powerhouse hormone. We have the ability to really push our workouts, lift heavy, do more intense exercise, we can go lower carbohydrate if that’s within our toolbox. It’s also the time that we can get away with quite a bit of fasting. I find that the beginning part of the menstrual cycle, most women do pretty well. I say most women, as in most women, if you’re a lean 25, 30-year-old, I don’t want you fasting every day. Maybe you’re fasting once or twice a week.

Again, it’s about hormesis beneficial stress in the right amount at the right time versus after ovulation when the predominant hormone is progesterone. This is a great hormone, but this is a hormone that correspondingly we’re slowing down. I don’t mean slowing down like you’re going out to pasture, but you’re going to do more yoga, you’re going to do more Pilates, less intense exercise, a little more discretionary carbohydrate? Yes. this can make a big difference for women in their PMS symptoms and understanding that you may want to start reducing the amount of hours in, that you’re in a fasted state, and the five to seven days preceding the menstrual cycle, no more than 12 hours of digestive rest.

If women abide by that prevailing therapeutic approach, they can very successfully fast. Again, thin lean women, you should not be fasting every day. It’s not necessary. Then lean active women. If you have PCOS, you’re insulin resistant, you can get away with a bit more flux. Again, understanding that week before your menstrual cycle, you want to back off a bit versus women in perimenopause, which is 10 to 15 years and average age of menopause in the United States is 51. Late 30s, early 40s most women are there, whether they realize they’re not.

As we are becoming less stress resilient at this stage of our lives, this is when I talk to women about how important it is, yes, you can fast, but in the caveat, in addition to what I just mentioned about the women in peak fertile years, you have to sleep, you have to manage your stress. You’re not as stress resilient as your ovaries are producing less progesterone and your adrenal glands are having to step in to help with this whole physiologic process. Understanding that anti-inflammatory nutrition becomes a bigger issue. Understanding the right types of exercise.

I don’t like to see perimenopausal women or menopausal women doing five days of CrossFit and then understanding why they become weight loss resistant. Again, protective of the luteal phase and then menopause 12 months out of menstrual cycle, I think women in menopause and men have the easiest time with fasting because there’s not as much hormonal fluctuation.

Again, the little caveat is the lifestyle piece for menopausal women is important. If you’re not sleeping through the night, please don’t add gasoline to the fire and try to do a 20-hour fast. You’re just adding more stress to the body. Please make sure you manage your stress and the anti-inflammatory nutrition and the right types of exercise.

Let me define right types of exercise for middle-aged women. You got to lift, you have to strength train. Chronic cardio days are over. Because as we are getting older, we are losing muscle mass. It’s a process called sarcopenia.

It’s not a question of if, but when and as you are losing muscle mass, you are losing insulin sensitivity. All of a sudden I get these 48, 49, 50-year-old women who say, I’ve done nothing different, and all of a sudden I’m weight loss resistant and a lot of it has to do with this loss of muscle mass.

Muscle mass is important to plug for metabolic health as well as we’re talking about this. Those are kind of the prevailing philosophies I have about cycling women, peak cycling, peak fertility, perimenopause, menopause, and menopausal women generally have the easiest time as it pertains to life stage provided that they dial in a lifestyle piece.

How to fast for weight loss

Ari: Got it. I have one more topic that I want to talk to you about, which is weight loss. I think this is probably the most common reason that people are interested in fasting or intermittent fasting so we need to make sure to talk about it before this podcast ends.

Obviously, if you eat less, you will lose weight. If you fast, if you don’t consume food, you will certainly lose weight. The questions of course, and that’s also true if you adopt a calorie restriction diet. Of course we know that across the board, we’ve had decades of studies now showing that calorie-restricted diets, whether they’re low in carb or low in fat or vegan or meat-based or whatever, result in profound amounts of weight loss.

Of course, we have the behavioral problem with adherence and a huge portion of people who go on weight loss diets end up regaining, and that component of adherence and choosing a diet, choosing an approach that you can stick with and not revert back to your old habits becomes essential to maintaining the weight loss. That’s a whole other piece of this story that’s really important whenever we’re talking about weight loss.

Now, having said that, there are people who have espoused the idea that fasting creates unique effects on weight loss and that time-restricted feeding creates unique effects on weight loss and there are people, particularly people who are in what is the self-proclaimed evidence-based fitness movement, a lot of nutrition and fitness gurus in that community who have seemingly been obsessed with trying to debunk the idea that fasting or time-restricted feeding has any unique benefits beyond just straight calorie restriction.

And there are studies I think that one can cite to support either direction so those, that group that I was just describing tends to cite studies that say, see the calorie-restricted diet did the same thing as far as weight loss and improvements as metabolic health compared to the time-restricted feeding. Therefore, there is no magic time-restricted feeding, the benefits of that approach purely are the result of people lowering their calorie intake, so what is your take on that landscape of research and do you think that there are unique metabolic health benefits to time-restricted feeding or fasting that go beyond just calorie restriction?

Cynthia: Well, I think that there’s some degree of rigid dogma in the health and wellness space. Whatever camp people embrace, whether it’s SECO whether it’s fasting, there’s a kind of evolving toxins as the impact on metabolic health and that’s what’s driving weight loss resistance.

I think what it all comes down to is, I’m not a calorie counter, I never have been, that has never been my methodology. I always say count macros. When I’m talking to my patients and I’m asked these kinds of questions, I think our bodies are so much more sophisticated than to assume that it’s just about calories or it’s just about fasting, it’s really all about this combination of different changes.

It’s a multifactorial reason for why I believe that fasting really has a lot of benefits, it’s kind of retraining our relationship with food, it’s retraining the way that we think about food. We do talk about reduction in insulin levels. We’re lowering our risk for these metabolic health issues, whether it’s hypertension, whether it’s dyslipidemia, whether it’s cardiovascular disease, Parkinson’s, dementia, etcetera and so I always like to reflect on the fact that what we are unfamiliar with, sometimes we mock and sometimes we decide to, we want to compartmentalize everything, we want to make it so easy, so effortless but I do fervently believe that fasting is more than just physiologic, it’s psychological, it’s spiritual. Fasting is a practice.

When I tried to comparatively say like the SECO people and there are plenty of them out there and they might very well be listening, there’s a lot more to fasting than just eating less often, there’s a whole lot more of it, that becomes this whole other kind of methodology and strategy that those of us that believe in it and have seen amazing results with our patient populations and our clients and whoever we’re working with or just even personally, is more than just a calorie.

The other thing that I would add is when we’re not tracking calories and we’re tracking macros as an example, it allows us to look at our food very differently, have a different relationship. I find that for a lot of women who have been part of this diet culture that I certainly kind of grew up in, the fat phobia years that kind of unfortunately, I propagated some of that as a new clinician and I now I know better. I think it’s important for people to understand that our relationship with food is complicated and I think that fasting, at least for someone that’s got a healthy mindset can be a really unique way for people to reinvestigate their relationship with not just eating but eating less often.

Ari: As far as the actual weight loss effects, is your impression of the body of literature that there are unique additional benefits beyond just the calorie restriction effects and the improvements in metabolic health that come from eating fewer calories. Are there additional metabolic health benefits that come from time-restricted feeding?

Cynthia: I do believe that there is. I think that if we look at the people that are counting calories and we look at the individuals that are eating less frequently, I do like to talk about just the research that I’m familiar with that talks about upregulation of autophagy in this unfed state that really looks at the reduction in inflammation, things that, per se are not up-regulated by just counting your calories.

It’s not to suggest that calories don’t matter because let’s be honest, they do. But it’s more complex than that. I think it’s an oversimplification that you just count every bean that you eat and somehow that’s going to magically yield to weight loss.

In fact, I see a lot of people that are doing either or maybe they’re not even– unknowingly, they’re eating very low calorie and also fasting and then they become weight loss resistant. There’s a whole lot more to this than just counting calories and just eating less frequently. But I do think there are unique attributes and characteristics to fasting that make it a superior option to just sitting down with an app and counting every single morsel of food that you put in your mouth.

How to break through weight loss plateaus

Ari: On the subject of weight loss resistance and this is my last question to you, let’s say someone does get to that state, they’re in the condition that you just described, they’re eating a low-calorie diet, they’re also pulling a lever of intermittent fasting and then they plateau and their bodies become resistant to losing further weight, what is your strategic approach to helping people break through those plateaus?

Cynthia: I think it’s important because I think a lot of women, maybe they have not been weight loss resistant until they get into perimenopause or menopause. It’s an endless source of concern. For many women, we really start with the basics to be completely transparent. Are you sleeping through the night? I always say if I can’t get you to sleep at night, I cannot get you to lose weight.

We look at the changes in leptin and ghrelin and blood sugar response all related to poor-quality sleep. How many women are having hot flashes or they fall asleep, they can’t stay asleep or their relationship with alcohol and sleep really erodes things.

We always start with sleep. That’s usually the first thing to work on. I look at stress management, we know we’re less stress resilient as we’re getting older. Finding a stress management style that works for you and really leaning into that. Meaning if it’s meditation, if it’s grounding work, if it’s connection to nature, if it’s a PMF mat– I’m a gadgets person, I’m a data geek, I love to track metrics so I’ve got lots of things that are at my disposal but for every person that’s listening, it could just be as simple as meditation before bed or getting off electronics.

So sleep, stress management, getting your labs checked, it’s not just checking sex hormones, it’s looking at gut health, it’s looking at your inflammatory markers, it’s looking at your lipids, it’s looking at so many different metrics, stool testing, saliva testing, dried urine testing, can be very helpful.

It’s also understanding that as we are losing muscle mass, we’re losing insulin sensitivity so if you’re still doing 60 minutes on the treadmill every day or looking haggard running through your neighborhood, understanding that you could take that time, even 30 minutes of strength training is going to help your body maintain or at least try to build some muscle that will help with insulin sensitivity and very likely will be helpful for weight loss reduction.

I also just think about toxins we’re exposed to our environment or personal care products and our food. I cannot overstate how important it is for people to look at this, we have escalating rates of infertility in the United States, we have endocrine-mimicking chemicals that can offset the receptors for these hormones that are supposed to be optimally utilized.

I think about the role– and this is something I’m learning more about through Gabor Mate’s work, the role of trauma and how if you have adverse childhood events, a certain amount of there’s scores that you can check online, you can take those tests, we know that individuals that have lived with quite a bit of trauma history as a child, they’re not only at risk for autoimmune conditions, especially women but also at risk for weight loss resistance. These things in this degree of inflammation depending on what we do in our personal lives, professional lives has a lot of net benefit.

There’s a lot to unpack, as it pertains to weight loss resistance. It’s never just one thing, I find it certainly multiple things. It’s things as simplistic as putting your macros together wrong. Most women are eating 40 grams of protein a day that I see. All of a sudden telling them I want you to eat that much protein in one meal, I want you to back off on the unhealthy carbs, I want you to have those healthy fats, I will just want you to read food labels, that all can be very, very beneficial as a good starting point.

There’s usually those are the low-lying fruit that can all contribute. But I find it’s never just one thing. It’s usually multiple things but the sleep piece is critically foundationally important to our health.

Ari: Yes, agreed. Cynthia, this has been wonderful, your wealth of knowledge. I’m very impressed with your command of the literature on this topic and tell people where they can find your book, find more about your work and follow you.

Cynthia: Thank you so much. It’s been a pleasure. Probably easiest to start with my website. So You have access to my book, my podcast Everyday Wellness, for which I hope that we’ll get ourselves rescheduled and get you back on the podcast.

I’m very active on Instagram. I’m a little snarky on Twitter. I do you have a free– I have to forewarn everyone I grew up in New Jersey so sometimes some things slip out of my mouth. I have a free Facebook group called Intermittent Fasting Lifestyle backslash my name. It’s a great community of men and women that just want to learn more about metabolic health.


Show Notes

00:00 – Intro
00:53 – Guest Intro
02:06 – The benefits of intermittent fasting
05:40 -The difference between time-restricted feeding and intermittent fasting
10:41 – How fasting duration affects your health
14:40 – How hormetic stress may stimulate autophagy
19:00 – Potential harms of fasting too long
23:28 – Why you need to individualize fasting to your situation
26:30 – The best fasting window
33:05 – Metabolic flexibility
38:35 – Keys to start fasting
43:40 – How to set up fasting based on your gender and age
48:31 – How to fast for weight loss
55:35 – How to break through weight loss plateaus
59:18 – Outro


Recommended Podcasts

Like this article?

Share on Facebook
Share on Twitter
Share on Linkdin
Share on Pinterest

Leave a comment

Scroll to Top