The Truth About Fasting: Separating Fact from Fiction

Mark Hyman, MD

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Dr. Mark Hyman
Coming up on this episode of The Doctor's Farmacy. You gotta start your first meal protein. Protein will help you live longer, be healthier, and the key thing is when you break your fast and you eat protein, it turns on your body's capacity to stimulate the growth and the building up of new muscle, not if you have sugar for breakfast. Before we jump into today's episode, I'd like to note that while I wish I could help everyone via my personal practice, there's simply not enough time for me to do this at this scale. And that's why I've been busy building several passion projects to help you better understand, well, you.

If you're looking for data about your biology, check out function health for real time lab insights. If you're in need of deepening your knowledge around your health journey, check out my membership community, Hyman Hive. And if you're looking for curated and trusted supplements and health products for your routine, visit my website, supplement store, for a summary of my favorite and tested products. Welcome back to another episode of The Doctor's Farmacy and health bites where we take juicy little bites into current health topics. When it comes to fasting, there's a lot of misinformation floating around.

And one of the most common myths is that fasting slows down your metabolism. But here's the truth. It all depends on how you fast. It all depends on how you fast. Long term fasting can have its drawbacks, like slowing down your metabolic rate as your body conserves energy or losing muscle.

But when it comes to short term fasting, the opposite is often true. It can actually give your metabolism a boost. In today's episode, we're diving deep into the science behind fasting and metabolism. We're gonna break down the differences between long term and short term fasting, and we're gonna explore how your body responds to each, and we're gonna debunk some of the biggest myths out there. So whether you're curious about intermittent fasting or time restricted eating or just wanna better understand how fasting impacts your hormones, your energy levels, and overall health, well, this episode is packed with insights to help you make informed choices.

So let's cut right to it. The first myth, myth number 1, fasting slows your metabolism. Well, many people believe that fasting slows your metabolism, and this might be true in some extreme cases. But the truth is it depends on the type of fasting you're doing. Long term fasting could have negative implications for your metabolism.

But with short term fasting, the opposite actually seems to be true. It actually seems to speed up your metabolism. So let's break it down. What is long term fasting? Well, long term fast typically range from a full day to several days without food.

Long term fasts have lots of benefits. It stimulates autophagy or cellular cleanup. It promotes the burning of fat, clean fuel instead of carbs for energy. And it can be super beneficial for weight loss, improving insulin sensitivity and lowering body inflammation. About 48 hours or more into a fast, your metabolism starts to shift.

The body may start to conserve energy, which results in a downregulation of metabolism. Now this is an adaptive response to prolonged calorie restriction, but because your body doesn't know when your next meal is coming, your metabolism slows to conserve energy. Now why is long term calorie restriction a problem? Well, over time, long term calorie restriction without adequate nutrition causes your body to go into what we call starvation mode. And this puts stress on your body.

In most cases, a little stress. Isn't a bad thing. It can actually be good thing, but prolonged stress is not necessarily good. Now there's a concept known as hormesis. This is kind of a good stress.

It's basically a stress that doesn't kill you but makes you stronger. And you get that with short term fasting, with exercise, a sauna, cold plunge, and that turns on all your longevity switches. Now calorie restriction is a form of hormetic stress, and it has its benefits. But there's also bad stress, which occurs when we undergo this stress for too long, and the slower metabolism could be the result. So what caused your metabolism to slow down during long term fast during calorie restriction?

Well, hormones change leptin and ghrelin change during prolonged fast. Your body undergoes changes in the appetite suppressing hormone called leptin and the appetite stimulating hormone called ghrelin or ghrelin or however you wanna say it. But basically, that's the hormone. Leptin is a hormone that's produced by fat cells and helps regulate metabolism and hunger. Kinda tells you when you're full.

When calorie intake is restricted, leptin levels go down, and that tells your body that energy stores are low. Now this is a good thing for people who have a common condition called leptin resistance, which is when you basically have too much food and your leptin levels go up and up, and it's you become resistant to the effects of leptin. And when you do fasting, it actually makes your leptin sensitivity better. So you're not resistant to its effects, which makes you feel full. And then that stimulates the body's ability to regulate your appetite more naturally.

It also improves insulin sensitivity and leads to weight loss. Now in the long term, in nonobese individuals, when you fast too long, it can result in a slowdown of your metabolic rate. And that means you're gonna try to conserve energy because your body thinks you're starving. Algraline, which is the hunger hormone, will initially increase during fasting, making you feel hungry. Its job is to stimulate appetite and food seeking behavior, which is a good thing.

When you're hungry, you're gonna go looking for food. Now during longer fasts, more than a few days, then grain levels start to decrease. And studies examining 3 day fasts have found that 24 hour mean grain levels actually declined over the fasting period. In other words, you know, a lot of people who are fasting report a decreased hunger as you continue to fast. So you might be hungry the 1st few days and then after a while, you're not hungry.

Now when you're less hungry, you eat less than the body then adapts by slowing down your metabolism. Now another thing happens when you fast too long. Your thyroid doesn't work out so good. A slower metabolism is in large part related to your thyroid hormone levels. See, thyroid is the master control center of your metabolism.

When you undergo long term fast or calorie restriction for longer periods of time, it actually lowers your thyroid hormone production and therefore your resting metabolic rate, meaning you burn less calories just sitting around. T3 is the active thyroid hormone, and it plays a really important role in regulating your metabolism during longer fasts. The body aims to conserve energy and the way it does that, it reduces the production of the active thyroid hormone called t 3. So you don't burn as many calories That slows your metabolism, and your body can conserve energy and become more efficient using those fewer calories. Now this is good when we didn't have a lot of food around and we conserved energy, and it was sort of built in to protect us from food scarcity, but we don't have that anymore.

Now as your metabolism starts to slow down, the body becomes resistant to weight loss, and then it makes it harder to continue losing weight without further reducing calories. You get kind of a plateau. Now, of course, if you're, you know, not eating at all, your body is gonna continue to waste and you'll starve and die. But if you calorie restrict, initially, you'll lose weight, but then you hit a plateau. Now it's a survival mechanism.

It's gonna help your body conserve energy in times of scarcity. Now short term fasting is good because it doesn't result in a slow metabolism. It does the opposite. It actually boosts your metabolism. So what's short term fasting?

Well, there are many different types of short term fast. A few of the main ones include what we call intermittent fasting. Now that's an umbrella term for various fasting schedules where you alternate between periods of eating and fasting. There's a lot of different methods out there. There's something called the 16:8 method, which is 16 hours of fasting and 8 hours of eating.

So let's say you eat between, let's say, 8 in the morning and 4 in the afternoon. Right? And then you're done. And then you take the next 16 hours off, and you go to bed, and you sleep, and you wake up, and you eat. There's something called the 52 method, which is eating normally for 5 days and dramatically reducing your calories for 2 days.

Or there's alternate day fasting, or you can do even shorter fast, like 12 to 14 hours. Now intermittent fasting focuses on on more extended fasting periods, which may include skipping meals or limiting calorie intake for whole days. But I I think the better way to think about this is what I call time restricted eating. And it's a specific form of intermittent fasting that emphasizes eating all your daily meals within a consistent window of time, like 8 hours, 10 hours, and fasting for the rest of the day, like 14 to 16 hours. Unlike other forms of intermittent fasting, time restricted eating focuses more on aligning eating with the body's natural circadian rhythms and ensures all meals are consumed within a set time from each day.

You can do 12:12 fasting, 12 hour feeding window where all your calories are consumed followed by the 12 hour fasting period where food intake is restricted. So, basically, you wouldn't be fasting after dinner from 8 PM to 8:8 AM the next morning. I mean, everybody could probably do a 14 hour fast. Basically, you eat dinner. You finish at, let's say, 6 o'clock.

That means you can eat breakfast at 8 o'clock the next morning. The calorie restriction is not a requirement. It can be a side effect of condensing your daily food intake to a shorter time window because you just don't have enough time to eat that many calories. And that's why time restricted eating is so popular. It helps you eat fewer calories, which then leads to weight loss without having to change your diet.

But optimizing your diet quality leads to even more benefits. We're going to discuss that later, but there are some studies that show that regardless of what you eat, it works, but it works better if you eat more healthy food. Now short term fasting can actually reset and even boost your metabolism when you do it correctly. It does this by supporting fat burning and by supporting metabolic health, and it modulates several fat burning hormones, especially insulin. Now high insulin levels block fat loss, and it makes it harder to lose weight.

And the intermittent fasting can change that. Insulin is the fat storage hormone. And when you have a high insulin level, you can't lose weight and the fat gets locked in the fat cells, so you can't even burn them. Research shows that intermittent fasting can help lower your fasting insulin, which is especially meaningful for those with prediabetes, type 2 diabetes, or those with underlying metabolic dysfunction. And that, my friends, applies to 93.2% of Americans.

That means somewhere in that spectrum of mild insulin resistance to prediabetes to type 2 diabetes, 93.2% of Americans fall in that group, which is terrible. And most of us would benefit from listening carefully to this podcast and actually applying the principles here. And a melon a big study, what we call meta analysis, which is a study of other studies, found that intermittent fasting reduced insulin levels by an average of 13,000,000 units per liter in patients with metabolic syndrome. And that is a lot. Okay.

That's a lot. A review of the literature found that intermittent fasting and alternate day fasting could reduce fasting insulin levels in prediabetics by up to 31% after 8 weeks. That's a good thing because high insulin levels lead to heart attack, strokes, cancer, dementia, obviously, diabetes eventually. Now some studies have even shown that patients were able to reverse their need for insulin therapy just by therapeutic intermittent fasting when they're supervised by their doctor. I've seen this over and over when people eat healthfully, when they cut out sugar and carbs, when they narrow their eating window to 12 minuteimum, but ideally 14 to 16 hours, it has dramatic effects.

The other thing that happens when you do intermittent fasting is you increase the level of something called norepinephrine, kind of like adrenaline, right? This is your stress hormone. It's involved in the fight or flight response. And when you enter it and fast, it increases adrenaline, which is a good stressor because it tells your body to mobilize stored fat to burn them for energy, which is a good thing. It also stimulates human growth hormone, which is really important.

It helps with fat loss, and it helps to preserve muscle mass. And this brings me to another common myth about fasting. As you heard me talked about, I cofounded a company called function health, which gives you access over a 110 biomarkers for 4.99 membership a year. Now when you wanna see the impact of your fasting regimen on your hormones and biomarkers, you should for sure check out function health before and after you do your intermittent fasting regimens. And we're gonna put the link in the show notes.

But if you go to function health.comforward/mark, you can jump the wait list, which is about 300,000 people right now. So little tip for me. Okay. Let's get into the 2nd myth. Myth number 2, fasting causes muscle loss.

Your muscle is 3 times, maybe even more metabolic active than fat and requires a lot of energy to maintain it. Muscle contributes to about 20% of your total daily energy expenditure. For every extra 1 pound of muscle on your body, you burn 6 to 10 extra calories per day. So that's why strength training is so great. It actually makes you burn more while you're just sitting around doing nothing.

Now during prolonged fasting or calorie restriction, the body actually breaks down muscle, which is not good, especially if you're not resistance training or if you're not eating enough protein. Now this loss of muscle is not a good thing because then you get a slower metabolism since fewer calories are burned. And that's why I don't regularly do prolonged fast. When I do this, I'm relatively thin, healthy, fit. Don't have any swim resistance.

I have to be careful because I will lose significant amounts of weight and muscle if I do too long fast, but I can do 12 to 14, sometimes 16 hours without a problem. When you're fasting, you don't want to lose muscle. You want to build muscle and maintain your muscle, not lose it. Why? Because muscle is the organ of longevity.

It's linked to a longer health span and your lifespan. When you have a lot of muscle, it's linked to a longer health span, meaning you're healthy longer and to a longer lifespan. And it does that through a lot of different mechanisms. It lowers the risk of something called sarcopenia, which means less muscle, and that's really this hidden disease of aging that affects so many people. Basically, turning your muscle from a filet mignon in your thirties to a Wagyu rib eye in your sixties or seventies, that's bad news.

Even if you don't gain weight, you can lose muscle, stay the same weight, and you become basically skinny fat. Also, when you look at people who don't build muscle and strength training, they get osteoporosis and you can get chronic diseases like obesity, Alzheimer's type 2 diabetes. Now when you have enough muscle, it helps protect against chronic disease by improving your metabolic health. It improves insulin sensitivity and muscle acts like a sponge. It soaks up sugar and excess fuel from your bloodstream.

Now, having muscle also benefits your brain, which is kind of your biggest muscle and not actually muscle, but it's the thing that you use to run your life basically. And if it's not working well, it's a problem. And so having healthy muscle also leads to improve cognition, to mood, to mental health and lots more. And that's why we got to combine resistance training with any time restricted eating or intermittent fasting regimen to preserve muscle and get the maximum benefits. When you just do it without the strength training, I don't think it's a good idea.

Body composition is really important and this is really how much fat and how much muscle do you have. Right. Very simple thing, but it's, it's often not looked at. It's one of the most important diagnostic tests we can do. Now, when intermittent fasting is combined with resistance training, meaning strength training, your lean muscle mass is also preserved, which is a good thing.

Now, some studies suggest that resistance training while intermittent fasting improves body composition by increasing muscle mass and decreasing body fat. That's a good thing. And and so this is really where we want to go. We don't want to fast and lose muscle because that's a bad thing. Now there's a randomized controlled trial of 41 overweight obese adults.

They found that combining intermittent fasting with calorie restriction and regular protein intake, about 25 to 50 grams per meal over 8 weeks, led to greater reductions in total body fat, in visceral fat, in other words, the inflammatory belly fat, and total body weight compared to just calorie restriction alone. So the message here is if you're going to do calorie restriction or time restricted eating or intermittent fasting strength, training is non negotiable intermittent fasting in this study combined with time, protein intake and calorie restriction positively shifted the gut microbiome to bacteria that help burn fat and improve your metabolic health more than just calorie restriction alone. Yep. Your gut bacteria actually affect your weight. It's quite remarkable, but you got to make sure you're getting a protein.

That's really key. And the ideal amount, if you're active is about a gram per pound of ideal body weight. So if you're a 120 pounds, it's about a 120 grams a day. It can be 0.7 to 1, but somewhere in that range. And this really helps you fight muscle loss that happens during prolonged fast.

Now here's a tip. Break your fast with a meal that contains protein to halt the muscle protein breakdown from your overnight fast, and then you stimulate muscle protein synthesis. So this is really important. I want you to hear this. When you fast, even if it's just overnight, right, from 12 hours, from, let's say, 8 at night till 8 in the morning, you gotta start your first meal with protein.

If you do what we do in America, which is eat sugar for breakfast, cereal, muffins, bagels, croissants, sweetened sugars, sweetened teas, sweetened coffees, I mean, everything that we typically eat in America, it's going to kill you. I promise you faster than if you eat protein. Protein will help you live longer, be healthier. And the key thing is when you break your fast and you eat protein, it turns on your body's capacity to stimulate the growth and the building up of new muscle. Not if you have sugar for breakfast.

Fasting only becomes a problem for muscle mass if you're not substituting calories and carbs with protein and also adding resistance training. Okay. What's the 3rd myth around fasting? Fasting increases the risk of heart disease. Well, there's been a lot of buzz around articles attempting to debunk the benefits of fasting.

Some even linked into an increased risk of heart disease, which doesn't sound so great. Now one study in particular caused confusion by suggesting that time restricted eating, specifically an 8 hour window, which is a 16 hour fast, right, was linked to a 91% higher risk of cardiovascular death according to the American heart association. So that sounds terrible. Now for those with existing heart disease, the study claimed that eating in a window of 8 to 10 hours per day was associated with a 66% higher risk of death from a heart attack or stroke. But let's kinda wait there for a minute and break this down.

Let's look at the limitations of the study, which were quite significant. First, the data came from about 20,000 US participants from what we call the NHANES study, the national government study, the National Health and Nutrition Examination Survey. Dietary information was based on 2 self reported questionnaires and relied on participants' memory, which is not really good. The data on dietary food frequency questionnaires shows that they're pretty unreliable. And there's a lot of bias.

Right? People often misremember what they ate. They underestimate what they ate. They overestimate the good stuff. They underestimate the bad stuff.

Let's face it. We're all human, and we don't like to tell the truth. So it's really not an accurate way to evaluate someone's diet. And then there's just a lot of data on this, and I I've written a lot about this. But it's it's something we need to really be aware of.

These these food frequency questions can be helpful for looking at maybe patterns or associations, but they don't prove anything. 2nd, the study didn't account for what we call confounding factors that have influenced participants' health or the decision to engage in time restricted eating. In other words, maybe the people who have tried to do this were already really sick. And so that's why they did it to improve their health, but they were already really sick and already had a high risk of heart attacks. And maybe it wasn't the time restricted need at all.

The other thing was that overall diet quality wasn't really paid attention to. So, I mean, for example, some of the participants might do their fast, but then when they eat, they just eat the standard American diet or have fast food or French fries or sugar or crap. Right? So is it fasting that caused a problem or is it the rest of their diet? And third, the researchers focused only on meal timing and cause of death.

They didn't provide other health data such as things about your critical health factors that determine your risk, like how many calories are consumed during the feeding window? What was the quality of the calories? Was it whole foods, or was it the standard American ultra processed diet, which is what we mostly eat in America? 60% of our diet is ultra processed food. Of course, there's lifestyle factors that they didn't account for.

Stress, smoking, alcohol. And by the time of day they're eating is also important. Fasting during the day and binging at night is more problematic than eating during the day and fasting at night. In other words, what if you didn't eat all day and then you had a giant meal at dinner and then kept eating till you go to bed? That's gonna have a very different effect on your metabolism.

So this is the premise of time restricted eating or eating within your circadian rhythm. While the headlines might sound scary, it's important to understand, like any study, what the limitations are before you come to any conclusions. So is fasting dangerous for some people? Yes. True.

Who are those people? Well, if you're pregnant, not a good idea. If you're breastfeeding, if you're trying to get pregnant, if you have thyroid issues, if you're type 1 diabetic or you're underweight or malnourished, all those things, or you're frail or you're elderly or chronically ill, maybe not such a good idea. Now fasting can trigger hormonal changes that cause your body to be stressed, not something you want while you're pregnant or trying to get pregnant. Women who are trying to get pregnant with PCOS, which is polycystic ovarian syndrome, or reproductive issues could benefit from time restricted eating or at least eating within a 12 hour window.

I think that's just basic. Like the standard should be everybody should take a 12 hour break from eating every day. Right. Finish dinner at 8. Don't eat until 8 in the morning.

Finish at 7. Eat breakfast at 7 in the morning. That's not so hard. That's why I call it breakfast break fast because we should be breaking the fast, not just eating until we go to bed and then eating first thing in the morning. Now, intermittent fasting can also help reset the body's internal clock.

So what does that mean? Well, this is your circadian rhythm. When you fast, it's really important to fast with your circadian rhythm, not against it. So what do I mean by that? Well, human beings are biologically programmed to digest and absorb food best earlier in the day.

This is known as circadian eating. Doctor Sachin Pedda is a lead scientist in circadian rhythm research. That means doing daytime things when the sun comes up, like eating, moving, and working, and nighttime things when the sun goes down, like winding down and sleeping. Duh. Right?

Doctor Panda discovered that thousands of genes turn on and off at specific times of the day, which affects lots of different bodily functions. Your circadian rhythm affects your body's ability to digest your food. Now insulin sensitivity, which is really important, is highest in the morning, and it decreases during the day. Our digestive system produces less saliva, stomach acid, and digestive enzymes at night, which is why late night eating is a bad idea, and yet it's what we do mostly in America. What happens when you eat late?

Well, it causes blood sugar swings throughout the night, makes weight loss harder. It messes up your REM sleep, which is your dream sleep, and increase the risk of type 2 diabetes. It also puts energy towards digestion. It takes energy away from rest and repair. Because at night, your body's supposed to rest and repair and heal.

But when it's digesting, it can't do that. And time restricted eating might also help extend your lifespan. A study on firefighters show that 14:10 time restricted eating, meaning 14 hour fast, 10 hour eating window for 6 weeks improved oxidative stress and reduce something called advanced glycation end products, which is a marker of aging, inflammation, and cardiovascular disease. Now eating an earlier dinner or fasting for at least 3 hours before bed can improve sleep. It can support weight loss.

It can improve your immune function and your metabolic flexibility and prevent chronic disease. So lots of benefits. So are the benefits of fasting due to not eating to calorie restriction or eating within your circadian rhythm? Well, it's probably a combo effect. So as we've explored today, fasting doesn't have to be a metabolism slowing monster.

It's often made out to be when approached correctly, especially with short term fasting methods, like intermittent fasting, time restricted eating, you can actually boost your metabolism. It can help you burn more fat, and it can improve your overall health. It's all about finding the right balance for your body and your lifestyle. So whether you're looking to improve your metabolic health, manage your weight, or simply give your body a reset, fasting can be a powerful tool when used wisely. The key takeaway, don't fall for the myths and understand the science behind fasting.

Listen to your body. Your body is the smartest doctor in the room. I always say that Don't listen to doctors. Don't listen to the science. See how you feel and how you respond, and then learn from what your body is telling you.

Also, remember, fasting is just one piece of a puzzle. Combining with a nutrient dense diet, with regular exercise, with stress management, it's gonna give you the best results. Thanks again for joining me today on The Doctor's Farmacy and health bites. Don't forget to rate, review, and follow The Doctor's Farmacy wherever you get your podcast. Stay healthy, stay informed, and I'll catch you next Friday for another episode of Health Bites.

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