Effects of Fasting & Time Restricted Eating on Fat Loss & Health | Huberman Lab Essentials
Date: 2025-08-28 | Duration: 00:38:22
Transcript
0:00 Welcome to Huberman Lab Essentials, where we revisit past episodes for the most potent and actionable science-based tools for mental health, physical health, and performance. I’m Andrew Huberman, and I’m a professor of neurobiology and ophthalmology at Stanford School of Medicine. What we’re going to talk about today is how intermittent fasting, aka time-restricted feeding, impacts weight loss, fat loss in particular, muscle maintenance, loss, and gain, organ health such as gut
0:30 health and liver health, the genome, the epigenome, inflammation, sickness, recovery and healing from sickness, exercise, cognition, mood, and lifespan. So, let’s talk about eating and what happens when you eat, and let’s talk about fasting or not eating and what happens when you fast. If ever there was a topic that is controversial, especially on the internet, it is that of diet and nutrition. So, I’m wading into this
1:00 with a smile and in eager anticipation of all the “but this” and “but that” and “wait, but this showed that.” Here’s the deal. We need to precisely define what it is that we’re talking about when we talk about nutrition. I’m going to give you an example of a study that was published a few years ago, 2018, by a colleague of mine at Stanford. Chris Gardner is a terrific professor of nutrition and has done a lot of important studies on how
1:30 nutrition impacts different aspects of health. This paper where Chris is the first author—it’s Gardner et al. 2018 JAMA—looked at weight loss in people following one particular diet versus another particular diet. And this was a 12-month weight loss study. So it was focused specifically on weight loss, although they looked at some other parameters as well. And the basic conclusion of the study was that there was no significant difference in weight
2:00 change between people following a healthy low-fat diet versus a healthy low-carbohydrate diet with significantly more dietary fats in them. This caused a lot of ripples in the world of nutrition and nutritional science and certainly in the general population because anyone that understands diet and nutrition would immediately say, “But wait, there are all sorts of different implications of eating one type of diet, say low-carbohydrate, higher fats versus a
2:30 higher-carbohydrate, lower-fat diet.” And indeed there are. This study was focused specifically on fat loss and on weight loss. So as we discuss time-restricted feeding, we need to be very precise about what are the effects of time-restricted feeding and of eating in particular ways at particular times. We are going to emphasize again whether or not the study was done in mice or in humans, in athletes, in men and women, or both. But the study from Gardner and
3:00 colleagues is a beautiful study and really emphasizes that if one’s main goal is simply to lose weight, then it really does not matter what one eats, provided that the number of calories burned is higher than the number of calories ingested. However, anyone out there who understands a little bit of biology or a lot of biology will agree that there are many factors that impact that calories burned
3:30 part of the equation. Some of those are obvious. For instance, amount of exercise, type of exercise, basal metabolic rate—how much energy one burns just sitting there. I’ve talked before on this podcast about NEAT, non-exercise activity thermogenesis, where if people bounce around a lot and fidget a lot, they can burn anywhere from 800 to 2,000 calories per day. So, their basal metabolic rate is actually much higher simply because they’re fidgeters, whereas people who tend to be more stationary have a lower basal metabolic rate on average. There’s
4:00 great science to support this. Metabolic factors and hormones are also very important. Hormones such as thyroid hormone, insulin, growth hormone, and the sex steroid hormones, testosterone and estrogen—those levels will also profoundly influence the “calories out,” the calories burned component of the calories in, calories out equation. So if out there on the internet or in listening to a particular podcast or speaker somebody says, “This is
4:30 the ideal diet,” or “Calories in, calories out does not matter,” or “Calories in, calories out is the only thing that matters,” I think it’s very important to understand that there are some foundational truths, such as calories in, calories out, but that of course hormone factors and the context in which a given diet regimen is taking place are exceedingly important. So there’s no way that we can drill into every aspect of a given feeding plan or feeding schedule
5:00 that would allow us to tap into every aspect of the list that I read out before: weight loss, fat loss, muscle, organ, genome, epigenome, inflammation, exercise, cognition, mood, and lifespan. But today we’re going to be very precise about how time-restricted feeding—it’s very clear from both animal studies and human studies—can have a very powerful and positive impact on everything from weight loss and fat loss to various health parameters. This is a beautiful
5:30 literature that’s emerged mostly in the last 10 or 15 years. So there is a perfect diet for you, and today I’m going to arm you with the mechanisms and understanding that will allow you to define what that perfect diet is and will allow you to eat on a schedule and to eat the things that are going to best serve your goals. Some simple rules about eating: First of all, when you eat, typically your blood glucose, your blood sugar, will go up. Also, insulin levels will go up. Insulin
6:00 is a hormone that’s involved in mobilizing glucose from the bloodstream. How much your glucose and insulin go up depends on what you eat and how much you eat. In general, simple sugars—including fructose from fruit, but also sucrose and glucose—will raise your insulin and blood glucose more than complex carbohydrates, things like grains, breads, and pastas. And grains, breads, and
6:30 pastas will raise your blood glucose more than fibrous carbohydrates like lettuce and broccoli. Protein has a somewhat moderate or modest impact, and fat has the lowest impact on raising your blood glucose and blood insulin. So what you eat will impact how steep a rise in blood glucose and insulin takes place. And there are a number of factors that are related to your individual health that will also dictate how steep and how high that rise in glucose and
7:00 insulin will be. The longer it’s been since your last meal, the lower typically your blood glucose and insulin will be, and the higher things like GLP-1, glucagon-like peptide-1. Glucagon being a hormone that’s also secreted when you are in a fasted state or a low blood glucose state. It’s involved in mobilizing various energy sources from the body, including fat through what we
7:30 call lipolysis, also using carbohydrates and potentially even using muscle as a source of energy. So that’s a fire hose of information about what happens when you eat and don’t eat. But just think of it this way: blood sugar and insulin go up when you eat. They go down when you don’t eat. And other hormones go up when you don’t eat. So there are hormones associated with the fasted state and there are hormones associated with the eating and having just eaten state.
8:00 Now the most important thing to understand is that, like everything in biology, this is a process that takes time. So insulin and glucose go up when we eat, and it takes some period of time for them to go down. Even if we stop eating, they will remain up for some period of time and then go back down. It takes time. This is very important because if you look at the scientific literature on fasting, on time-restricted feeding,
8:30 it’s absolutely clear that the health benefits—not just the weight loss benefits, but the health benefits from time-restricted feeding—occur because certain conditions are met in the brain and body for a certain amount of time. And that gives us an anchor from which to view what eating is in terms of how it sets conditions in the body over time. And if that sounds overly analytic, I promise you this is the simplest and best way to think about
9:00 any eating schedule or any eating plan. So I think it’s fair to say that in the field of nutrition there are a few landmark studies that serve as really strong anchors for building our understanding of what to eat and what not to eat and when to eat depending on our goals. The Gardner study that I mentioned earlier is one such study in that it says if your goal is weight loss, it really does not matter what foods you consume provided that you
9:30 consume a submaintenance caloric diet. However, I want to emphasize again that sets aside issues of adherence, meaning how easy or hard it is to adhere to a given diet. Some people find it much easier to follow a high-fat, low-carbohydrate diet. Some people follow a different diet because it’s much easier for them to follow. And some people are concerned with mental performance and athletic performance. So, that study doesn’t say there’s a best diet. What it says is that what you consume is less important than the
10:00 amount of food that you consume, at least for the sake of weight loss, not necessarily for the sake of health. Now, the study that I’m going to refer to next is what I would consider the second major pillar of nutritional studies. This was a paper in mice that set the basis for studies in humans that came later. And the title of this paper is “Time-restricted feeding without reducing caloric intake prevents metabolic diseases in mice fed a high-fat diet.” So
10:30 the title tells us a lot. It says that what’s varied in this study is not what these mice ate; it was when they ate it. One of the most important things to take away from this study was that mice that ate a highly palatable high-fat diet—a great tasting diet—but only during a restricted feeding window of each 24-hour cycle maintained or lost weight over time. Whereas mice that ingested the same diet, same amount of calories,
11:00 but had access to those calories around the clock gained weight, became obese, and quite sick. And as an additional second point, the mice that restricted their feeding window to a particular portion of eight hours of every 24-hour cycle actually showed some improvement in important health markers. And what was even more incredible is that mice that only ate during a particular feeding window also experienced some
11:30 reversal of some prior negative health effects. Not only did restricting food to a particular phase of the 24-hour cycle benefit things like lean body mass and fat loss and a number of health parameters that I’ll talk about in a moment, but it also anchored all the gene systems of the body and provided a more regular, stable so-called circadian rhythm or 24-hour rhythm. You may be surprised to learn that 80% of the
12:00 genes in your body and brain are on a 24-hour schedule. That is, they change their levels going from high to low and back to high again across the 24-hour cycle. And when those genes are high at the appropriate times and low at the appropriate times—meaning their expression is high and low at the appropriate times and therefore the proper RNAs and proteins are made because DNA encodes for RNA, and RNA is translated into proteins—
12:30 when that happens, your health benefits. When those genes are not expressed at the right times, when they’re high or low at the wrong times of each 24-hour cycle, that’s when you