159: Why Exercise Alone Won’t Help You Lose Weight

Most people exercise to lose weight, but that's the wrong reason to do it. When you start exercising, the calorie math predicts that you should lose weight, but over time the body adapts, and the scale often changes far less than you might expect. The scale can't show what's really happening. After all, you may be losing fat while gaining muscle. You may be reducing dangerous visceral fat around your organs. And every time your muscles contract, they can pull glucose from your blood without requiring insulin. Exercise also increases your muscle's ability to burn fat and can improve insulin sensitivity for up to 2 days after a single workout. So don't judge exercise only by the number on the scale. Exercise to improve your strength, your metabolic health, and your longevity. In other words, Exercise to be strong, eat smart to be lean. This is Lecture 159 of The Metabolic Classroom. Welcome back to The Metabolic Classroom. I'm Ben Bikman, metabolic scientist and professor of cell biology. Exercise is among the most commonly prescribed interventions for weight loss, and unfortunately, it's among the least effective ones. Now, don't misunderstand me. This is not an argument against exercise, but I am making the case that we should reframe it, at least. As much as I'll make the case that exercise isn't a great intervention for weight loss, I still want to defend it, and I think you'll see why over the course of this mini lecture. All right, let's get started. The recommendation to exercise for weight loss comes from a calculation that anyone can really perform these days. An hour-long walk, for example, can cost a little over 300 calories. Now, of course, that can depend a lot on the size of the body that is doing the walking. And if you do that session, though, if we just take this simple math and you do that 5 times over a week, that comes to about 1,500 calories, which when you balance that against the roughly 7,000 calories stored in a kilogram of body fat, that should lead to a predicted loss of about 5 kilograms across 6 months. Hopefully you can follow that math a little bit. Now, this has been tested, this simplicity of the calculus of the calories. The clearest test of this came from a 6-month trial in over 400 sedentary, overweight, postmenopausal women, and they exercised under very strict laboratory supervision, and they did so at one of 3 exercise doses. Now, every session was measured. It wasn't just reported from the individual subjects, and the investigators were able to compare what each woman should have lost against what she actually lost. At the lower doses of exercise, that was about 70 minutes weekly and then 135 minutes weekly, the calculation was pretty decent. The actual losses of body weight closely matched the predictions, and it was 1 and 2 kilograms respectively. Now, at the highest dose, that was about 200 minutes weekly. Women were predicted to lose almost 3 kilograms, but instead they only lost 1.5. So about half of what was expected. Now, only the highest dose fell short of prediction, which means the proportion of predicted weight loss actually obtained declined as the prescribed dose rose. So the mathematics started to fall apart as the intensity or the frequency of the exercise went up. The implication of this kind of goes contrary to the standard guidance, which typically recommends 200 to 300 minutes weekly in order to get weight loss. Larger meta-analyses find that the energy deficit created at the start of an exercise program declines ever closer to zero over roughly a year. Let's get in to some of those compensatory mechanisms now. So if the calories are being expended during the exercise session, which it is, but the fat isn't being lost like you would expect it to, the energy has to be going somewhere. Or it has to be made up for somewhere. As it turns out, the body has more than one way to take back some of that energy that is lost during the exercise. The first way is the one that almost nobody anticipates. So take that 45-minute walk or so that I mentioned a moment ago, the one that costs about 300 calories. By the end of the day, the total energy that you've burned is not actually 300 calories higher than it would have been if you hadn't done it. It's actually closer to about 200 because about 100 of those calories end up coming back somehow. They've— or they've gone missing, if you— if I can say that another way, they went missing because the body ended up spending less energy on everything else. You see, most of what we burn in a day has nothing to do with physical activity, or at least exercise. It goes toward the constant work of just keeping a body running. The process of autophagy or replacing old cells or building or repairing various proteins or just maintaining the electrical gradients that you need, that every cell depends on, all of the shifting around of the electrolytes that all costs energy. But that, that background energy expenditure is not fixed. The body can change it. It can expend more energy in these impossible to quantify ways or impossible to notice, or it can expend less. So when you add a demand at one end of this energy formula, the body can trim the caloric spending or expenses at the other. In other words, the body can pay for part of the walk by doing a little less of its kind of background upkeep. On average, that recovery runs about 28% of what was spent. But some people recover far more than others, and the compensation rises with body fat from roughly 28% that is recaptured by slowing the body down in other ways. And that happens in the leaner people, but it can go up to 50% in the heaviest people. So someone carrying a great deal of fat is getting back almost half the energy they spend trying to lose it. Which means an hour of effort ends up yielding something closer to 30 minutes of actually— of actual caloric deficit if you're just counting the calories. Now, whether people become fat because they're strong compensators or become strong compensators because they're fat, that has not been settled, unfortunately. Hopefully you can follow my logic there. But either way, though, the practical consequence is the same. Exercise works progressively worse as a fat loss tool in exactly the people who are most often told to use it for that purpose. All right, so that's the first route of this, um, compensatory energy change that I mentioned. The second is, uh, behavioral, actually, and I think it's more important than the consensus scientific view currently makes it look. The intuitive view is that a hard training session leaves a person moving less for the rest of the day. Maybe they're taking the elevator when they would have taken the stairs, or they're sitting longer at their desk when they would have just spontaneously been getting up more and just doing stuff. Compensation appears more pronounced as the exercise dose goes up, which matches the dose pattern that I mentioned earlier, and more pronounced in people carrying more fat. And that again matches that kind of metabolic gradient that I, that I described just a moment ago. Now, fatigue is the most obvious mechanism here. A greater fatigue does predict measurably less movement over the following hours. So if you're really, really worn out, it just stands to reason you're going to just do a little less. So my view is that both routes are operating. The body trims its own background work or energy spending, and the person moves less. And neither is a decision that anyone is necessarily aware that they're making. These are not conscious decisions. That second one, at least, is partly modifiable. Nobody can choose to stop reducing their kind of baseline metabolic cost of just maintaining the body. But spending the afternoon on the couch because you spent the morning training, that is something you can control if you're looking for it, if you're aware of it. And so are other permissive behaviors actually that tend to cluster around exercise. For example, the sense that a workout has been earned or allowed you to earn something else that you wouldn't have indulged in before, particularly things that we may eat and drink. So let's talk about that now. So this is the second general explanation for why exercise may not lead to very clean, predictable weight loss, and that is appetite, because over time, exercise does increase hunger. But the evidence is a little more nuanced than you may expect. There are absolutely studies that find conflicting evidence, but there is evidence that increased physical activity will influence energy and intake later. But there's a time— there's a window to this, or a temporal effect. The acute physiology makes it a little complicated. After an hour of vigorous running, in one particular study, they found that circulating ghrelin levels, a hunger hormone, were significantly suppressed over the following hours. So a demanding session does not leave a person hungrier immediately afterward. Indeed, based on these— on this hormone signal, it would suggest it leaves the person transiently less hungry. Immediately following the exercise, they are less interested in eating. Now, across longer intervals, the picture becomes more, well, two-directional. Over 12 weeks of supervised exercise in overweight and obese adults, fasting hunger and average daily hunger both increased, while at the same time, the satiating power of a, of a set breakfast actually increased. So training raised the background drive to eat, but it simultaneously improved the capacity of a given meal to suppress that drive. Now, in another 12-week trial, mean weight loss matched prediction closely enough to suggest no compensation had occurred at all. However, the inter-individual difference was enormous. Some individuals over this 12-week period lost 15 kilograms, while others gained a few kilograms. We could divide this into, I think, two groups of people: compensators and non-compensators. The compensators are the ones who lost very little and maybe even gained a little bit, whereas the non-compensators were the group that lost a lot of, uh, several kilograms. What separated them was intake. Compensators increased daily energy intake by several hundred calories and reported more hunger, whereas the non-compensators, they decreased their intake by a few hundred— well, a little over 100 calories, and they reported no change in appetite. Those differences are enough to erase any exercise-induced energy deficit entirely. But they're small enough that the typical dietary assessments, let alone the level of just being conscious of it, might not detect and indeed would not reliably detect. So the intake explanation is correct for some individuals, but it's going to be wrong for others. And we don't really— there are currently better— we're better able to measure the missing energy than we are actually to predict who's going to be a compensator or a non-compensator. So who's going to compensate for that exercise, those calories expended by eating more, and who's not going to compensate? Again, we can quantify that, but we can't really predict who's going to fall into which camp. Okay, so far I've been presenting the idea that focusing on exercise as a way to leverage a caloric balance in your favor is not as simple as most people think. So let's get a little more positive and come back to a common theme for me, and one that still matters even in this topic. An enormous health benefit of exercise is improvements in insulin and glucose. Skeletal muscle takes up glucose through a transporter held within the cell, these intracellular storage compartments, until it is recruited to the cell surface. So this is a doorway that can be in the surface of the cell, but only when it's told to be. Or to say that all another way, it's a door that stays closed until it's told to open. Insulin opens it, but muscle contraction also opens it. In fact, in mice that are bred or genetically lack the insulin receptor in skeletal muscle, if you can get those muscles to contract, it can increase glucose uptake to a totally normal level. Despite the muscle not having any capacity to respond to insulin whatsoever. A contracting muscle therefore can clear, can clear glucose from the blood without imposing any demand on the pancreas. Those beta cells can stay perfectly quiet because the exercising muscle can take care of the glucose by itself. Exercise does not raise insulin in order to move glucose. Indeed, exercise lowers insulin through the sympathetic nervous system. And this is perfectly fine because the exercise is able to move the glucose into the muscle without the need of insulin. This is what I mean when in the past I've described exercise as insulin sparing. It is not insulin stimulating. It is, if anything, insulin inhibiting. The effect persists well beyond that single exercise session or the moment Dr. Ben Bikman will elevate or enhance the ability of insulin to stimulate glucose uptake for up to 2 days after the exercise session. So the muscle becomes so much more responsive to insulin that the body just doesn't need to make as much of it. And again, this can persist for 2 days. That's a pretty remarkable effect. You can imagine stacking those benefits on top of each other as you're training daily. So sustained training reinforces this even further, actually, by increasing the number of those transporters, those doors that the muscle, um, has to allow the glucose to come in. And exer— exercise is the most— in fact, it's the most potent stimulus for increasing the number of those glucose transporters. So you're expanding the muscle's capacity to con— to take in and consume that glucose. Whether it's through exercise or indeed whether it's through insulin stimulation. So you're making— so each exercise session can really, in a way, accomplish two things. There's the acute reduction in the need for insulin, and there is an ongoing incremental increase in the response to insulin as you're making more glucose transporters or more of those glucose doors. All right. So nonetheless, if exercise is a poor tool for shedding body weight. The fair question is why anyone should bother. But the answer is that the body— body weight is the wrong thing to be measuring in the first place in this conversation. Let's just start with overall fitness. When researchers pooled all available studies that measured cardiorespiratory fitness and then track who lives and who dies, unfit people are— have twice the level of mortality, so dying from anything, as people who were considered fit. And that holds regardless of their body weight. So no matter what their body status, fit people are half as likely to die, or their, their all-cause mortality is half that as the unfit people. So overweight people who are fit die at rates similar to lean people who are fit. That is very important. So fitness tracked survival. Weight, once you account for fitness, doesn't really matter. It didn't track in this study. So it's better to be fat and fit than skinny and sick. In a study following 140,000 adults across 17 countries, grip strength— yes, that humble little simple measurement of just squeezing a device as hard as you can— predicted death from any cause better than blood pressure. In fact, they went a little further. Getting grip strength came with about a 16% higher risk of dying from anything. Just think about that for a moment. We screen everyone for blood pressure. Every time you're going in for an annual wellness visit or any kind of visit, they're going to measure your blood pressure, but nobody gets screened for strength. And strength is the better predictor of just dying from anything. 30 to 60 minutes a week of muscle strengthening and activity is, is associated with something like a 10 to 20% lower risk of dying from any cause. So this is evidence that isn't even measuring strength, it's just measuring habits. And just the simple act of strength training was lowering the all-cause mortality by up to 20%. Now, one explanation for all of this is metabolic. Muscle is the largest site of glucose disposal in the body. More muscle means more capacity to clear glucose, and it means clearing it with less insulin. With less insulin comes greater insulin sensitivity, and with greater insulin sensitivity comes a reduced risk of dying from anything. Well, any chronic disease, certainly. But exercise also changes the way our body stores fat. And of course, this is a way that the scale can't tell you. Across a lot of these trials, waist circumference can fall even if body weight isn't. And in men in particular, men who exercise without losing a single pound of weight still found reductions in their visceral fat. In fact, significant reductions. All right, so hopefully I've convinced you that exercise is important. In fact, let me add one other sentiment to this. There's a just general reality of living in this chaotic world that if you have more muscle, you can simply protect your body better. For example, a fall. One of the most common reasons that an older person will start to accelerate in their decline of overall health is that they fall and break something. Well, they probably tried to stop their fall, but they didn't have enough muscle to generate enough power to slow their body down. They couldn't grip the counter well, or they couldn't push against the floor quickly enough to slow their fall. And so they end up hitting the floor with much greater force and they end up breaking something and then they accelerate their own demise. Okay, so I've convinced you, I'm sure, that exercise is important, but let me give you now a little bit of advice. So first, train for strength. And I unapologetically suggest prioritizing strength training over exercise or over aerobic training. Unapologetically, strength training trumps aerobic training. I would suggest at least 2 sessions a week. And within those 2 sessions a week, you are focusing on these major movements of pushing and pulling, and you're squatting or hinging. The push and the pull actually applies as much to the lower body as it does upper body. You just can't envision the pushing and the pulling as easily. So that's why I'm saying squatting, which is a push. Or hinging down and back up, which is a pull, but also just carrying something like doing farmer carries where you're carrying a kettlebell down around the block or something. These 5 movements— again, the push, the pull, the squat, the hinge, and the carry— can put you in the range where the mortality benefit is going to show up. And notably, a meaningful amount of the benefit happens at just around an hour of exercise a week. And then when it comes to aerobic exercise, do not look at aerobic exercise as some magic fat-burning portion of your program. In fact, if anything, fat burning appears to be greater minute over minute with resistance or strength training. So instead, look at aerobic exercise as just helping build your mitochondrial pool and function, as well as your cardiorespiratory fitness. Of course, doing both strength training and aerobic is wonderful. Even if the aerobic is a long walk. But if you're very, very time limited, most people are, I would suggest focusing on the strength training over the aerobic training. Again, if you have to pick. Now, third, change what you're measuring. Rather than stepping on the scale, look at your waist circumference or your circumference of other areas like your upper thigh or your upper arm or your chest. Also focus on what you can lift and how well you're moving, how well you recover, and maybe even some clinical markers, of course, like your fasted insulin levels or even your fasted lactate as a sign of your mitochondrial fitness or function, if you can get it. Body weight may not move enough on the scale to keep you motivated, and it's not necessarily measuring what you think it is anyway. So the case for exercise is very strong. Again, some of the benefits: you're clearing glucose more easily, you're making your body more insulin sensitive, you're increasing your— activating your mitochondrial biogenesis, visceral fat goes down, strength and fitness, which predicts survival, are going— are improving as well. Now, much to our chagrin and our collective confusion, perhaps, none of this is going to show up in the number that you're seeing on your scale when you step on it each morning. To close this, maybe the best way to sum up this mini lecture is this: exercise to be strong, eat smart to be lean. Class dismissed. Until next time, more knowledge, better health.