Where does our fat go when we exercise? - CrowdScience, BBC World Service

change, a thickening of the mid-center and just this general slowing down, so I don't want that to happen to me, so I work very diligently and very hard to keep a really low BMI and keep my energy up. Lili is right. People often seem to grow older as they age, but why? The usual response is "Oh, well, my metabolism is slowing down," but is that true? Lucky for us, someone did some research and discovered an answer that may surprise you. Children burn a lot more calories than we would expect just from their body size and we think that's because of all the work their body

does

to develop, their organs and their immune system and all this other developmental work they have to do. do and then of course in old age

when

we get over 60 our metabolism starts to slow down.

This is Herman Pontzer. I am a professor of evolutionary anthropology and global health at Duke University. And Herman discovered that, aside from these big changes that he just described, actually for most of our adult lives our metabolism remains fairly stable. The stress that builds up as we get older is knowing that you get a job, you have kids, and you have all these things on your plate that can also make you feel different. I think we tended to blame all those changes we call metabolism. When we look at actual measurements of calories burned per day, it's actually not metabolism, but all these other things, so chances are it's those other changes that push us to eat a little more, for example, and to gain weight or feel different from what we do. we did it

when

we were younger.

So what can people do to combat obesity? Lili spends hours in the gym. That must be helping, right? Well, Herman also investigated this question and, again, the results were totally unexpected. I had the opportunity to work with a community called Hadza. They live in northern Tanzania and have a very physically active hunter-gatherer lifestyle. They walk miles and miles every day to get wild game or plant foods from their landscape, so it was a good opportunity for us to ask how many calories your body needs to burn each day to maintain that lifestyle and we were hoping it would be a very good one. , because again they are very physically active and we are usually told that the more active you are, the more calories you burn each day, but to our surprise, what we found was that they burn the same amount of calories every day as men and women in America , Europe and other industrialized countries.

In other words, although they get more physical activity in a day than most Americans do in a week, there is no difference in how their bodies burn calories. We know they walk a lot, so they must burn a lot of energy walking. They are not more efficient for walking or anything like that. Instead, their bodies adapt to other places during the course of the day. Therefore, they are saving energy on non-activity tasks, allowing them to keep their total calories burned the same as you and me, even though they are much more physically active. These findings are very contradictory.

How can someone who runs four times a week burn the same amount of total calories as someone who lives a more leisurely lifestyle? I think it's fair to say that researchers in this field still don't have a full explanation for what's happening here, but Herman thinks there are some clues, and they have to do with how the body adjusts the energy it spends on other things. processes. We have some evidence that it saves energy on things like immune function. So, for example, if you are very physically active, if you

exercise

regularly, you will have lower levels of inflammation.

Inflammation is simply the type of immune system that works harder than it needs to over time, and inflammation is also bad for you, so when you

exercise

your body reduces the amount of inflammation it produces. People who exercise also have a lower stress response, producing less adrenaline and cortisol over the course of the day than people who do not exercise. Studies are being done in this area, but so far no one has provided conclusive answers. Still, even though it's the opposite of what we might have expected, think about it another way and it makes sense. Many things about the human body have to do with maintaining a balance from body temperature to hormone levels, so why wouldn't it also find a way to compensate and keep our metabolism constant over time?

Herman's studies look at population data and it appears that when body weight is taken into account, people's metabolisms are quite similar, yet there is some variation and we all know those lucky people who seem to eat what they want and stay healthy. thin People who simply say they have a very high metabolism. I asked Herman what his data says about this. As far as we can tell, the feeling of whether your metabolism is fast or slow has more to do with your relationship with food and how food causes your brain reward system to turn on and how it makes you feel and whether you find it difficult or not. eat the right amount or you may find it easier to overeat.

That seems to be what people really mean when they talk about their metabolism and how fast or slow it is because unless they've measured it, you don't really know if you have a fast or slow metabolism. So, according to Herman, people who naturally burn more energy are no more or less likely to gain weight than the rest of us, and all of this has big implications for the way we think about exercise, diet, and weight loss. If you want to control obesity, you must focus on diet. The foods we eat are the most important lever we have to try to change and regulate our weight.

Obesity is primarily a diet problem. It's not so much an exercise problem. Now, the other thing we can say is that exercise is really good for us. It's not a great way to lose weight, but it's important for other aspects of your health. If exercise

does

n't change your metabolism and you don't really feel like going on a diet, you may be wondering what else you can do to keep your fat stores at a healthy level. Well, believe it or not, the answer may lie in having more fat, but it's a different type of fat than what we've looked at so far and it's pretty special.

We have white fat, which most people think of when they think of fat, and we have developed or evolved this tissue as a way to store excess calories. In addition to white fat, which stores energy, mammals also have another type of fat called brown fat and interestingly, instead of storing energy, this type of fat can dissipate energy or convert it into heat and we now know that brown fat does not It not only generates heat but can also help protect against weight gain and many of the complications associated with obesity. This is Paul Cohen. I am an associate professor at Rockefeller University in New York.

He studies the other type of fat: brown fat. Now, these cells function in a totally different way than white adipocytes: they produce heat. So in a way, they're a little more like the ghee candle we lit at the beginning of the show. When we're born, we have a lot of brown fat (those tender little rolls on babies' backs and necks that make them so cute and help keep them warm when they're small and hairless), but until recently scientists thought that adults barely had any. brown fat. It turns out that we actually do, although only in very small quantities.

The exact amount of brown fat humans have is not actually known, but it can be measured in grams, while white fat can easily make up more than a third or more of a person's total body weight. My group looked at a cohort of more than 50,000 people who had undergone the type of scan that can detect brown fat in humans, and by looking at a patient's brown fat status and then linking it to all the information in their records electronic doctors, we were able to determine that brown fat is associated with a lower likelihood of suffering from a wide variety of chronic diseases, including type 2 diabetes, hypertension, lipid disorders and coronary artery disease.

So, ironically, having more brown fat could also protect against obesity. Paul says we don't really understand how brown fat has these effects, but there is some evidence that increasing the amount of brown fat we have in adulthood or increasing its activity could influence our metabolism. So how could we do this? One way to produce more brown fat would be to identify and study the precursor cells that give rise to brown fat cells and see if we can increase the number of these precursor cells or do something so that more of these precursor cells become fat cells. brown .

However, another possibility would be to work with the existing brown fat that we have and try to develop interventions or pharmacological approaches that increase the activity of the brown fat. We already know that a very effective way to increase the activity of brown fat is exposure to cold; However, most people of course don't like the cold very much, so I don't know if it will be a lasting solution, but if we understand more about the biology, then there is a chance we can recapitulate some of the same effects of cold exposure with medications or other interventions without people exposing themselves to the cold.

Obviously I'm biased, but I think there's a lot to learn from studying how fat works, both white and brown fat, and I think in the end it's really a window into human metabolism with really strong and obvious links to a whole variety of diseases. serious. diseases. For something that gets such a bad rap when we normally talk about it, fat actually does a lot more than most of us ever thought. It protects us from disease, tells our brain when we are hungry, generates heat and, of course, stores energy; However, in many ways, fat remains a mystery and researchers are still figuring out how it does all the things it does.

While we know that having too much fat is bad for our health, it also seems like our ideas about how to lose weight have been totally off base for decades. Who could have thought that something as seemingly mundane as fat could be so full of surprises? We have come to the end of this edition of CrowdScience from the BBC World Service. Today's question came from me, Lili Clever, in California. The show is presented by Anand Jagatia and produced by Marijke Peters. If you would like the team to investigate an idea, you can email them at

crowdscience

@bbc.co.uk.

Thanks for listening.