Managing the weight and diet of individuals is one of the essential points of any public health policy, but also of personal well-being. Indeed, the foods we eat and their impact on our body mass are important, because all this acts directly on our morphology and our physiological processes. Obesity and malnutrition are among the great scourges of the 21st century, and while science is beginning to better understand the impact of our environment and our daily activity on our organism, certain myths related to human metabolism still exist.
Obesity is a sign of personal failure. A slow metabolism condemns irreparably to weight gain. Intense and continuous physical exercise helps to lose weight. We have all heard these claims before, and yet they all have one thing in common:they are wrong. These myths about metabolism still persist, and we offer to clear up some of them.
It's the core belief of almost every workout featured in magazines or on the internet:exercise more to burn more calories. In the short term, that's okay — you burn energy while you exercise, and if you start a new workout routine, you'll burn more calories, at least initially. But recent studies show how dynamic and adaptive our metabolisms can be.
In 2010, researchers studied exactly how many calories our primitive ancestors, who were hunter-gatherers, were likely to have burned each day. They spent weeks with the Hadza people in northern Tanzania, conducting the first study to measure calories burned over the course of a day in a modern hunting and gathering community.
Subsisting on wild plants and game, without guns, machines or pets is a physically demanding way of life. Hadza men log 19,000 steps each day hunting and gathering wild honey, while women log 12,000 steps collecting wild tubers and berries, often with a child on their back in a sling.
Yet despite getting about five times more physical activity each day than the average Western lifestyle, researchers found that Hadza men and women burned the same number of calories daily as sedentary office workers. in industrialized populations. It's not just the Hadza:farmers and gatherers in other small-scale societies, with equally high daily workloads, have the same daily expenses as people in high-income countries.
It seems that our body works to keep the daily number of calories burned within a narrow range, regardless of our lifestyle. Workouts are subject to the same metabolic adjustment. Measured daily expenditures for study participants increase steadily at the start of a new training regimen, but these gains decline over time. Their bodies adapt, so within a few months the daily energy they burn is only slightly higher, and sometimes exactly the same, than before they started training.
Even those who manage to increase the amount of energy they burn through exercise still struggle to lose weight. A recent review of 61 exercise studies, with more than 900 participants, presents evidence that will be familiar to many. Weight loss often starts out great at the start of a new exercise regimen, but fades over time, so that a year or so later the weight lost is a fraction of what we would all expect. the calories burned by training.
In a long follow-up study, men and women in the United States burned 2000 calories per week during supervised exercise sessions for 16 months. After nine months, the men had lost around 5 kg, after which their weight leveled off. The women in the study lost no weight for the entire 16 months. Neither men nor women lost what the researchers expected based on their exercise load, despite the fact that their daily energy expenditure increased slightly.
The reason is extremely simple:when you burn more calories, you consume more. You may not intend to, of course, but that is the problem. The complex physiological systems that work subconsciously to regulate your hunger and satiety do an exceptional job of matching energy intake to expenditure. As a result, the amount of weight you can expect to lose with exercise alone over the course of a year is 2 kg or less.
Not losing weight? Do not abandon ! Exercise may not change your body mass, but that's not what it's for. Humans evolved as hunter-gatherers, and a high dose of physical activity has been a regular part of daily routine for over 2 million years. Our bodies are built to move, and there are good reasons the Hadza avoid heart disease and diabetes, despite burning the same amount of calories as sedentary people. Regular exercise keeps our hearts healthy, our muscles strong and our minds sharp, especially as we age.
Curiously, recent studies suggest that metabolic adjustments that hinder weight loss are a major reason why exercise is so good for us. It seems that our body reacts to increased daily activity by reducing the energy spent on other tasks. For example, the immune system calms down, which reduces inflammation, which is important because we know that inflammation is a serious risk factor for cardiovascular disease and many other health problems.
People who exercise regularly also respond to stressful events with smaller surges of the stress hormones cortisol and adrenaline, reducing their risk of stress-related illness. Even reproductive hormones seem to be produced more sensibly. Comparisons of estrogen and progesterone in women and testosterone in men generally show reduced levels in adults in physically active populations.
These reductions do not appear to impair fertility, but they have been linked to a lower risk of reproductive cancers such as prostate and ovarian cancer, as well as breast cancer. Exercise seems to hone in on all the unseen tasks our bodies do throughout the day, helping to protect us from heart disease, diabetes, and cancer.
Like most other biological traits, the amount of energy burned in a day varies from person to person. The daily energy expenditure of two people of the same age and gender with the same lifestyle can easily differ by 500 calories or more. Surprisingly, this variation in energy consumption does not predict a person's weight. Obese people have on average the same daily energy expenditure as thin people. Body size is an important factor because a larger body tends to burn more calories per day simply because it has more active cells.
If size is not taken into account, obese people burn more energy. Weight gain and obesity are not the products of a slow metabolism. So why do some people find it easy to stay in shape while others struggle? Although there is probably no single answer, a major factor seems to be how our brains are wired. For the most part, weight gain happens slowly over months and years, reflecting tiny errors in the regulation of energy intake.
The vast array of processed and manufactured foods available to us overwhelms the neural reward systems that have evolved to handle raw, unprocessed foods. Our brains err on the side of overconsumption. Support for this view has come from recent work on the physiology of hunger and satiety, as well as advances in the genetics of obesity. Of the hundreds of genes associated with obesity in humans, the vast majority are most active in the brain. The variants you wear may affect your ability to control your weight.
The battle against obesity is often framed as a test of willpower, pitting the virtues of exercise and food control against the vices of gluttony and laziness. Metabolic science says otherwise. Stores are full of ultra-processed foods, loaded with added sugars and oils. Recent work from the National Institutes of Health from the United States have shown that eating ultra-processed foods leads to weight gain, although we don't yet know exactly why.
These foods are on the rise worldwide. In the United States and the United Kingdom, they represent more than half of the food consumed. In wealthy countries, ultra-processed options often dominate the foods available in low-income neighborhoods and those where the majority of residents are from minority groups, contributing to health and nutrition inequities. In low- and middle-income countries, the growing reliance on ultra-processed foods has helped fuel the global rise in obesity and related diseases.
These diseases, including heart disease, stroke, diabetes and other non-communicable diseases, kill more people worldwide than infectious diseases. This shows why obesity has become a crisis that disproportionately affects economically disadvantaged people, including people of color.
Recent scientific breakthroughs are a call to action. Obesity is not a choice, but that doesn't mean our choices don't matter. We can start by removing ultra-processed foods from homes. We do not need to wait for societal changes in our food environment to act in our daily lives. And we must learn from the Hadza and others to incorporate physical activity into our daily routines. Exercise doesn't necessarily make us thin, but it keeps us alive.
