午夜福利1000集合

THE universe of good reasons for putting a live guinea pig in an insulated metal pot is small. I can think of only one: in France, in the winter of 1782, the chemist Antoine Lavoisier and his polymath friend Pierre-Simon Laplace placed their unwitting subject into a double-walled metal chamber, the world鈥檚 first calorimeter, and sealed the lid. They had packed snow into the space between the walls, and by comparing the rate at which the guinea pig鈥檚 body heat melted the snow to the rate of carbon dioxide it exhaled, they discovered metabolism 鈥� the 鈥渇ire of life鈥� that drives our very existence. At last, science had a physical measure of the life force that enables us to grow, reproduce and move. Physiologists like myself have been counting calories ever since.

Today, a widespread obsession with fitness and body weight has led to a new era of calorie counting. Diet books and magazine workouts promise a kind of shiny metabolic nirvana of calories burned, villainous foods avoided, waistlines melted and health and vitality restored. The reasons they fail 鈥� and they almost always do 鈥� are as varied as the schemes themselves, but the common theme is a fundamental misunderstanding of metabolism. Yes, diet and exercise are critically important for our health, but they don鈥檛 work in the ways we are usually taught. Our bodies aren鈥檛 simple calorie-burning engines that we can easily manipulate to keep us looking trim and feeling good. They are complex and dynamic metabolic systems meticulously shaped by evolution for survival and reproduction.

My own metabolic research has taken me and my colleagues across the globe, , and . We have also explored the expenditures of our closest living relatives 鈥� chimpanzees, bonobos, gorillas and orangutans 鈥� taking the tools of metabolic science out of the lab and across the tree of life.

Recent studies from my lab as well as from others鈥� have reshaped our understanding of how our bodies burn calories, and how exercise and diet affect metabolism and health. In an era of obesity, diabetes and heart disease, societies struggling with these issues would be happier and healthier if we built these advances into our public health programmes and personal routines. We can start by recognising 鈥� and tossing aside 鈥� seven of the biggest metabolic myths that hold us back.

鈥淒iet and exercise simply don鈥檛 work in the ways we are usually taught鈥�

1. Exercise burns through calories and boosts metabolism

It is the bedrock belief of pretty much every workout routine featured in magazines: exercise more, burn more calories. In the short term, it is correct 鈥� you burn energy while you are exercising, and if you start a new workout routine, you will burn more calories, at least in the beginning. But recent studies show just how dynamic and adaptive our metabolisms can be.

In 2010, my colleagues and I decided to investigate exactly how many calories our evolutionary ancestors, who were hunter-gatherers, were likely to have burned each day. We spent weeks with the Hadza people in northern Tanzania, conducting the first study to measure the calories burned during a day in a modern hunting and gathering community.

As you might expect, subsisting on wild plants and game, with no guns, machines or domesticated animals, is a physically demanding way to live. Hadza men log 19,000 steps each day hunting, and gathering wild honey, while women log 12,000 steps collecting wild tubers and berries, . Yet despite doing than the average for Western lifestyles, we found that Hadza men and women burn the same number of daily calories as sedentary office workers in industrialised populations. It isn鈥檛 just the Hadza: farmers and foragers in other small-scale societies, with equally high daily workloads, have the same daily expenditures as people in high-income countries. It seems our bodies work to keep the daily number of calories burned within a narrow range, regardless of our lifestyle.

And your new workout routine? It will be subjected to the same metabolic adjustment. Daily expenditures measured for participants in exercise studies routinely increase at the beginning of a new workout regimen, but those gains . , so that within a few months, the daily energy they burn is only marginally higher, and sometimes exactly the same, as before they started working out. The boost is a bust.

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2. Exercise will make you lose weight

Even those who do manage to increase the amount of energy they burn through exercise typically still find it hard to lose weight. A of 61 exercise studies, totalling more than 900 participants, lays out the grim evidence that will be familiar to many.

Weight loss often starts off well at the beginning of a new exercise regime, but it fades over time, so that a year or so later, the weight lost is a vanishing fraction of what we would expect from all the calories burned through working out.

In one of the longer trials, men and women in the US burned 2000 calories per week during supervised exercise sessions for 16 months. After nine months, the men had lost around 5 kilograms, after which their weight plateaued. Women in the study lost no weight over the entire 16 months. based on their exercise workload, despite the fact that their daily energy expenditures had edged up slightly.

The reason for this is frustratingly simple: when you burn more calories, you eat more calories. You might not mean to, of course, but that is the problem. The complex systems working subconsciously to regulate your hunger and satiety do an exceptional job of matching energy intake to expenditure. What else would we expect from half a billion years of evolutionary tuning, where losing weight was generally a sign of impending doom? As a result, the amount of weight you can expect to lose from exercise alone over the course of a year is or less.

鈥淓xercise seems to fine-tune the unseen tasks our bodies do all day鈥�

The misunderstood science of metabolism
Join Herman Pontzer at our online event on Thursday 25 February, or catch up on demand:newscientist.com/events

3. Your workout programme isn鈥檛 succeeding unless you are losing weight

Not losing weight? Don鈥檛 give up! Exercise might not change the number on your bathroom scales, but that isn鈥檛 what it is for. Humans evolved as hunter-gatherers, and a heavy dose of physical activity was an inescapable part of the daily routine for more than 2 million years.

Our bodies are built to move, and there are good reasons why the Hadza avoid heart disease and diabetes, despite the fact that they burn 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.

Intriguingly, recent studies suggest that the metabolic adjustments that frustrate weight loss are a big reason why exercise is so good for us. My lab and others are working to track down the precise nature of these changes, but it seems our bodies respond to increased daily activity by reducing the energy expended on other tasks. For example, , reducing inflammation, which is important because we know that inflammation is a serious risk factor for cardiovascular disease and a range of other health problems.

People who exercise regularly also respond to stressful events with , reducing their risk of stress-related disease. Even reproductive hormones seem to be produced more judiciously. Comparisons of oestrogen and progesterone in women and testosterone in men . These reductions don鈥檛 appear to harm fertility, but they have been to a lower risk of such as prostate and ovarian cancer, as well as breast cancer. Exercise seems to fine-tune all the unseen tasks our bodies do throughout the day, .

4. Calories don鈥檛 matter

Gaining weight is fundamentally a physics problem: when we eat more calories than we burn, those extra calories pile up as fat. Since it is futile trying to boost the energy we burn each day with exercise (or superfoods, or ice water, or the latest gimmick), the primary cause of being overweight or obese is clearly diet. We gain weight because we eat too much.

Yet counting calories has become pass茅. We are told it is the types of foods we eat, or the way that we eat them, that get us into trouble, and that the 鈥�calories in, calories out鈥� view of the world is for suckers. Low-carb evangelists tout ketogenic diets 鈥� which rely heavily on fat, rather than carbohydrates 鈥� as a way to lose weight without cutting calories (some even claim you can eat more). Intermittent fasting fanatics promise much the same.

These and other weight-loss regimes du jour get a few things right. First, . When people claim to consume around 2000 calories per day, the real number, from thousands of adults living their normal routines, is closer to 2400 calories a day for women and 3100 for men, on average.

With all the metabolic sleight of hand our bodies perform, tracking calories in and out can feel hopeless. However, just because we are bad accountants doesn鈥檛 mean that calories are a meaningless currency. All weight-loss diets work by reducing calorie consumption, the concept is simply hidden behind different guises.

Which brings us to the other thing these diverse diets get right: we are an adaptable species, able to thrive on a broad range of diets from carnivore to vegan. Meat-based diets work wonders for many, but so do plant-based ones and everything in between. The science over the past couple of decades is clear that any diet can help you lose weight if you stick to it, and there is no single one that is easier to adopt. A 2005 study in the US randomly assigned 160 adults with obesity to four different diets, and found in the ease with which people adhered to their assigned diet, nor in the weight loss and health benefits obtained.

If you are attempting to lose weight, the trick is to find a diet that you can maintain without feeling miserable. Foods high in protein and fibre . It also helps to avoid crash diets that can cause our clever, evolved metabolisms to hit the brakes and reduce daily energy expenditure.

For anyone who has found a diet that works for them, stick with it. But don鈥檛 expect it to work for everyone. There is no singular, naturally perfect human diet, which brings us to the next metabolic myth.

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5. Humans evolved to eat a Paleo diet

Striving to eat the kinds of foods our ancestors ate makes intuitive sense. But emulating ancient diets, the idea behind the fashionable Paleo or 鈥渃aveman鈥� diet movement, in which people eat similarly to how our ancestors did in the Palaeolithic era, isn鈥檛 as straightforward as it might appear.

Cast your mind across the dizzying array of cultures on this planet, and consider the staggering variety of foods we eat. Clearly, there is no single human diet today, and it would be laughable to claim otherwise.

Yet when we consider our Palaeolithic past, somehow it has become reasonable to suggest that cultures around the globe, over millennia, ate a single, uniform, 鈥渘atural鈥� diet. Even stranger to those of us in the fields of human ecology and anthropology, some popular Paleo-style diets, for instance the so-called carnivore diet, suppose that ancestral diets were heavy on the meat, with only a few grams of carbs each day and essentially no sugar.

My team鈥檚 work with the Hadza community, along with ethnographic accounts gathered over the past century of other hunter-gatherer groups and much older evidence from the fossil and archaeological records, paints a very different picture.

First, hunter-gatherer diets are (and were) just as diverse as diets in industrialised populations, with lots of variety among groups and through time in the proportions of meat and plants, fat and carbs. Some diets, particularly those of Indigenous people in the Arctic, are meat-heavy; others, especially in warmer climates, are plant-heavy. The Hadza eat a balance of plants and game, as well as a huge amount of .

Second, outside the Arctic, there is no evidence for meat-heavy diets among hunter-gatherer groups today or in historical records. Even in the Palaeolithic, we see plenty of archaeological and fossil evidence for a balance of plants and meat in the diet.

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6. A slow metabolism dooms you to obesity

Like most other biological traits, the amount of energy burned in a day varies from person to person. Daily energy expenditure in two people who are the same age and sex, and have the same lifestyle, can easily differ by 500 calories or more. Surprisingly, that variation in energy use doesn鈥檛 predict someone鈥檚 weight. People with obesity have the same daily energy expenditure, on average, as those who are slim. That鈥檚 after accounting for body size, since a larger body tends to burn more calories per day simply by virtue of having more cells at work. If we don鈥檛 correct for size, people with obesity . Weight gain and obesity aren鈥檛 products of a slow metabolism.

So why do some people find it easy to stay trim while others struggle? Although there is probably no single answer, a major factor seems to be the way our brains are wired. For most, weight gain comes on slowly over months and years, reflecting tiny errors in the regulation of energy intake. The vast array of processed and engineered foods available to us overwhelms neural reward systems evolved to handle unprocessed wild 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 carry are .

7. Obesity and weight gain are a sign of personal failure

As powerful as our genes are, DNA isn鈥檛 destiny. Today鈥檚 gene pool is essentially unchanged from that of our great grandparents鈥� generation because genetic change is slow. They didn鈥檛 face a global obesity crisis. What鈥檚 different in much of the world is our environment, specifically our food environment 鈥� the access we have to specific foods. In engineering our industrialised world, we have surrounded ourselves with foods that drive us to overconsume. The battle with obesity is often framed as a test of willpower, pitting the virtues of exercise and portion control against the vices of gluttony and sloth. New metabolic science says otherwise. Shops are stocked with ultra-processed foods, laden with added sugars and oils, symphonies of sweet and savoury that overwhelm our Palaeolithic brains.

Recent work at the US National Institutes of 午夜福利1000集合 has shown that , although we don鈥檛 yet know precisely why. These foods are on the rise worldwide. In the US and the UK, . In wealthy countries, ultra-processed options often dominate the foods available in low-income neighbourhoods and those where the majority of inhabitants are from minority groups, contributing to inequities in health and nutrition. In low and middle-income countries, the growing dependence on ultra-processed foods has helped to . Those maladies, including heart disease, stroke and diabetes, and other non-communicable illness, kill more people globally than infectious disease. This shows why obesity has grown into a crisis that disproportionately affects the economically disadvantaged, including people of colour.

Recent breakthroughs in metabolic science are a call to action. Obesity isn鈥檛 a choice, but that doesn鈥檛 mean our choices don鈥檛 matter. We can start by getting ultra-processed foods out of homes. We don鈥檛 need to wait for societal changes in our food environment to take action in our daily lives. And we need to learn from the Hadza and others to weave physical activity into our daily routines. Exercise won鈥檛 make us thin, but it will keep us alive.

From guinea pigs in metal pots to detailed studies of obesity genes, the science of metabolism has advanced over the past two centuries with new approaches and new technologies. Yet some of the biggest advances in recent years have come from societies like the Hadza, modern communities that hold on to ancient ways and provide a window into our collective past. Our bodies were shaped by evolution to be clever, adaptable and dynamic. We will need that same flexible creativity to tackle the obesity crisis.

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