The physical activity paradox

Mevasad led our group exactly with the hot hawk - Dave Reichlen from the University of Arizona, a 12-year-old Hadza boy named Neze, and I - outside the camp. Dave and I were pretty much useless in this operation. Mawasad invited us to join as a friendly gesture and to help carry the slaughtered animal back to camp if the search efforts were successful. As anthropologists who study ecology and human evolution we were delighted to have the opportunity to follow them, as the tracking skills of the Hadza people are legendary. It was certainly better than wasting a long day at camp with our scientific equipment.

After an hour of strenuous walking on a non-road, in a golden sea of ​​chest-high grass, dotted with flowering thorny acacia trees, we arrived at the part where the giraffe was injured. Navigating this in itself was quite complicated. It was as if someone had led you into the center of a 4000 dunam wheat field to show you where a toothpick had previously been dropped, and then he picked it up with aplomb. Since then, hour after hour, we have been following the wounded animal with the cruel sun beating down on our heads, always following faint signs.

And still no giraffe. At least I had water. Right at noon today we sat down to rest in the shade of some bushes. Mawasad tried to guess where the wounded creature could have gone. I had almost four liters of water left, which was enough, according to my calculations, to get me through the afternoon heat. Mawasad, as is the way of the Hadza people, did not bring water with him at all. When we got ready to start the search again, I offered Mosad water. He glanced at me, smiled and drank the entire contents of the bottle in one gulp. When he was done, he casually handed me the empty bottle.

It was karma. Dave, me andBrian Wood, an anthropologist at Yale University, spent the last month living with the Hadza people and conducted the first direct measurements of energy expenditure in a population of hunter-gatherers. We recruited about 25 women and men from the tribe, including from Wased, and asked them to drink water enriched with two rare isotopes, Deuterium וOxygen 18, from small and horribly expensive bottles. Analyzing the concentration of these isotopes in the urine samples of each participant will allow us to calculate the daily production rate of carbon dioxide in each body and accordingly the daily energy expenditure. This approach, known as the double labeled water method, is the most reliable in public health sciences for measuring calorie utilization in daily life. The method is direct, safe and accurate, but requires that each of the participants drink all the enriched water to the last drop. It cost us a lot of effort to make it clear to them that they are not allowed to spit and that they must finish the entire dish in its entirety. It seems that Mosad got the message right.

However, regardless of Mawassed's sly humor, my colleague and I learned a lot in our work with the Hadza people about the burning of calories in the human body. By combining findings from researchers who examined other populations, we came to some surprising insights about the metabolism in the human body. Our data indicates that, contrary to popular belief, humans tend to use the same number of calories regardless of their level of physical activity. And yet, we evolved during evolution to utilize much more calories than primates others close to us. These results help explain two seemingly separate but actually related puzzles: one, why exercise doesn't generally help with weight loss, and the other, how the uniquely human traits emerged.

The calorie economy

Researchers interested in human evolution and the quality of the environment often focus on energy expenditure because energy is essential to every biological process. You can learn a lot about any species from measuring its metabolism: life is essentially a trick designed to turn energy into offspring. Every feature of us living creatures is fine-tuned through natural selection to get the most evolutionary return on every calorie invested. Ideally, the population under study lives in the same environment in which the species originally evolved, with the same environmental pressures that shaped its biology still operating. Such conditions are difficult to achieve with humans because most people have become disconnected from the daily task of obtaining food in a wild landscape. In the last two million years or so, humans and their ancestors lived and evolved as hunter-gatherers. Agriculture only began about 10,000 years ago; Industrialized cities and modern technology only exist for a few generations. Populations like the Hadza, one of the last surviving hunter-gatherer populations in the world, serve as a key to understanding the evolution and function of our bodies before the appearance of cows, cars and computers.

Hadza life requires physical activity. Every morning the women leave the grass huts in the camp in small groups. Some of them carry swaddled babies on their backs and are searching for berries or other edible food. Wild tubers are a key component of the Hadza diet, and women can spend hours digging to get them out of the rocky ground using sticks. Men go miles every day hunting with their handmade bows and arrows. When hunting is not available, they use simple axes to cut tree branches, sometimes at the top, up to 12 meters high, to harvest wild honey. Even the children participate in the activity. They haul buckets of water back from the nearest water hole, which can be up to two kilometers or more from the camp. At dusk, the people make their way back to the camp, sit on the ground and chat around small cooking fires, share the day's experiences and watch over the children. And this is how the days pass in the dry and wet seasons for thousands of years.

But forget any romantic notions of paradise lost. Hunting and gathering are calculated and dangerous crafts, it is a high risk game where the currency is calories and failure means death. People like Mevasad use up hundreds of calories a day hunting and tracking, a gamble they hope will pay off in the form of venison. Fateful knowledge here is just like physical fitness. While other predators rely on their speed and strength to catch their prey, humans must calculate their steps, take into account the prey's behavior and scan the landscape for signs of prey. And still the Hadza men go hunting for big prey like giraffe only once a month. They would have starved if the Hadza women hadn't implemented a complementary and complex strategy, using their encyclopedic knowledge of local plant life, to bring home loot every day. Foraging in such a complex way, based on sharing, is what made humans extraordinarily successful and is at the core of what made us so unique.

Researchers of public health and human evolution have long assumed that our hunter-gatherer ancestors used more calories than people living in cities and towns today. It is impossible to imagine another situation, considering the amount of effort that people like the Hadza put in. Many in public health have gone so far as to claim that the global epidemic of obesity in the developed world stems from this decrease in daily energy expenditure, with all unused calories slowly accumulating as fat. One of our motivations for measuring Hadza's metabolic rate was to determine the extent of the decrease in energy and to see how much we Westerners lack in daily energy expenditure. When I returned to my home in the USA after a hot and dusty season in the field, I lovingly packed the vials of Hadza people's urine in dry ice and sent them to Baylor College of Medicine, one of the best laboratories in the USA for doubly marked water, and I imagined the tremendous amount of calories they would discover.

However, a funny thing happened on the way to the mass spectrometer that determines the ratio of isotopes. When the analysis of the results came back from Baylor, the Hadza people looked like anyone else. The men ate and used about 2,600 calories a day and the women about 1,900 calories a day, just like adults in the US and Europe. We examined the data in every way imaginable considering body size, body fat percentage, age and gender. We found no difference. how is it possible? What did we miss? What else did we get wrong in understanding human biology and evolution?

Lies that the accelerometers told me

It seems so obvious and self-evident to us that people who exercise use more calories that we have accepted this paradigm without question and without experimental evidence. But from the 80s and 90s of the 20th century, with the discovery of the double-labeled water method, the experimental data often challenged the popular belief in the fields of medicine and nutrition. The results of the Hadza, as strange as they may be, are not thunder on a clear day but rather the first cold drop of rain that fell on the neck after the rain clouds had accumulated for years without anyone noticing.

The earliest studies of doubly labeled water among traditional ranchers in Guatemala, The Gambia, and Bolivia showed that their energy expenditure was generally similar to that of urbanites. In a study published in 2008, she did Amy Locke, a public health researcher at Loyola University in Chicago, went one step further and compared the energy expenditure and physical activity of rural Nigerian women with the energy and activity of African-American women in Chicago. In her research, as in the case of the Hadza people, no difference was found between the populations, despite the great difference in their activity levels. Following this study, Laura Doggs, also from Loyola, along with Luke and others analyzed data from 98 studies conducted around the world and showed that people in developed countries, spoiled with all the modern comforts, expend the same amount of energy as people in less developed countries whose lives involve much higher activity.

Humans are not the only race where the rate of energy expenditure is constant. Following the research on the Hadza people, I led a joint experiment to measure daily energy expenditure among primates, the group of mammals that includes monkeys, apes, lemurs and humans. We found that captive primates living in laboratories or zoos use the same number of calories each day as primates in the wild, despite the obvious differences in physical activity. In 2013, Australian researchers selected sheep and kangaroos showed that their energy expenditure was similar, whether they were in cages or freely grazing in a meadow. And in 2015, a Chinese team reported on similar energy expenditure in pandas in zoos and in the wild.

To get a more detailed picture by comparing individual people instead of comparing population averages, I joined Luke and her staff, including Dougs, to examine activity and energy expenditure in a large, multi-year clinical study called DISH. More than 300 participants wore fitness bracelets 24 hours a day, with their daily energy expenditure measured using doubly labeled water. We found that the relationship between the physical activity, recorded in the bracelets, and the metabolism was very weak. On average, "armchair potatoes" tended to spend about 200 calories less per day than moderate exercisers: the type of people who do a few exercises a week and make sure to climb the stairs. More importantly, energy expenditure was constant at the higher activity levels: the most active people used the same number of calories as the moderately active people. The same phenomenon that caused a similar expenditure of energy by the Hadza people and other populations was also discovered among individuals in the study.

How does the body adapt to high levels of activity while maintaining constant energy expenditure? How can the Hadza people spend hundreds of calories in daily activity and yet use the same number of calories a day as people in the US and Europe who spend most of their time sitting? We still don't know for sure what the reason for this is, but the price of the activity in itself does not change: we know, for example, that elderly Dazah people use the same number of calories in walking one and a half kilometers as Westerners do. It's possible that highly active people change their behavior in sophisticated ways that save energy, such as sitting instead of standing and sleeping better. But an analysis of the METS data shows us that although these changes may contribute, they are not enough to explain the stability measured in the rate of daily energy expenditure.

Another interesting possibility is that the body makes room for the energetic cost of the increased physical activity by reducing the number of calories allocated to many tasks of the body, which are not visible to the eye but consume most of the daily budget: the maintenance work that the cells and organs do to keep us alive.

Saving energy in these processes can free up space in the daily energy allowance and allow us to spend more energy on physical activity without increasing the total daily calorie expenditure. For example, exercise often reduces inflammatory activity of the immune system, as well as levels of reproductive hormones, such as estrogen. In laboratory animals, increasing the level of activity does not affect the daily energy expenditure, but causes a decrease in the number of their ovulation cycles and slows down the tissue repair process. Extreme activity can lead animals to eat their mammalian offspring. Humans and other creatures seem to have developed several strategies to keep daily energy expenditure stable.

All these facts indicate that obesity is a disease that results from gluttony and not from laziness. People gain weight when the number of calories they eat exceeds the number of calories they expend. Since daily energy expenditure has not changed throughout human history, the main culprit in the modern obesity epidemic is the number of calories we consume. This should not be breaking news. An old saying in public health is that "you can't run faster than a bad diet," and experts know from personal experience and a lot of data that going to the gym to lose weight is frustratingly ineffective. But the new findings help explain why exercise is such a poor tool for weight loss. It's not that we didn't work hard enough. Our bodies conspired against us from the beginning.

But we still have to exercise. This article cannot be used as a note from mom to get you out of gym class. Exercise has a variety of well-documented benefits, from strengthening the heart and immune system to improving brain activity and healthier aging. In fact, I suspect that metabolic adaptation to activity is one of the reasons exercise keeps us healthy, by diverting energy from activities like inflammation that become destructive if they go on too long. For example, chronic inflammation is linked to diseases of the heart and blood vessels and autoimmune disorders.

The food we eat clearly affects our health, and physical activity combined with dietary changes can help shed extra pounds when reaching a healthy weight, but the facts show that it is best to see nutrition and exercise as different tools with different strengths. Exercise to stay healthy and vital and focus on nutrition to maintain your weight.

Energy budgets and evolution

Although the current science of metabolic adaptation is helping to clarify the relationship between exercise and obesity, researchers are still left with larger existential questions. If daily energy expenditure is constant, how could humans evolve and be so radically different from our great ape relatives. Nothing in life is given for free. Resources are limited, and investing more resources in one channel obviously means investing less in others. It is no coincidence that rabbits reproduce at an amazing rate, but die young; All the energy invested in offspring means less energy invested in body maintenance and longevity. Tyrannosaurus rex paid for its large head and many dangerous teeth and its powerful hind limbs at the price of its weak front arms. Even dinosaurs aren't perfect.

Humans have underestimated the basic evolutionary principles of austerity. Our brains are so big that as you sit and read this article, the oxygen from every fourth breath is only needed to feed the brain. And yet, humans have larger babies, reproduce more often, and exercise more than any of our ape relatives. The Hadza camps are full of happy, healthy, frolicking children and healthy women and men in their 60s and 70s. Our energetic extravagance presents an evolutionary conundrum. Humans are so similar to other great apes genetically and biologically that researchers have long assumed that our metabolisms are also similar. But if our rate of energy expenditure is maintained, as we have learned from the Hadza study and other studies, how can such a rigid metabolism process all the calories necessary to support the valuable features of man?

As a result of the extensive comparative research on energy expenditure in primates, my colleagues and I began to wonder if the set of energetically expensive traits that allowed humans to develop was the result of a large-scale evolutionary change in metabolic physiology. We found in the same study that primates use only half the amount of energy expended by other mammals. The reduced metabolic rate corresponds to the slow rate of their growth and reproduction. But it is possible that inversely the faster reproductive rate and other valuable traits in humans are related to the evolution of an increased metabolic rate. All that was needed to test this idea was to get a group of rabid chimpanzees, Bonobos Sly, lazy orangutans and mischievous gorillas who will carefully drink double-marked water without spitting and provide a few urine samples. In a show of scientific strength, my colleagues came out Steve Rose וMary Brown, both from the Lincoln Park Zoo in Chicago, to the mission and worked with veterinarians from more than a dozen zoos across the US. The test lasted about two years, and they accumulated enough data on energy expenditure in great apes to provide a solid comparison between them and humans.

As expected, humans use more calories per day than any of our great ape relatives. Even after taking into account the effects of body size, activity level and other factors, humans consume and expend about 400 more calories per day than chimpanzees and bonobos. The differences between humans and gorillas and orangutans are greater. These extra calories represent the extra work our bodies do. They support the big brain, the birth of more babies and the maintenance of the body for a longer period of life. Not only do we eat more than all other great apes (and we do eat more); And as we all know very well, the result of accumulating excess calories in a body that is not equipped with the tools to use them is obesity. Our bodies, down to the single cell level, have evolved to utilize energy faster and do more than our ape relatives. The evolution of humans is not entirely without trade-offs: our digestive system is smaller and less expensive than that of the other great apes who need large, energetically expensive intestines to digest the fiber in a plant-based diet. But the fateful changes that make us human are the result of an evolutionary change in our metabolism.

shared destinies

At some point in the afternoon, our course veered towards the camp and Wassad looked ahead instead of searching the ground. We arrived home without a giraffe. Herein lies the fundamental danger in one's energy-consuming strategy: returning home empty-handed is both more likely and more consequential. Many of the high-energy foods we need to fuel our faster metabolisms are scarce in nature by their very nature, thus increasing the cost of the energy needed to find food and increasing the risk of starvation for both the men and women who go out to find food and the children who stay in the camp.

To Mawasad's delight, the humans have developed some tricks to keep hunger away from them. We are the only species that has learned to cook, which increases the caloric value of many foods and makes them more digestible. Our control of fire turns inedible root vegetables into nutritious starch bombs. We also evolved to be fat. We know all this well from the obesity crisis in the West, but even Hadza adults, emaciated by any human standards, carry twice as much fat as idle chimpanzees in zoos. Our tendency to store fat, as problematic as it may be in the modern age, probably evolved simultaneously with the faster metabolism as an energy store that allows us to survive in difficult times.

As the sun set, heavy and orange, just above the trees, we returned to camp, Dave and I to our tents and Vasad and Neze to their family cabins, each of us glad to be home. Despite the lost giraffe, no one was hungry that evening. Instead, with utter modesty, the camp used our species' most sophisticated and powerful weapon to fight hunger: sharing. Sharing food is such a basic feature of the human experience, the thread connecting barbecues, birthdays and bar mitzvahs, that although we take it for granted, it is unique and a necessary part of our evolutionary heritage. Other apes do not share and share.

Apart from all the nutritional requirements and the constant preoccupation with obesity, perhaps the most profound effect of increasing energy expenditure is this human need to work together. The development of a fast metabolism binds our destinies together, and requires us to cooperate or die. As I sat with Dave and Brian and we described our days of adventure over tinned sardines and chips I realized I wouldn't want any other arrangement. No giraffe, no problems.