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Anthropological study

Desk workers use as much energy as hunter-gatherers

The Hadza tribe's daily energy expenditure questions our assumptions about exercise and weight. Photos: hadzafund.org/Brian Wood, flickr.com/Phillie Casablanca
African tribespeople use similar amount of energy per day as sedentary Westerners, despite much more active lifestyle
Findings suggest diet is more important than exercise for maintaining healthy weight
Study is first to directly measure energy expenditure of hunter-gatherers

The cause of the obesity epidemic, says conventional wisdom, isn’t complicated: we eat too much, of the wrong things, and we don’t do enough exercise afterwards. However, research by a group of anthropologists on an African hunter-gatherer tribe suggests that it might be even simpler than that.

An article, “Hunter-Gatherer Energetics and Human Obesity”, published in PLOS One, has challenged that the common view that the widespread adoption of machines, cars, and desks has led to abnormally low calorie expenditure in Westerners, and that this has been a primary cause of obesity in developed nations.

The study found that the Hadza tribe, who live a similar lifestyle to our Pleistocene ancestors, burn a similar amount of energy per day to sedentary Western workers, despite engaging in significantly more physical activity.

“There’s certain set points that humans have adapted to as a species, and we think that energy expenditure is one of those set points” Dr Herman Pontzer, study lead author

The finding led the study’s lead investigator, Dr Herman Pontzer, of Hunter College, New York, to hypothesise that energy expenditure may be a relatively stable, constrained physiological trait, influenced more by genetics than by an individual’s environment and lifestyle.

“There’s certain set points that humans have adapted to as a species, and we think that energy expenditure is one of those set points,” said Pontzer.

“Nobody would argue with the idea that unhealthy weight gain happens when energy intake exceeds energy expenditure. That’s physics. The question is, which side of this equation is more important? Our data suggest it’s food intake which is causing these big differences in levels of obesity.”

The paper is the first to directly study the energy expenditure of hunter-gatherer tribes. It challenges assumptions made by previous studies, which have attempted to estimate their energy expenditure by measuring their activity levels.

The findings came as a surprise to Pontzer and his team. “We kind of expected those estimates to be borne out,” he said. “We had no reason to think otherwise.”

Study

The investigators studied 13 men and 17 women aged 18-75 from the Tanzanian Hadza tribe.

The men of the tribe hunt game and gather honey, while women gather plant foods, using tools similar to tribes from the Pleistocene era. They live highly active lifestyles, the women walking on average 5.8 km/day and the men 11.4 km/day. Around a third of their diet consists of meat, with the rest made up of honey, berries, tubers, and vegetables. They eat no processed food.

Their energy expenditure was measured through the doubly-labelled water method, in which participants are given water marked with uncommon isotopes of hydrogen and oxygen. The amount of the marked water used in the creation of carbon dioxide during respiration can be calculated by measuring the concentration of the isotopes in urine samples.

This rate of respiration was compared with measurements taken from Western workers, as well as figures from previous studies investigating agricultural and market economies across the world.

As expected, the Hadza group were very lean, with body fat percentages on the low end of the ordinary healthy range among Western populations.

However, when they measured their total energy expenditure, they found that it was “statistically indistinguishable” from Westerners.

Performing multivariate comparisons of total energy expenditure controlling for fat free mass and age, the researchers found that Hadza women’s energy expenditure was similar to that of Western women and Hadza men’s total energy expenditure was similar to Western men; lifestyle had no effect on total energy expenditure.

Exercise

“Activity is probably still really important. But not for obesity.” Dr Pontzer

Pontzer emphasised that his findings did not diminish the importance of exercise to a healthy lifestyle. “These are snapshots of habitual energy use in two populations living a normal daily lifestyle,” he said. “After your body’s adapted to a certain lifestyle, you can throw a new exercise regime into that, and that might end up changing your energy expenditure at least for the short term until your body adjusts again.”

The Hadza tribe had minimal levels of heart disease, diabetes, and other cardiovascular diseases, leading Pontzer to hypothesise that physical activity might be a part of keeping illness at bay. “Activity is probably still really important,” he said. “But not for obesity.”

The Hadza

The team lived with the Hadza tribe for two months to perform their study, although some in the group have been working with the tribe for almost a decade.

“You can’t parachute in,” said Pontzer. “If someone came into your living room and said hey, would you mind giving me a urine sample and wear this GPS for the next couple of weeks, you’d tell them to go jump in a lake.”

In the future, Pontzer hopes to be able to put their work on energy expenditure and diet into a larger context, including measuring the Hadza’s health over their lifespan: the study population, he said, was too small to measure possible subtle effects of age on energy expenditure.

As well as providing research data, Pontzer also hopes that further study will be beneficial to the Hadza. He has set up a site, Hadza Fund, with his colleague Brian Wood, to promote responsible tourism in the Hadza homeland and raise awareness of their culture.

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