When children gain excess weight, the culprit is more likely to be eating too much than moving too little, according to a new study of children in Ecuador. The study compared the lifestyles, diets and body compositions of Amazonian children who live in rural, foraging communities with those of other indigenous children living in nearby towns. The results have implications for the rising rates of obesity in both children and adults worldwide.

The in-depth study found that the rural children, who run, play and forage for hours, are leaner and more active than their urban counterparts. But they do not burn more calories, a finding that implicates the urban children’s modernised diets. The findings also raise provocative questions about the interplay of physical activity and metabolism and why exercise helps so little with weight loss, not only in children but us, too.

Childhood obesity is of pressing global interest, since the incidence keeps rising. Researchers point to increasing childhood inactivity and junk food as drivers of youthful weight gain. But which of those might be more important remains murky and matters, as obesity researchers point out, because we cannot effectively respond to a health crisis unless we know its causes.

That question drew the interest of Sam Urlacher, an assistant professor of anthropology at Baylor University in Waco, US, who for some time has been studying the Shuar people. An indigenous population in Amazonian Ecuador, the traditional Shuar live primarily by foraging, hunting, fishing and subsistence farming. Their days are physically demanding, their diets heavy on bananas, plantains and similar starches, and their bodies slight. The Shuar, especially the children, are rarely overweight but are also not often malnourished.

Were their wiry frames a result of their active lives, Urlacher wondered? As a postgraduate student, he had worked with Herman Pontzer, an associate professor of evolutionary anthropology, whose research focusses on how evolution may have shaped our metabolisms and vice versa.

In Pontzer’s pioneering research with the Hadza, a tribe of hunter-gatherers in Tanzania, he found that, although they moved frequently during the day, hunting, digging, dragging, carrying and cooking, they burned about the same number of calories daily as much-more-sedentary Westerners. Pontzer concluded that, during evolution, we humans must have developed an innate, unconscious ability to reallocate our body’s energy usage. The result is that our average, daily energy expenditure remains within a narrow band of total calories, helpful for avoiding starvation among hunter-gatherers but disheartening for those of us in the modern world who find that more exercise does not equate to much, if any, weight loss.

Urlacher wondered if similar metabolic trade-offs might exist in children too, including among the Shuar. So, for a 2019 study, he precisely measured energy expenditure in some young Shuar and compared the total number of calories they incinerated with existing data about the daily calories burned by relatively sedentary (and much heavier) children in the US and Britain. Although the young Shuar were far more active, they did not burn more calories, over all.

Young Shuar differ from most Western children in so many ways that interpreting that study’s findings was challenging. But Urlacher was aware of a more-comparable group of children — among Shuar families that had moved to a nearby market town. Their children regularly attended school and ate purchased foods.

For the newest study, he and his colleagues measured the body compositions and energy expenditure of 77 Shuar children aged 4-12, both rural and relatively urban, while also tracking their activities with accelerometers and what they ate.

The urban Shuar children proved to be heavier than their rural counterparts. About a third of them were overweight by World Health Organization criteria. They were also more sedentary. But all the children, rural or urban, active or not, burned about the same number of calories every day. What differed most were their diets. The children in the town ate far more meat and dairy products than the rural children, along with new starches, such as white rice, and highly processed foods, such as candy. In general, they ate more. This diet, Urlacher and his colleagues conclude, contributed most to their higher weight.

“Exercise is still very important for children,” Urlacher says. “But keeping physical activity up may not be enough to deal with obesity.”

NYTNS