A new study comparing the lifestyle of Westerners with that of hunter-gatherers challenges the idea that the current obesity crisis is due to lack of physical activity. The researchers suggest the more likely explanation is over-consumption of calories, particularly due to the presence of energy-dense foods in the Western diet.
The researchers write about their findings in a paper published online in PLoS ONE on 25 July.
They describe how they compared the daily energy expenditure of the Hadza, a population of traditional hunter-gatherers living in the open savannah of northern Tanzania, with that of Westerners.
The study is the first to use direct measurements of energy use in hunter-gatherers instead of estimates.
The researchers found, despite the fact the Hadza spend their days trekking long distances to forage for wild plants and hunt game for food, as did all our ancestors until about 10,000 years ago, they burned no more calories per day than adult Americans and Europeans.
The team ran several analyses, taking into account the effects of body weight, body fat percentage, age, and gender, and found no difference in the daily energy use of the Hadza hunter-gatherers and Westerners.
The study overturns current theories that the rising obesity crisis is due to lower levels of energy expenditure from having lifestyles that are less physically active.
This means, therefore, that the more likely explanation for the rising levels of obesity in Western cultures, is driven by increased calorie intake from food consumption. The researchers draw particular attention to the increasing presence of energy-dense food in the Western diet.
However, in their discussion, the researchers admit to being puzzled by the results:
“The similarity in TEE [total daily energy expenditure] between the Hadza and Western populations is counterintuitive given the Hadza’s physically active lifestyle and elevated PAL [physical activity level].”
They conclude their results, together with those of “other investigations of forager physiology, suggest that interactions between metabolic physiology, physical activity, and the environment are more complex than often thought”.
Lead author Herman Pontzer, who runs the Human Evolution and Energetics lab at Hunter College in New York City, told the press the results highlight the complexity of human energy expenditure:
“It’s not simply a function of physical activity,” he says, explaining that:
“Our metabolic rates may be more a reflection of our shared evolutionary past than our diverse modern lifestyles.”
Pontzer and colleagues suggest even active, “traditional” lifestyles of people in developing regions may not protect them against obesity if they also start eating more calories.
“Thus, efforts to supplement diets of healthy populations in developing regions must avoid inundating these individuals with highly-processed, energy-dense but nutrient-poor foods,” they urge.
They call for more studies to test their suggestions, and also investigate other hunter-gatherer tribes, and they also note:
“Data from other primate species are needed to fit the human metabolic strategy into a comprehensive evolutionary context.”
And they urge people not to go away from this study with the wrong message: exercise and being physically active are still important for staying healthy, they say.
The study received financial support from the National Science Foundation, Washington University, and the University of Arizona.
Note: The article has been amended to remove a potentially confusing use of the term “metabolic rate”.
Written by Catharine Paddock PhD