Last year I wrote of a meta-analysis on energy expenditure in industrialized countries vs developing countries, that counter-intuitively found that it was on average not different. Today a study by Herman Pontzer and colleagues published in PLOS ONE adds to this debate on whether food or being sedentary is primarily fueling societal obesity, by comparing a Western population with a modern hunter-gatherer population, the Hadza. And again the results are unexpected.
Here is a summary of the study with the main findings:
Total daily energy expenditure was measured over 11 days in 30 Hazda adults with the gold standard doubly labeled water method. Physical activity level was calculated from total energy expenditure and an estimated basal metabolic rate using measured weight and height and equations developed from previous research. Resting metabolic rate was measured with a portable system on 19 subjects, and walking cost was measured on 14 subjects, using GPS to measure distances.
Here is a description of the Hadza from the paper:
While no living population is a perfect model of our species’ past, the Hadza lifestyle is similar in critical ways to those of our Pleistocene ancestors. The Hadza hunt and gather on foot with bows, small axes, and digging sticks, without the aid of modern tools or equipment (e.g., no vehicles or guns). As in many other forager societies , there is a sexual division of foraging effort; Hadza men hunt game and gather honey, while Hadza women gather plant foods. Men’s forays are typically longer than women’s, as reflected in their mean daily travel distances (see below). Women typically forage in groups, while men tend to hunt alone . As is typical among traditional-living Hadza, over 95% of their calories during this study came from wild foods, including tubers, berries, small- and large-game, baobab fruit, and honey .
The collected energy expenditure measurements in the Hadza were compared to previous research in various populations using doubly labeled water and new measurements taken in 68 free-living participants enrolled in other studies over a 2-week period. Population and individual comparisons were classified by “hunter-gatherer” (the Hadza is the first population that can be classified as such that has doubly labeled water measures), “Western” (Europe or US), “market economy” (Westerners & non living in market economies), or “farming”.
Body fat in the Hadza population was lower than a Western sample, as expected. However, total energy expenditure (TEE) were not significantly different than Westerners after adjusting for fat free mass and age. Analyzing variation in TEE among individuals, the Hadza women were similar to Western women (p=0.67), Hadza men similar to Western men (p=0.68). There were similar results when the Hadza were compared to the all “market economy” population, and also when male and female data was combined. Importantly, the variation in body fat within the Hadza and between populations was not correlated to TEE or physical activity level (p=0.08 and 0.55 respectively).
Analyzing variation in TEE among population means, Hadza compared to the market economy population was not different (p=0.73), but compared to previous data collected from farming populations, the Hadza had a lower TEE.
Estimated physical activity levels (PAL) were higher in Hadza men and women compared to Western men and women (p=0.001 and 0.05 respectively) when adjusted for age. Hadza men walked much farther per day (11.4 +/- 2.1 km/day vs 5.8 +/- 1.8 km/day) which is consistent with other hunter-gatherer research, according to the authors.
The energy cost of walking was not different between Hadza and Western subjects, which was included to check if muscle efficiency was different between them.
So this is the first measurement of energy expenditure in a hunter-gatherer population and it isn’t different than Western measurements, which adds further challenge to the hypothesis that low activity is a cause of obesity in Westernized countries. However, as the authors note, evidence is mixed on physical activity and weight gain prevention, so perhaps the increase in physical activity level in the Hadza despite not burning more total calories than Westerners might help to prevent at least some weight gain. Physical activity level also of course could influence disease risk independently of weight.
A caveat though: because there isn’t more data on other hunter-gatherer populations, we can’t be positive that the Hadza are representative of energy expenditure in such lifestyles.
The concluding passages in the paper are particularly interesting to consider:
We hypothesize that TEE may be a relatively stable, constrained physiological trait for the human species, more a product of our common genetic inheritance than our diverse lifestyles. A growing body of work on mammalian metabolism is revealing that species’ metabolic rates reflect their evolutionary history, as TEE responds over evolutionary time to ecological pressures such as food availability and predation risk [49,50]. In this light, it is interesting to consider human TEE as an evolved trait shaped by natural selection. Humans are known to have greater TEE than orangutans , a closely related ape, but have low TEE compared to other eutherian mammals [50,51]. Data from other primate species are needed to fit the human metabolic strategy into a comprehensive evolutionary context.