Eat smarter, don’t work out harder?

There have been several recent studies reemphasizing diet over exercise (timely because Americans are kind of fat, on average). A new piece in The New York Times looking at the Hadza of Tanzania, who are hunter-gatherers, seems to reiterate this point, Debunking the Hunter-Gatherer Workout:

We found that despite all this physical activity, the number of calories that the Hadza burned per day was indistinguishable from that of typical adults in Europe and the United States. We ran a number of statistical tests, accounting for body mass, lean body mass, age, sex and fat mass, and still found no difference in daily energy expenditure between the Hadza and their Western counterparts.

How can the Hadza be more active than we are without burning more calories? It’s not that their bodies are more efficient, allowing them to do more with less: separate measurements showed that the Hadza burn just as many calories while walking or resting as Westerners do.

We think that the Hadzas’ bodies have adjusted to the higher activity levels required for hunting and gathering by spending less energy elsewhere. Even for very active people, physical activity accounts for only a small portion of daily energy expenditure; most energy is spent behind the scenes on the myriad unseen tasks that keep our cells humming and our support systems working. If the Hadza’s bodies somehow manage to spend less energy in those areas, they could easily accommodate the elevated energy demands of hunting and gathering. And indeed, studies reporting differences in metabolic-hormone profiles between traditional and Western populations support this idea (though more work is needed).

As noted in the paper the researchers used urine samples to ascertain caloric expenditure. I’ve seen this in surveys of overweight people. One result was that many overweight people inadvertently under report their consumption (e.g., not writing down snacks that they ate) by as much as 1/3, so this is an essential check on self-reports. A reader brings up this relevant point:

They hypothesize that the Hadza save energy on unspecified activities other than walking and carrying loads. (Presumably digestion also is taking more energy for them?). Should we hypothesize that they spend less energy thinking? Any other good venues for energy efficiency?

BTW the paper starts out from stating that we “evolve as hunters and gatherers”, and IMVHO this statement requires a lot of qualifiers. For too much evolution happened after switch to agriculture to leave it unqualified, what do you think?

Here the reader is alluding to the fact that our brain utilizes ~25 percent of our caloric intake. In terms of energy it’s incredibly “expensive.” This is why it’s ridiculous to wonder if the human brain is a spandrel of some sort. Domesticated animals tend to have smaller brains than their wild ancestors, probably because they’ve offloaded some of the more cognitively intensive tasks to humans.

As for the issue with EEA, I think that’s a valid point. I checked over their references, and the authors don’t note the rather numerous studies since the mid-2000s which indicate that metabolism has been one of the major targets for natural selection in the Holocene (last 10,000 years). For example, Adaptations to Climate-Mediated Selective Pressures in Humans, or, Adaptations to climate in candidate genes for common metabolic disorders. If I had to bet I think the authors of the PLoS ONE paper are on to something, but they need to be careful to generalize from the Hadza, Western populations. In fact, I would be very curious to see a similar survey of the Bushmen of South Africa, and the Pygmies of the Congo. Probably the results would be the same, but it would still be informative to check to see if in fact these deeply diverged human lineages tended toward the same metabolic housekeeping and accounting. If so, then that might be the ancestral state.

By the way, I’ve been recording my own weight sporadically since the summer of 2010. I publish it on Facebook now and then, so I thought readers would be interested. I recommend this sort of thing to everyone, before more advanced self-quantification tools come online:

Source: Discover Magazine – Gene Expression