The Thrifty Human Machine: "Constrained Total Energy Expenditure"

(Now we'll see if the small and mixed readership of this blog can tolerate my continued trend towards an esoteric and narrowed focus.)

In the past decade, Herman Pontzer of Hunter College in NY has made enormous contributions towards our understanding of how human metabolism is tuned. His work from the last few years is presented in digestible fashion in the latest issue of Scientific American (article here ) and, if you find this post interesting, I highly recommend you try to get past the paywall and read this thing.  Below I'm going back to the research he summarizes in this Sci American article to make more sense of it and consider its evolutionary relevance.  Note: I love Pontzer's work so may this critical deconstruction not be misinterpreted as anything but a thought exercise from a fan and grateful student of evolutionary physiology.

"Constrained total energy expenditure"

Ponzter's big contribution is the finding that, contrary to the widespread belief that exercise boosts your metabolism, it actually does the opposite.  How did he reach this anti-dogmatic conclusion?  By carefully measuring the total energy expenditure (TEE) per day in various groups of people of varying activity levels.  For some extra evolutionary relevance and panache, he gathered data from the Hadza, a traditional foraging peoples who we like to think live in a manner similar to our earlier ancestors.  Compared with data from farming peoples and modern Westerners, here's what he found:

What matters is total energy expenditure per kg. of body weight (a 6 ft 5, 300 lb man will burn more calories than me-- but that doesn't necessary tell us anything about his metabolic rate, just that he has more tissue consuming energy).  The solid line in the above graph shows the regression for TEE vs. body weight in Western (American) men; the dashed line is for Western women.  Notice that if we were to make such lines for the Hadza men and women (the closed and open red circles, respectively), these lines would be similar to the Western ones.  Per kg of body mass, the Hadza aren't burning many more calories than Westerners, despite having more a more active lifestyle than we do.  A puzzling finding.

A similar phenomenon was found with a big study looking at the activity habits and TEE in Americans.  Let's make sense of the graph below: 

from Pontzer et al., 2016

Look at the top, black line.  This shows how total energy expenditure (TEE) was related to how physically active the subjects were; physical activity was measured with little devices that record movement.  This is a crude measure-- "counts per minute, per day".  More on this later.  Anyway, notice that for the most active people in the study, the line stops climbing and levels off.  There is NOT the linear relationship between activity and total calories burned that we'd expect.  At higher activity levels, people are burning less calories than you'd expect.

The key finding from these 2 studies is that doing a moderate amount of exercise does not significantly increase how many total calories you burn in a day (TEE).   This was true in Hadza vs. Americans, and within the American sample too.  This is counter-intuitive.  Pontzer hypothesized that our bodies somehow compensate for the energy we use during exercise by reducing energy spent elsewhere in other body tissues.  A reduction in basal metabolic rate (BMR; the calories needed to run the body, without any physical activity) has been observed in trials of people doing moderate exercise, and Pontzer's data is a mega-confirmation of this.  Where exactly this reduction in BMR comes from is unknown, but in women, we know that ovarian and estrogen functions are lowered with exercise.  Likely, the body's metabolic engine tunes down energetic needs just about everywhere in response to a moderately-active lifestyle.  Pontzer calls this "constrained energy expenditure": Humans (and other primates, and maybe many animals?) have evolved through natural selection to conserve calories by turning down overall energetic needs when activity levels increase.  

from Pontzer et al., 2016.

This is a big finding, and actually it makes a lot of sense. It's especially interesting to me because primates overall are characterized by a slower metabolism (lower BMR, lower TEE) as compared with other mammals.  But humans have reverted back to the more primitive mammalian characteristic of a hotter engine: we have a higher TEE than other primates (see graph to the right).  So we are unique among primates in that we are high-octane.  But, it still makes sense that our bodies respond to moderate activity by reducing calories spent elsewhere. We evolved to require and burn lots of calories--more than other primates-- probably because a) we have big brains;  and b), as I've been harping on for a while, we are endurance creatures.  We evolved to be active, to cover lots of ground, for whatever reason (and these reasons are hotly debated).  But it makes sense that, in response to exercise, our bodies will reduce energy expended elsewhere.

Together this work has been interpreted to mean that exercise is a rather crappy way of losing weight, and that our sedentary lifestyles are not to blame for diabetes and obesity.  Rather, it's that we eat too much.  Mostly that makes sense: someone who hits the eliptical for 30 minutes 3 days a week is only burning an extra few hundred calories per week, calories that are a) probably made up for in extra food intake and b) compensated for by this "constrained energy expenditure" phenomenon.

What's missing?

As awesome as this new finding is, there are a few caveats, I think.  First, while clearly our bodies do reduce overall metabolism to compensate for exercise, we also compensate behaviorally.  I find myself unwilling to move much at all after a 2.5 hour trail run. (This is changing now that I consciously choose to remain active despite being tired from training.)  Ponzter's group made similar observations in Americans and Hadza: we lie down more, move less, and are generally lazier after we exercise.  This helps to account for the fact that active people don't burn as many calories as you'd expect.  Pontzer notes this in his papers but I think it's a bigger factor than is admitted.

Also, look at the 2nd graph.  "Counts per day"-- the number of movements picked up by the accelerometers worn by subjects-- is only poorly reflective of activity level.  See the red data points at the bottom of that graph? They show the relationship between counts per day and calories actually burned through physical activity.  The relationship is very weak-- less than 1% of the variation in calories burned can be explained by counts per day.  The Pontzer group touts this as evidence for their constrained-energy hypothesis: higher activity level does not equal proportionally-greater total energy expenditure.  Perhaps, though, this is evidence that their activity tracking system doesn't accurately measure physical activity.

More importantly, notice who's not included in these studies-- a group with potential evolutionary relevance: serious endurance athletes.  Look at the top graph again.  That black circle?  That's me-- I added roughly where I fall for an average training day.  As a fairly typical serious endurance athlete, I would be the highest calorie-burner in that study.  Even in the bigger study (see the 2nd graph) an endurance athlete would be the biggest calorie spender... these people were not included in either study, and I strongly suspect that a bunch of data points from us folks would make that body mass/TEE line look a lot more linear.  That is, while the body clearly compensates for activity by reducing energy spent elsewhere, there's only so much compensating the body can do when you're burning 1700 cal/day through exercise.  I think that the constrained energy model is, well, constrained at higher levels of activity, even though the studies didn't test the right people to demonstrate this.  What's the relevance of this?  Well, moderate exercise may not be good for losing weight, but 12 hrs/week of training certainly is.  And, because I think that early humans were (at least at times) more active than anybody in these studies, we're missing a piece of the story.

In short, these studies didn't include people who are highly active.  Even the Hadza only burned a modest number of calories per day through activity, and the most active Americans burned only 200 more than sedentary Americans.  That itself is a finding-- that hunter-gatherers don't really work all that hard physically (in terms of calories)--but it does stifle the main finding a bit, in my opinion.

"Life is essentially a game of turning energy into kids."  A great quote from the Scientific American article.  This recent work on metabolism is shaping our understanding of how our engines are tuned, no doubt...I hope future research here includes my relevant-if-weird subgroup of humans. 

March 2017 update: Pontzer has published a fantastic new paper on this topic with a bit more synthesis and discussion of evolutionary implications:
http://onlinelibrary.wiley.com/doi/10.1002/evan.21513/full

References

Pontzer, H., Brown, M. H., Raichlen, D. A., Dunsworth, H., Hare, B., Walker, K., ... & Plange-Rhule, J. (2016). Metabolic acceleration and the evolution of human brain size and life history. Nature.

Pontzer, H., Durazo-Arvizu, R., Dugas, L. R., Plange-Rhule, J., Bovet, P., Forrester, T. E., ... & Luke, A. (2016). Constrained total energy expenditure and metabolic adaptation to physical activity in adult humans. Current Biology, 26(3), 410-417.

Pontzer, H., Raichlen, D. A., Wood, B. M., Mabulla, A. Z., Racette, S. B., & Marlowe, F. W. (2012). Hunter-gatherer energetics and human obesity. PLoS One, 7(7), e40503.