Thursday, January 26, 2017

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 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.

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. 


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 Biology26(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 One7(7), e40503.

Wednesday, January 11, 2017

The Man Bun: Energetic and Thermoregulatory Costs.
That's right-- it's time that everyone's burning question is finally explored: what are the costs associated with having a man bun?  Not the costs to his social standing, which are mixed, nor his manhood, also mixed.  But rather, energetic and thermoregulatory costs, particularly while locomoting.  I fully expect an NIH grant to explore this further.  You're welcome, science.

During particularly sweaty workouts I have questioned the wisdom of having extra hair attached to my dome.  Is it trapping heat?  Preventing sweating?  Adding excess weight? Let's try to get at some answers.

1. Energetic consequences of the man bun

All that extra hair only adds a trivial few ounces of weight.  It's not going to make much energetic difference to the whole-body cost of running.  It's like wearing slightly thicker shorts-- a few ounces there isn't going to add up to much.  However, the head moves around a bit when you run, so these extra few ounces on top of the head could count for more. Unlike extra weight at, say, the waist, which has to be carried forwards with each stride (plus a bit of up and down), weight at the end of a bouncing extremity incurs an extra cost through every cycle of its independent movement.  So a bouncing man bun is like wearing a heavier pair of shoes: there's a cost to carry the weight forwards, like any body weight, but also the cost of moving the part around during its extra movement cycle.

But it's not like heavier shoes.  Most of the work done to stabilize your head while running is performed by the nuchal ligament (right).  Unlike muscles, ligaments don't need energy to do their work, so the human head is restrained while running with little energy cost.  This structure is found in other animals that hold their head up all day, and in running animals; indeed, in humans, it may have developed largely because of running.  So, while a heavier shoe incurs a cost every time the foot is lifted, the man bun's weight is mostly controlled by the passive nuchal ligament and incurs little cost.  The extra weight of a sweaty man bun will add to this, but not much, and only if other muscles must be recruited.

2. Thermoregulatory consequences of the man bun

Except in very cool conditions, heat buildup is among the biggest factors limiting endurance performance (this is now the subject of my research).  Running generates massive amounts of heat; a trained athlete running a 1-hour race will elevate their metabolism ~19x over resting levels and heat is a byproduct. (In fact, you don't need to wait for summer to become heat acclimated: running even in moderate temperatures will get you partway there, as the endogenous (internally-produced) heat overwhelms your body even more efficiently than ambient heat.)  Because the body can only tolerate a small increase in internal temperature, much of our biology has been shaped in part by the need to dissipate heat: huge numbers of sweat glands (unusual and rare among animals), hairlessness (also weird), profusions of blood vessels supplying the skin, even our very body shape.  I'll note here that our unique heat dumping biology implies very strongly that our evolutionary past included strenuous heat producing activity.  Sustained running, or run-walking, could explain it, and few other things can.  Even carrying a heavy load in very hot weather, walking produces only half the sweat that running can; so arguing that our ancestors did not rely somehow upon running necessitates accepting that our cooling ability has huge safety margins, excess capacity.  And such excess capacities in complex and costly physiological systems are rare in the animal world.

Back to the head.  The brain lives here and is very temperature sensitive.  In fact, the body's chief heat-related concern during exercise is to cool the brain, as even a small temperature increase spells neural death.  In the cold, you lose roughly as much heat from the surface of the head as you do other parts of the body.  In hot conditions and during exercise the forehead sweats a lot, maybe even a bit more than other body regions, but overall the head isn't more of a radiator than you'd expect for a body part with its surface area to volume characteristics.  The brain is cooled primarily by taking hot blood elsewhere in the body for sweating, not simply by local sweating at the head, though other neat hypotheses abound: perhaps our larger sinuses or extra blood vessels permit brain cooling, or perhaps sweat on the scalp releases heat and then is re-absorbed into the skin instead of evaporating.

Machado-Moreira et al., 2008
So what about hair?  Does it prevent heat loss? Wearing a hat does, but hair?  You sweat everywhere on your head, though sweating under the hair isn't particularly effective because hair blunts evaporation and the sweat glands get clogged up (unless the weird hypothesis above holds true).   A 2008 experiment (left) confirmed earlier findings that the forehead sweats more than other head regions, with the top of the head having the lowest sweat rate.  Still, any sweat is helpful, the skin under head hair does do some sweating, and hair prevents sweat from doing its job (but again see the weird idea above).  However, this experiment involved shaved heads.  I suspect that the observed sweat rates would be lower with hairy subjects, maybe much lower, as sweat glands on a hairy scalp may clog and stop sweating very quickly.

The evidence is incomplete but it would seem that any hair on the head will blunt sweating to some degree.  But hair has another thermoregulatory purpose: it blocks solar radiation, shielding the brain from external heat.  In full sun, this heat-shielding mechanism might outweigh heat-increasing penalties incurred from reduced head sweating, having a net positive effect on body cooling.  In less sunny conditions I bet no hair is the best bet.  A man bun shouldn't be any worse than any length hair because it is up in a, well, bun, away from the forehead and neck and other sweat-producing skin regions.

What's the verdict?  A man bun will have a negligible effect on the energy cost of running and removes perhaps 10 square inches of skin real estate from the body's sweating arsenal.  This effect isn't any worse with any length hair, but in a competition where seconds count, this tiny added heat stress could matter.  For summer racing in the sun having some hair, even a man bun, is good, but a race entirely in the woods might call for a shaved head.

But none of this is as costly as the stigma that we man-bunners carry around.  Time to go to Whole Foods where I'll blend in.


Lieberman, D. E. (2015). Human locomotion and heat loss: an evolutionary perspective. Comprehensive Physiology.

Machado-Moreira, C. A., Wilmink, F., Meijer, A., Mekjavic, I. B., & Taylor, N. A. (2008). Local differences in sweat secretion from the head during rest and exercise in the heat. European journal of applied physiology104(2), 257-264.

Takeshita, K., Peterson, E. T., Bylski-Austrow, D., Crawford, A. H., & Nakamura, K. (2004). The nuchal ligament restrains cervical spine flexion. Spine29(18), E388-E393.

Wheeler, P. E. (1984). The evolution of bipedality and loss of functional body hair in hominids. Journal of Human Evolution13(1), 91-98.

Friday, December 9, 2016

Racing 2016, part 2

A final missive on running and racing 2016.
Summer mountain racing is my favorite season.  In fact, these are the only races that really motivate me now.  But I did't perform to quite the standard I'd hoped for this summer.  In the interest of keeping my trade secrets*, it suffices to say that by August I had made some adjustments to my training and I was PR'ing in key workouts (which again are trade secrets, unless you ask in which case I'll probably tell you).  A jump in fitness makes the daily escape to the woods even more enjoyable- you're more locked in to the trail and to your primal nature-- and it makes a guy like me excited to test himself in competition.  Especially at 34...being in PR shape in your mid-30's is something to grab hold of.  Rage against the dying of the light, etc. 

*In addition to what's on Strava, I'll happily divulge this much about the training adjustments I've made: Slightly less running (achieved through cutting out some fluff) and more mountain biking. 

Mansfield Double Up
For my late summer race I chose the adventurous Mt. Mansfield Double Up, an epic and technical 3-hour adventure over Vermont's highest peak (twice), a race for me to experience more than really compete in.  I don't have the skills to run trails like this-- I hiked probably 80% of this course.  But it was too tempting to pass up, a rare, first-of-its-kind event.

My recollections of this day have already become shrouded in nostalgia and misty fog, as the summit itself was during the race, and I don't think I have the words to describe what it felt like to move hard on this terrain: Epic? Free? Intimidating? All of these.  Also, humbling: I was strong on the later climbs but the technical hiking/running/scrambling wasn't my bag and I got my ass kicked by 14 people.  Two men put on a clinic--Ferenc and some Elf who skipped by me on the terrifying Subway trail, pictured below; it was impressive.  (This trail is where I caught myself from falling off the mountain and decided there and then to "just finish.")  The weekend was fantastic, spent with Stoneman and Ferenc, and I finally got to try Heady Topper.

The subway trail on the side of Mansfield's western ridge;
several thousand foot drop to the right. (
Unfortunately the nervous, downhill rock-slab shuffling that I had to employ to get down the mountain (twice) wrecked my quads and I missed what likely would have been my best race of the summer: the Race to the Top of Vermont, held the weekend after the Double Up on the same mountain but up the toll road.  I know from a few workouts in August that I could have competed for the win but my pathetic technical downhilling at the Double Up left me too wrecked to try.  (I even planned The Double-- the running race, then a screaming descent back down, then racing back up in the bike race.)  Through September, workouts continued to progress and I worked myself up into the best shape I've seen in many years, but due to various factors I didn't get to race at all.  Even now, in December, this stings.

Mt. Monadnock

I refuse to carry my phone when I run so
I pilfered this picture from 
Yeah, this isn't a race, but a quasi- FKT (fastest known time) attempt.  This 3100' peak is 75 minutes from my house, I've never climbed it, it's the most-climbed mountain in North America (#3 in the world), and it has a very legit and well-tested FKT from 2001.  Stoneman and I rolled up there on a Monday in early October in hopes of getting a day without crowds (mostly successfully, except for the Mom that I shoved aside when her 8-strong brood wouldn't move).  

With some minimal research and a short warmup I hammered the trail as best I could.  The first third of this ~1.8-mile trail was runnable; the second third became steeper and had lots of rock step-ups, partially runnable; and the last third was steep, unrunnable exposed slab, as nasty as any of the other alpine summits in New England, with a few flatter sections of boulder-hopping.  All of it was rather wet, making traction a problem, and I was predictably ill-equipped for the techy rock hopping near the top.  The record of 24:44 was safe-- I summited in 27:30 and Aaron hit 33:04.  With dry rock (it was wet enough that we had to hike 75% of the route down) and more familiarity with the best lines (I had lost some time looking up for blazes and routes through the rock) I could probably get under 26 minutes.  But that last minute-plus to beat the record will require more skills and less fear on those rock-hopping sections.  The views up top were awesome and this was a fantastic way to spend a Monday but I was thinking ahead to a future attempt.

North Face Race to the Summit

This well-kept secret is a 2.1-mile blast up southern Vermont's Stratton Mountain via ski slopes and service roads.  Ferenc was not thrilled to see me in the parking lot (this was his well kept secret and solid payday, for several years running) but we warmed up together and it's always good to see this guy.  The short and direct course took us into the clouds, which by partway up served to mask the lead I was developing as well as the shrill anaerobic hell that comes with 22 minutes of flat-out mountain running.  This was my most confident race of the year-- short, all uphill, immediate suffering with no time for doubt or slow fatigue.  It's not often I can beat national class dudes so this was alot of fun.  We ran back down and enjoyed peak-foliage Vermont views, like tourists.  No, not like tourists, not at all, actually.

Mt. Toby Trail Race
My local trail race.  In 2013 I put up a good time and I mistakenly thought I could chase the course record this year.  However, despite several hard climbs, this trail race is fast, with mostly good footing.  It benefits someone who has maintained a long fast stride, which I haven't done these past few years, and despite hard interval training this fall I wasn't comfortable running near 6 minute pace for almost 90 minutes.  I won but put up a time 5 minutes slower than last time.  If I really want to run fast trail races, I'll have to build long tempo running back into my training, and I really really hate that stuff.

After a 6 week off-season consisting of 50% training volume I am now working back towards full-tilt.  Unfortunately this coincides with the onset of winter, which tends to push me back onto the roads more than I'd like, so motivation is harder to come by.  But the running goals I have these days will be more elusive as I push into mid and then late 30's, so I will continue getting out the door even if I'd rather just get fat.