Burn - New thinking on the human metabolism

Just listened to the latest Science of Ultra podcast that features Herman Pontzer, PhD and his study of metabolism which can be found in his new book “Burn.” Not read the book but, if I understand it correctly, the research indicates that while increased exercise increases metabolic rates, over time, metabolic rates revert to the mean such that the metabolic rates of an endurance athlete who exercises consistently are pretty much the same as a sedentary human. Interesting stuff. Seems to strike against the “work out more to burn more calories” conventional wisdom. Honestly, still trying to understand it, think through it critically, and sort out the implications. Anyone had a chance to sort through this?

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I have seen similar research findings in a few other books, not really geared towards athletes - but rather more general regarding weight loss (the Obesity Code, Complete Guide to Fasting, Genius Foods, etc.).

The logic is sound. Our bodies are sophisticated and adapt pretty well. If we expose it to a stresser it will find a way, over time, to adapt to that stress.

So, in this example, caloric burn. It makes all the logical sense in the world that your body, indefinitely, can’t operate at a calorie deficit. If it did, eventually we would simply die. Eventually, the body will find a way to rectify the deficit. This is done through one of two ways - either eat more or burn less.

If we eat more then the deficit will be resolved through greater intake volume. More calories in excess of the deficit.

If we need to burn less…well, eventually the body (if we continue to run and exercise) will find a way to reduce the metabolic burn in non-exercise time. It will not allow ourselves to commit caloric suicide.

I think, if you step back and look at it from that perspective, it makes all the sense in the world. This is why long-term weight loss is so hard for most people to maintain…because your body, on the back-end, is either telling you to eat more (up intake) or burn less…

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Thanks for sharing! I’m gonna check out the podcast tonight. When I first got into running, I got pretty lean effortlessly but overtime I’ve seemed to go back to a more comfortable weight despite being fitter than ever. Changes to diet and volume made little difference, so I’ve started to accept that it’s where my body wants to be. This may be why!

Thanks again.

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No, but I can offer some insight.

Endurance training generally does not increase metabolic rate once the adaptation window of prior training sessions has closed. ie… less than a week. If anything, for the same muscle mass, I’d expect endurance athletes to burn slightly less kcal daily than non-endurance athletes because endurance athletes have spent so much time improving economy of movement. So, the fact that they’re reporting that once training ceases, metabolic rate returns to that of non-exercise counterparts, is good news!

To the contrary, resistance training which causes muscle growth, does cause greater metabolic rate because muscle mass is high kcal expenditure per unit mass tissue in comparison to most other tissues. Ironically, substantial muscle growth technically makes a person less efficient (burns more kcal for same activity) and this goes for at rest or sedentary conditions too.

Primary determinants of daily kcal expenditure are body weight and body composition. Outside of those two considerations, all other things, individually, make very little difference in daily kcal expenditure, by comparison, to those to those two.

Since endurance training doesn’t tend to promote muscle growth beyond very initial stages in sedentary populations (where any activity would), once you’re “lean” (whatever that means) further body composition improvement by way of increased muscularity is not going to happen as it might if more resistance training is included in the training.

That’s not to say anything against endurance training, just that you shouldn’t expect it to increase metabolic rate over time.

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Could it be that this is why we see such low heart rates sometimes in trained athletes? Maybe this is another way the body slows that metabolic burn.

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FYI: primary reason for lower HR in trained athletes = greater blood volume and greater heart size and contractile strength, (and maybe greater heart relaxation ability), but not lower metabolic rate.

Just thought I’d provide an answer here in case you found it helpful.