My husband, Andrew Rettek, has a blog you should read. As he’s gotten into fitness, he’s started following exercise science, which is the (very new!) field of running small controlled experiments on diet and exercise on athletes who do exactly what you tell them to, under strict observation.

This is in contrast to fields like nutrition science, which study the effect of the “intervention” of telling people (usually non-athletes) what diets and exercise programs to follow. Exercise science tends to have much smaller sample sizes, but it can come to more unambiguous conclusions because it’s testing the intervention itself, not people’s ability or willingness to follow it. Contra the “nobody knows anything about diet or exercise” conventional wisdom, we do know some things! It’s just…not very many things. And only about college athletes.

One surprising thing Andrew noticed is that organ size, especially liver size, has a lot to do with overall metabolism.

Larger Athletes’ Higher RMR is Partly From Bigger Livers

Athletes have higher resting metabolic rates than non-athletes; their bodies use more energy, even when they’re not exercising. That means they can eat more without getting fat.

Some part of this effect is due to higher muscle mass, which “costs” energy to maintain.

But muscle isn’t the only tissue athletes grow; bigger athletes also have bigger livers.1

In fact, muscle is a lot less metabolically expensive than other organs. Muscle consumes only 13 kcal/kg/day, while liver, brain, heart, and kidney consume 200, 240, 440, and 400 kg/kg/day respectively. In fact, 60-70% of resting energy expenditure2 in adults comes from these four organs, even though they’re only 6% of body weight.

If you look at “small”, “medium”, and “large” athletes3 (male, mean weights 147, 170, and 200 lbs), obviously the larger athletes have more REE than the smaller ones. What’s surprising is that most of this is due to non-muscle fat-free mass differences — i.e. organs, mostly liver. The difference between “small” and “medium” athletes’ REE is 73% due to this “residual mass”. The difference between “medium” and “large” athletes’ REE is 56% due to “residual mass”.

Same deal for female athletes4 — 76% of the REE difference between “medium” and “small”, and 36% of the REE difference between “large” and “small”, is due to increases in “residual mass”.

If you compare sumo wrestlers to untra...

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