What Do We Really Know About “Slow” Metabolism?

Some of the biggest and most read stories of 2021 were about new and better insights into human metabolism. Now it looks like the flood of information will flow into 2022. Already we have two new papers to digest. One, from evolutionary biologist Thomas Kraft and colleagues, offers new thinking on how humans evolved to gather and consume more energy than other great apes. The other, just published yesterday, gives us new insight into old thinking about fast and slow metabolism.

It turns out that so-called slow metabolism really is a thing. But it may be that it is not necessarily a risk for gaining weight or body fat over time.

Total Energy Expenditure (TEE)

Rebecca Rimbach led the publication of this latest paper from the IAEA DLW Database Consortium. This is the group that published the landmark study last year of daily energy expenditure across the span of human life.

In this new paper, they sought to document whether slow or fast metabolism is an individual characteristic that persists over time. For a long time, people have assumed that low total energy expenditure – a slow metabolism – might be one reason that some people are more biologically susceptible to obesity. So these researchers looked at repeated measurements of TEE in 347 adults and 47 children in their database.

A Durable Trait

For children, they did not find a repeatable pattern of low or high TEE. But for adults, they did. The authors conclude:

“Our analyses here show that having a “fast” or “slow” metabolism is a repeatable, durable trait for adults that is consistent over years. However, we find no evidence that subjects with lower adjusted TEE are at increased risk of gaining body fat, nor that higher adjusted TEE protects against weight gain. The causes and consequences of metabolic variation in humans remain a critical focus for future investigation.”

Note that last thought carefully. Like any good research, this raises more questions than it answers. This is especially true because it challenges the assumption that a slow metabolism puts a person at risk for obesity.

Implications of Getting Energy More Efficiently

Humans have evolved to gather energy more efficiently, say Kraft et al. But we don’t use it more efficiently. We simply use a lot of it compared to our evolutionary relatives. This has important implications they say:

“Humans’ high-cost but high-return strategy is ecologically risky, and we argue that it was only possible in the context of increased cooperation, intergenerational food sharing, and a division of labor. We contend that the time saved by human subsistence strategies provided more leisure time for social interaction and social learning in central-place locations, which is critical for cumulative cultural evolution.”

We are who we are because of the way we turn energy into children, as Herman Pontzer, senior author on this research, memorably tells us. And now we have the task before us of understanding what that means for our health as obesity continues rising.

Click here for the new paper by Rimbach et al and here for the paper by Kraft et al. For an informative thread about this from Rimbach, click here.

Masks Looking at a Tortoise, painting by James Ensor / WikiArt

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January 11, 2022