New Insights into Metabolism, Plus a Little Humility

An impressive new study published today in Science has researchers feeling a bit giddy. “Blown away” was the sentiment Rozalyn Anderson expressed to the New York Times about the study. “A pivotal paper,” said Leanne Redman, adding that it will shape textbooks for years to come. Herman Pontzer and a remarkable array of collaborators analyzed daily energy use by humans across the lifespan. The insights into metabolism coming from this analysis are the source of all this excitement.

So of course, some of the reporting has been a little over the top. In its headline, the Times suggests that what we think we know about metabolism is all wrong. Not quite. The real problem is with the hubris of people who presumed they knew it all to begin with. Really outstanding research like the Pontzer study gives us all a taste of humility about the limits of what we know.

Energy Use from Eight Days to 95 Years

For the first time ever, this research offers a serious accounting for the energy that humans use across their entire lives. They obtained this data from the doubly labeled water method in a diverse sample of 6,421 subjects. Their ages ranged from eight days to 95 years. They came from 29 countries.

For infants, the researchers found that energy use rose rapidly after birth until it peaked when they were a year old. From there, it declined slowly until it stabilized at approximately the age of 20. Then, through adulthood until about age 60, energy use is surprisingly constant – even in pregnancy. After 60, energy requirements decline significantly once again.

Weight Gain in Middle Age

Much of the this-changes-everything hype about this study revolves around weight gain in middle age. Earlier this year, Yang Claire Yang et al published fine work to document BMI across the course of life. In that data, the weight gain that many people experience through middle age is quite visible.

Some people presume this is because of slowing metabolism, but insights from today’s new study makes it quite clear that this presumption is wrong. Senior author John Speakman explains:

“Perhaps the most unexpected feature was the constancy of metabolic rate in both males and females between the ages of 20 and 60. This suggests that if you are experiencing middle age spread its more likely to be because you are eating more rather than expending less.”

Adding Insight and Humility

Without a doubt, Pontzer et al have published an impressive study. It doesn’t prove that “everything we thought about metabolism is wrong,” but it does add very important insights into human metabolism across a lifetime. It is a great accomplishment, says Anderson in her commentary with Timothy Rhoads alongside the study:

“The unprecedented scale and scope of the study is matched by the outstanding collaborative spirit that made it possible.”

Scientific knowledge grows in this way. New research leads us to question presumptions and biased mental models. Smart people let go of old ideas that don’t fit with the reality of solid, new data like this. Ultimately, this is how we overcome the unfounded biases that are so pervasive in obesity and nutrition. With facts.

Click here for the study and here for the commentary. For further perspective, click here, here, here, and here.

Dynamism of a Cyclist, painting by Umberto Boccioni / WikiArt

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August 13, 2021