fitness · Science

Surprising new research on human metabolism: overview and some questions

CW: brief discussion of weight-blaming and shaming of people over 40.

Ah, conventional wisdom! We rely on it, use it to advise and direct other people, and conveniently forget times it doesn’t work for us. I decided to look up some good examples of conventional wisdom that are clearly not wisdom (in fact not even knowledge, as they’re arguably false). Here are some:

You get what you pay for.

One financial site pointed out that, in many circumstances, we get what we don’t pay for. That is, paying for something is a loss for us, and paying more is a greater loss. We have to pay close attention when paying more to determine if it’s a good deal. Hmmm…. Good thinking.

Warren Buffet, famous rich money-making person agrees, saying “price is what you pay. Value is what you get”.

If you can’t stand the heat, get out of the kitchen.

This saying suggests we have two options when things get difficult: push on through, or abandon our plans. No– I don’t think so. Informal logic classifies this as a bona-fide fallacy: the fallacy of false dilemma. When faced with a crisis or barrier, there are almost never only two options. We can slow down, enlist others, shift timelines, take a break and regroup, recast the parameters of the project… I could go on.

Here’s great advice on what to do if you’ve got too much heat/spice in food: add acid, veggies, nuts, broth, dairy, sweet or just make more. No need to leave the dish.

At last, we get to human metabolism and aging. Here’s the conventional wisdom:

No matter what we do, our metabolisms slow down over time, especially after age 40.

According to this view, our metabolisms slow gradually as we age and we experience a marked and continued slowdown of resting metabolic rate (insert all kinds of asterisks here, as this is super-complicated, and the conventional wisdom often conflates lots of distinct metabolic processes). In addition, there’s also conventional wisdom about ways we have control over our metabolisms.

Okay, if the convention wisdom about slow but inexorable decline in human metabolism is wrong, then what is right?

Short answer: we’re a long way from knowing in great detail how human metabolism works, and applying that to clinical medical and health practice.

Longer answer: a new study, combining very high-quality data on more than 6500 subjects from more than 40 testing sites, suggests a new four-stage model of human metabolism over the life trajectory. Veteran NY Times science write Gina Kolata sums it up here:

  • There’s infancy, up until age 1, when calorie burning is at its peak, accelerating until it is 50 percent above the adult rate.
  • Then, from age 1 to about age 20, metabolism gradually slows by about 3 percent a year.
  • From age 20 to 60, it holds steady.
  • And, after age 60, it declines by about 0.7 percent a year.

For those of you who prefer graphs, the original paper explains it below:

Graph of metabolism over the life span, showing sharp increase in energy expenditure through childhood, sharp drop in adolescence, plateau to age 60, then decline.

Here’s another conventional wisdom-buster from the paper that Kolata reports:

Once the researchers controlled for body size and the amount of muscle people have, they also found no differences between men and women.

But wait, there’s more myth-busting:

The four periods of metabolic life depicted in the new paper show “there isn’t a constant rate of energy expenditure per pound,” Dr. Redman noted. The rate depends on age. That runs counter to the longstanding assumptions she and others in nutrition science held.

Wow. So it’s not “calories in, calories out”, right? Right. That piece of conventional wisdom has been on its way out for a while now, and these results further explain the ways that view is wrong.


I have questions.

Media outlets are already using the results of this study to blame people over 40 for weighing more than they did when they were younger. From the BBC to beauty spas, the message is being put out there: increases in body weight after 40 aren’t because of slowing metabolism. Now insert implicit conclusion: it’s your/my/our own fault!

This was NOT one of the conclusions that came to mind for me. On the contrary: this study brings up lots of questions about the relationships among energy intake/expenditure, aging, activity, physical performance, cognition and mental acuity, and what’s in our future, given a better understanding of how the human machine runs over time. Two things I’d like to know:

How does the metabolism life trajectory graph look when it’s divided into weight groups? When it’s divided into other demographic groups?

We know there are many social determinants of health: that is, our environments and social/political/economic circumstances influence how long and how well we live. What effects do they have on the metabolic pathways over time?

One thing I believe firmly in and would promote as a piece of relatively new conventional wisdom is this:

Biology isn’t destiny.

We’re still very busy trying to figure out how the puzzle pieces of genetics, environment, behaviors, culture and community, economics, and justice (or lack thereof) fit together to predict, explain and promote human flourishing. This study gives a clear direction for new metabolic research. The puzzle, however, is far from completed.

Readers, did you hear about this in news or did you read the article? Did it strike you as good news? Did it change your views? I’d love to hear from you.

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