Midlife weight gain isn't due to a slowing metabolism

Most of us reach our final adult height at around age 18 to 20. Unfortunately, that doesn’t mean we stop growing! On average, Americans continue to gain a pound or two a year, every year, from the time they reach adulthood until age 60, when this trend starts to reverse. Of course, by then, a lot of damage has been done.

Although gaining a single pound or two over the course of a year isn’t going to make a big difference in your health, gaining thirty or forty pounds over the course of your adult life span can have a significant negative impact on your risk of diabetes, heart disease, cancer, and other conditions related to obesity such as knee pain and sleep apnea.

What drives weight gain in middle age?

There’s long been an assumption that this seemingly universal trend is due—at least in part—to a slowing of the metabolism in midlife. We’ve been told that our bodies’ engines simply rev a little faster when we’re young and that there’s not much that can be done. If we want to avoid gaining weight in midlife, we’re going to have to spend more time exercising and/or adjust our food intake to compensate for this inevitable slowing of the metabolism.

Now, if you refuse to go gently into this good night, you can find all kinds of special diets, workouts, and supplements promising to goose up your middle-aged metabolism. To the extent that any of these actually succeed in boosting your resting metabolic rate, the effect is likely to be quite small. As I’ve said before, trying to lose weight by boosting your metabolism is like trying to row a boat with a butter knife—you're going to be rowing for an awfully long time without moving very far.

See also: Is Your Metabolism to Blame?

But a new study is throwing all of these assumptions about midlife metabolism into the blender.

Surprise: Your metabolism doesn't slow down after 40

Researchers used a sophisticated technique (called the doubly-labeled water method) to measure energy expenditure in a diverse population of over 6,000 people. The subjects ranged in age from newborn to 95-years-old and came from a wide variety of ethnic backgrounds and cultures. And the results were almost exactly the opposite of what our observations might have led us to believe.

Not surprisingly, our metabolic rate is highest when we are babies, peaking at about 12 months of age. It then declines steadily until we reach early adulthood. But then our metabolic rate is remarkably stable from age 20 to 60. There is virtually no slowing in midlife.

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