Midlife weight gain isn’t due to a slowing metabolism

by Christina Gvaliant
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.

The next change seems to occur around age 60, when the metabolism does indeed start to slow. But remember that this is the same age when people typically stop gaining weight and start gradually losing weight. It’s basically the opposite of what you’d expect to see if you assume that age-related changes in body weight are driven largely by age-related changes in our metabolic rate.

To the contrary, this study shows that—all other things being equal—our metabolisms are actually no slower in our 40s and 50s than they were in our 20s and 30s. So why do so many of us gain weight and or struggle to lose it as we go through midlife? Clearly, all other things are not equal.

If it’s not our metabolism, then what is it?

We may simply be taking in more calories than we need. There are certainly plenty of opportunities to do that.

See also: Why we overeat

Perhaps we are also moving less. Even if we are making time for regular exercise, the rest of our day may be a lot more sedentary at age 40 than when we were younger. Think, for example, about how you spend time with your friends now, compared with how you might have when you were younger. Are your leisure activities more or less active than they used to be?

Perhaps we are mirroring what has been modeled for us in terms of how grown-ups behave. Or, we may move less because our bodies feel stiff or achy. (Unfortunately, inactivity simply accelerates this process.)

When we move and use our bodies less, our body composition also changes. We lose muscle mass, which further reduces both our capacity to move and the number of calories we burn at rest. So, in that sense, our metabolism may in fact be slowing down—but not due to our age. Rather, it is slowing as a result of lifestyle choices that we may be more likely to make as we age.

In fact, this study confirms that the amount of fat-free mass you have is the single most significant factor in how much energy you expend—even more significant than how many calories you burn through exercise. And much more significant than your age.

What you can do to prevent midlife weight gain

The good news here is that much more of this may be within our control than we previously assumed. But we may need to stop acting our age. Or, better put, stop following our society’s script for what middle age looks like.

My colleague Dr. Jonathan Su, who recently stepped into Brock Armstrong’s shoes as the new host of the Get-Fit Guy podcast, has lots of good advice for people of all ages and fitness levels on being more active—no matter what limitations you might be working with. Just this week, after listening to his show, I realized that I could be getting a lot more benefit from my morning walks by picking up my pace.

From a nutrition perspective, protein can be a key player in our quest to beat the midlife spread. Including the right amount of high-quality protein in our diets can help us maintain both our weight and our body composition in at least three important ways:

  1. Protein helps to temper the appetite, which can make it easier to curb over-eating.
  2. Protein also has a direct effect on metabolism through the greater thermic effect of food. This means that you burn more calories after eating protein than you do after eating the equivalent amount of calories from fats or carbohydrates. The effect is small but every little bit helps.
  3. Protein (in conjunction with exercise and movement) helps us build and maintain muscle tissue, which helps us burn more calories at rest.

To that end, I suggest including some sort of protein food in most of your meals and snacks  Animal foods, including eggs, fish, and dairy, provide the most bioavailable and concentrated sources of protein. Beans, legumes, grains, nuts, and seeds also provide protein. Just be aware that it takes more servings of plant-based foods to provide the same amount of protein you’d get from fewer servings of most animal-based foods.

My previous episode on protein density can help you manage your protein intake while also keeping an eye on total calorie intake. You may also want to check out my episodes on preventing age-related muscle loss and building more muscle from less protein as well as this post on building muscle on plant-based diets.

And finally, Brock Armstrong and I recently teamed up to create a special podcast series on beating midlife spread, which you can subscribe to here.


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