Throughout my 20s, whenever someone commented on how much I was eating relative to the size of my body, my get-out-of-this-conversation-free card was to cite my “high metabolism.” (At an old job, we lined up for catered lunch twice a week, which provided a lot of opportunity for voyeurism and commentary.)
But did I even know what my metabolic rate is? No. Did I know how it worked? Barely. All I know for sure is that when I turned 30, I stopped apologetically bragging about my so-called high metabolism — because it justifies something that doesn’t need to be justified (eating), but also because my metabolism is definitely, and obviously, slowing down.
I’ve long known that our metabolism slows as we age, though I never knew by how much, and what (if anything) moderated that decline. I knew it had something to do with how efficiently we converted food into energy (rather than fat), but what, exactly? I got in touch with Dr. Holly Lofton, director of medical weight management at NYU Langone Health, to learn more. Lofton tells me first that the type of metabolism I’m asking about is the basal metabolic rate, which she defines as the amount of energy it take to “maintain your muscle mass, your fat mass, your bones and all of the activities of your organs.” More basically, “how much energy it takes to keep us alive.”
Here’s the bad news: While many of us consider 30 as the age beyond which our bodies start slowly falling apart in all sorts of ways, Lofton says our metabolism actually starts to drop at age 25 — it’s just that many of us won’t notice until our 30s. Metabolism “actually decreases almost linearly with age,” she says. “The reason 30 is an important decade is because that’s the first decade in which we’re no longer increasing in bone production. So if we don’t increase muscle-mass production, overall metabolism goes down.” When we stop producing new bone, our bodies more quickly accumulate fat mass, which, unless it’s offset by new muscle mass, results in weight gain.
How much one’s metabolism drops after age 25 varies somewhat person to person, but Lofton says the typical rate is “2 percent or more” per decade. This may help explain why even active adults who feel they haven’t changed their lifestyle or eating habits may gain a little weight year after year — in order to offset such changes, Lofton says, we either need to increase activity or reduce calorie intake. If, for example, you ate 2500 calories a day at age 25, you may need to reduce that amount by 2 percent (or 50 calories) in order to offset that metabolic change and maintain your weight, says Lofton.
Of course, scientists debate the value in measuring one’s food intake by calories alone, and studies have shown that the human body tends to actively resist weight loss. When I asked Lofton if we might view the inevitability of our declining metabolism as permission to accept some weight gain as inherent to the aging process, she demurred, presumably not wanting to endorse weight gain broadly (though, again, the science around the health benefits of weight loss is very mixed). “Metabolism decrease is an inevitable part [of aging],” she says, “but we can offset it by being very, very careful. Monitoring your physical activity as well as your food intake and your metabolism takes a lot of work.”
Lofton adds that some of the quick, so-called metabolism-boosting fixes seen around the internet are, predictably, useless. “People ask me all the time, what if I drink hot water with lemon? No, that doesn’t work,” she says. “Well, what if I drink cayenne pepper? No.” Caffeine does boost one’s metabolism a little, she says, but only while you’re drinking it — so that’s only useful if you use that surge in energy to, say, go for a run right after you’re finished. There are no immediate solutions, and no reversals. Time marches on.