When it comes to weight loss (and gain), many of us believe our metabolism yields ultimate power over our success—and that there’s not much we can do to change it. After all, we all have that one friend who attributes his perpetually skinny frame to a ‘fast metabolism,’ just as we have that friend who blames her widening waistline on the sluggish metabolism she inherited from her mother. But are we really born with inner engines that run at different speeds—and does ours really determine our weight fate?
“Metabolism refers to your body’s process of converting calories into energy,” explains exercise and obesity researcher Tim Church, M.D., Ph.D., M.P.H, professor of preventive medicine at Louisiana State University and chief medical officer of ACAP Health Consulting. How fast or slow you convert those calories into energy, though, depends on a few factors—some of which you are born with.
First off, there’s how tall and naturally muscular you are. People with larger frames—who also tend to weigh more—actually have faster metabolisms than their smaller-framed friends. “The more you weigh, the more tissues you have, and the more tissues you have, the more calories you burn,” says Church. Then there’s whether you’re male or female. Men, who typically store less body fat, have more muscle mass, and are all-around larger than women, also typically have faster metabolisms because their muscle and size requires more calories to maintain than women’s generally smaller, less muscular frames.
Those metabolism factors are pretty much out of your control—but they’re not the only factors that determine the ultimate speed of your metabolism. The baseline number of calories your body needs to fuel essential functions, like breathing and circulating blood, is also determined by other factors, like your age (okay, also your of your control), your hormonal function, and your body composition (how much muscle versus fat you have). This metabolic baseline is called your BMR, or basal metabolic rate.
While your BMR is roughly how many calories you’d burn if you literally slept all day and didn’t move or eat anything, it only accounts for about 60 percent of your TDEE, or total daily energy expenditure, which is the total number of calories you burn per day and includes the energy you use to move around, exercise, and digest food.
In a nutshell, the more you move your body, the more energy it uses, and the higher your TDEE—meaning you have a ‘faster’ metabolism on days you exercise than on days you binge on Netflix.
Though it’s easier to boost your TDEE by moving more every day, it is also possible to boost your BMR over time, too. Remember when we said that the amount of muscle mass you have factors into your BMR? While you might be born with a more or less naturally muscular body than someone else, you can build more muscle mass and increase the baseline number of calories your body churns through every single day with strength training and proper nutrition (we’re looking at you, protein!). Research suggests muscle mass determines up to 60 percent of the variability in different people’s metabolisms, so putting in the work to build more is certainly worth your while.
By the time you’re an adult, lifestyle behaviors like being active and building muscle outweigh the aspects of your metabolism you’re born with. In a perfect world, weight loss comes down to a simple equation, says Church: Use more energy than you take in. However, if you’re faithful to your healthy routine, move your body regularly, and nourish it with the appropriate calories, but still aren’t seeing any changes in your waistline, give your doctor a call. Underlying health issues, like a thyroid disorder or diabetes, could be throwing your hormones out of whack and sabotaging your metabolism.