Many of us only have a vague answer to the question of 'what is metabolism?' Turns out (spoiler!), there's more to it than simply how much food you can eat without gaining weight. Scientifically, metabolism refers to all of the chemical reactions that happen within your body—processes like breathing, muscle functioning, building and maintaining bones, digesting food and turning it into energy, and even thinking. All of this requires energy—aka calories—so we measure metabolism as the number of calories used to keep the engine going. Sloths (like the cute, furry guy pictured here) have the slowest metabolism of all mammals—as much as 1.5 times slower than the average human's. Find out more about why some people have speedier metabolisms, how to estimate your metabolism and if there's anything you can do to speed it up.
Related: 4 Easy Ways to Boost Metabolism
Yes, some people's engines use more energy (what we call a fast metabolism) than others. You and a friend may be the same age and size, but she can indulge in a nightly scoop of ice cream and never gain weight, while the same habit would send the scale trending upward for you.
One factor may be your resting metabolic rate (RMR), which is the amount of calories your body uses just existing (sleeping, breathing). Surprisingly, this accounts for about 65 to 70% of the energy you burn in a day. The rest is energy used for physical activity and to break down the food you eat. And RMR can vary widely from person to person: one study of adults found that RMR ranged from 1,027 to nearly 2,500 calories per day. Researchers don't know exactly what's behind these differences, but how much lean muscle you have (as opposed to body fat) appears to play a large role because muscle and fat have different energy requirements (more on that in a minute). Other factors appear to be genetics, age and size (a taller body uses more energy than a petite one, for example).
Dina Metti, M.S., who teaches nutrition at San Diego State University, recommends the Mifflin-St. Jeor formula, which uses your weight, height and age to find your RMR. There are calculators that do the math for you. Your RMR is your baseline metabolism. From there, you can add in your activity level to get an idea of the number of calories you require each day, so you know how many you need to eat to lose, gain or maintain your body weight. Try this one from the Baylor College of Medicine. Some fitness centers also offer handheld metabolism-measuring devices, but Metti says skip them: they can be pricy, and an online calculator is just as accurate—and free.
Yes—to a point—with strength training. A review published in Current Sports Medicine Reports found that, on average, lifting weights increases RMR by about 7%, or an average of about 100 calories a day. This is because even at rest, muscle uses more energy than fat, burning about three times more calories—an estimated 6 calories per pound per day versus 2 for fat. Sure, 100 calories isn't huge, but it can make a difference for your weight over time. Think of strength training as one of the best ways to optimize the metabolism you were born with.