Your Body’s Metabolism—Explained

Get insight into how you burn calories day and night

By Alyssa Shaffer

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Photo by: Getty Images

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Did you know that as few as two nights of not getting enough sleep can slow your metabolism? Or that your body uses more energy to digest whole foods (like fruits and veggies) than processed, packaged foods? Below, Alyssa Shaffer—longtime health, fitness and nutrition writer and the author of Turn Up Your Fat Burn—addresses common curiosities about your body’s ability to burn calories.

Q: My friend can eat anything she wants and not gain an ounce, but I take one bite of a brownie and it goes straight to my hips. What gives?

A: Thank your parents for your brown eyes, your wavy hair and, in large part, your metabolism. Sorry to say, but your friend just naturally burns more calories than you do. That said, even though you’re born with a certain metabolism, you do have quite a bit of control over it, notes Dr. Westcott.

Here’s why: About 60% to 85% of the calories you burn during the day are spent on basic functions like breathing, digestion and circulation (this is called your resting metabolic rate, or RMR). Most women need at least 1,200 calories to support their RMR. The rest of your calorie burn is split between working out (going for a swim or a run) and everyday activity (walking to your car, carrying groceries). All of these areas (even your RMR) can be manipulated to make your everyday life more active (as we suggested on page 99), so you can eat more without putting on pounds.

Q: Do I burn more calories during the day than at night?

A: Yes. Although your RMR stays pretty consistent, you’re more active during the day so you’re burning more calories overall. Research actually shows that people who eat the majority of their calories later in the day tend to eat more calories, weigh more and have more body fat. Try making breakfast and lunch your bigger meals.

Q: Does dieting hurt my metabolism?

A: You may have heard that yo-yo dieting (gaining and losing weight intermittently) can permanently throw off your metabolism, but research shows that’s not the case. What does slow your metabolism is eating too few calories. Even just a day or two of a very low-calorie diet can have this effect, so make sure you’re taking in at least 1,200 to 1,500 per day.

Q: Could a medical condition cause my metabolism to slow down?

A: Possibly. People often blame weight gain on hypothyroidism (an underactive thyroid), but only a small percentage of people have it severely enough to make a noticeable difference in weight, says Dr. Garber.

How Many Calories Did You Burn Today?

You can get a pretty good estimate of your metabolism (how many calories you’ll burn throughout the day, not including working out) with some simple formulas.

If you’re mostly sedentary:

You rarely exercise, sit at a desk or at home for most of the day and drive to run errands.

Multiply your body weight by 12.

Calories burned by a 150-pound woman: 1,800

If you’re moderately active:

You exercise 3 to 5 days a week at a light-to-moderate intensity level (a 30-minute brisk walk); strength-train once or twice a week; stand a lot at your job or taking care of kids at home; you walk a few blocks to the store.

Multiply your body weight by 14–15.

Calories burned by a 150-pound woman: 2,100–2,250

If you’re very active:

You exercise at a moderate-intense to very intense level at least 6 days a week; strength-train at least 3 times a week; are on your feet or moving about for most of your day; you mostly walk to do errands and rarely drive.

Multiply your body weight by 16–18.

Calories burned by a 150-pound woman: 2,400–2,700
