Metabolism and weight loss: How you burn calories

Date Updated: 10/08/2022

Some people blame their weight on how their body breaks down food into energy, also known as metabolism. They think their metabolism is too slow. But is that really the cause? If so, is it possible to speed up the process?

It's true that the rate at which the body breaks down food is linked to weight. But a slow metabolism isn't usually the cause of weight gain.

Metabolism does help decide how much energy a body needs. But weight depends on how much a person eats and drinks combined with physical activity.

Metabolism: Converting food into energy

Metabolism is the process by which the body changes food and drink into energy. During this process, calories in food and drinks mix with oxygen to make the energy the body needs.

Even at rest, a body needs energy for all it does. This includes breathing, sending blood through the body, keeping hormone levels even, and growing and repairing cells. The number of calories a body at rest uses to do these things is known as basal metabolic rate, also called basal metabolism.

Muscle mass is the main factor in basal metabolic rate. Basal metabolic rate also depends on:

Body size and composition. People who are larger or have more muscle burn more calories, even at rest.
Sex. Men usually have less body fat and more muscle than do women of the same age and weight. That means men burn more calories.
Age. With aging, people tend to lose muscle. More of the body's weight is from fat, which slows calorie burning.

Besides the basal metabolic rate, two other things decide how many calories a body burns each day:

How the body uses food. Digesting, absorbing, moving and storing food burn calories. About 10% of calories eaten are used for digesting food and taking in nutrients. This can't be changed much.
How much a body moves. Any movement, such as playing tennis, walking to a store or chasing the dog, makes up the rest of the calories a body burns each day. This can be changed a lot, both by doing more exercise and just moving more during the day.

Daily activity that isn't exercise is called nonexercise activity thermogenesis (NEAT). This includes walking around the house. It also includes activities such as gardening and housework, and even fidgeting. NEAT accounts for about 100 to 800 calories used daily.

Metabolism and weight

You might want to blame a medical condition for slow metabolism and weight gain. But rarely does a medical condition slow metabolism enough to cause a lot of weight gain. Conditions that can cause weight gain include Cushing syndrome or having an underactive thyroid gland, also known as hypothyroidism. These conditions are uncommon.

Many things affect weight gain. These likely include genes, hormones, diet and lifestyle, including sleep, physical activity and stress. You gain weight when you eat more calories than you burn — or burn fewer calories than you eat.

Some people seem to lose weight more quickly and more easily than others. But everyone loses weight by burning more calories than are eaten. The bottom line is calories count. To lose weight, you need to eat fewer calories or burn more calories through physical activity. Or you can do both.

A closer look at physical activity and metabolism

You can't easily control the speed of your basal metabolic rate, but you can control how many calories you burn through physical activity. The more active you are, the more calories you burn. In fact, some people who seem to have a fast metabolism are probably just more active — and maybe fidget more — than others.

To burn more calories, the Physical Activity Guidelines for Americans recommends the following:

Aerobic activity. As a general goal, aim for at least 30 minutes of moderate physical activity every day. If you want to lose weight, maintain weight loss or meet specific fitness goals, you may need to exercise more.

Moderate aerobic exercise includes activities such as brisk walking, biking, swimming and mowing the lawn. Vigorous aerobic exercise includes activities such as running, heavy yardwork and aerobic dancing.
Strength training. Do strength training exercises for all major muscle groups at least two times a week. Strength training can include use of weight machines, your own body weight, heavy bags, resistance tubing or resistance paddles in the water, or activities such as rock climbing.

No magic bullet

Don't look to dietary supplements for help in burning calories or losing weight. Products that claim to speed up metabolism usually don't live up to their claims. Some may cause bad side effects.

The U.S. Food and Drug Administration doesn't ask for proof that dietary supplements are safe or that they work. Question the claims that are made. Always let your health care providers know about supplements you take.

There's no easy way to lose weight. To take in fewer calories than you burn, the 2020-2025 Dietary Guidelines for Americans recommends cutting 500 to 750 calories a day to lose 1 to 1.5 pounds (0.5 to 0.7 kilograms) a week. Add more physical activity to get to your weight-loss goals faster and maintain your weight loss.

A health care provider, such as a doctor or registered dietitian, can help you explore ways to lose weight.