Metabolism and weight loss: How you burn calories

By Mayo Clinic Staff

Some people worry that their weight is caused by the way 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 the body breaks down food is linked to weight. But slow metabolism isn’t usually the cause of weight gain.

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

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 the basal metabolic rate, also called basal metabolism.

Muscle mass is a key factor in basal metabolic rate. Basal metabolic rate also depends on other factors such as:

• 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 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 affect 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. You can’t change this 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. You can change this a lot, both by doing more exercise and by moving more during the day.

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

You might think a medical condition is a cause of slow metabolism and weight gain. But medical conditions rarely 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 called hypothyroidism. These conditions are uncommon.

Many things affect weight gain. These likely include genes, hormones, diet and lifestyle, such as 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 they eat. 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.

You can’t easily manage the speed of your basal metabolic rate, but you can affect 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.

The Physical Activity Guidelines for Americans recommends the following to burn more calories:

• Aerobic activity. As a general goal, aim for at least 150 minutes of moderate aerobic activity a week. Or get at least 75 minutes of vigorous aerobic activity a week. You also can get an equal mix of the two types.

  For even more health benefits, get 300 minutes a week or more of moderate aerobic activity. Exercising this much may help with weight loss or keeping off lost weight. But even small amounts of physical activity can be helpful. Being active for short periods of time during the day can add up and have health benefits.

  Moderate aerobic exercise includes activities such as brisk walking, biking, swimming and mowing the lawn. Aim for a brisk walk for 30 minutes five days a week to meet your goal. 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. One set of each exercise is enough for health and fitness benefits. Use a weight or resistance level heavy enough to tire your muscles after about 12 to 15 repetitions.

  Strength training can include the use of weight machines, your own body weight, heavy bags, resistance tubing or resistance paddles in the water, or activities such as rock climbing.

Don’t rely on 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 healthcare professionals know about supplements you take.

There’s no easy way to lose weight. To take in fewer calories than you burn, aim to cut 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 healthcare professional, such as a doctor or registered dietitian, can help you explore ways to lose weight.