Beagle Weight Loss Understanding the Role of Age in Metabolism

Weight loss

Metabolism and weight loss: How you burn calories

Find out how metabolism affects weight, the truth behind slow metabolism and how to burn more calories.

By Mayo Clinic Staff

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.

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Oct. 08, 2022Show references

1. Goldman L, et al., eds. Obesity. In: Goldman-Cecil Medicine. 26th ed. Elsevier; 2020. https://www.clinicalkey.com. Accessed Sept. 13, 2022.
2. Preventing weight gain. Centers for Disease Control and Prevention. https://www.cdc.gov/healthyweight/prevention/index.html. Accessed Sept. 13, 2022.
3. Perreault L, et al. Obesity: Genetic contribution and pathophysiology. https://www.uptodate.com/contents/search. Accessed Sept. 13, 2022.
4. Piaggi P. Metabolic determinants of weight gain in humans. Obesity. 2019; doi:10.1002/oby.22456.
5. 2015-2020 Dietary Guidelines for Americans. U.S. Department of Health and Human Services and U.S. Department of Agriculture. https://health.gov/our-work/nutrition-physical-activity/dietary-guidelines/previous-dietary-guidelines/2015. Accessed Sept. 17, 2022.
6. Physical Activity Guidelines for Americans. 2nd ed. U.S. Department of Health and Human Services. https://health.gov/our-work/physical-activity/current-guidelines. Accessed Sept. 19, 2022.
7. Dietary supplements for weight loss: Fact sheet for health professionals. National Institutes of Health. https://ods.od.nih.gov/factsheets/WeightLoss-HealthProfessional/. Accessed Sept. 17, 2022.

See more In-depth

Does metabolism matter in weight loss?

Do you know people who complain about having a slow metabolism and how they barely eat anything yet still gain weight? Or have you met people who complain about someone they know who can eat whatever he or she wants including large portions of junk food due to a fast metabolism and apparently never gain weight? In both cases the individual usually ends by saying, "It's not fair!" These scenarios raise several very good questions:

• What role exactly does metabolism play in weight gain or weight loss?
• Is your metabolic rate determined by your genes? If so, can you speed up a slow metabolism through exercise, drugs, or certain foods?
• Is the importance of metabolism just a myth? Is weight gain or loss purely due to "calories in and calories out?"

The answer to these questions involves a mix of nature (genetic make-up) and nurture (the environment).

What Is metabolism?

Metabolism, or metabolic rate, is defined asthe series of chemical reactions in a living organism that create and break down energy necessary for life. More simply, it's the rate at which your body expends energy or burns calories.

Our bodies burn calories in several ways:

• through the energy required to keep the body functioning at rest; this is known as yourbasal metabolic rate(BMR). Your BMR is partly determined by the genes you inherit.
• through everyday activities
• through exercise.

Metabolism is partly genetic and largely outside of one's control. Changing it is a matter of considerable debate. Some people are just lucky. They inherited genes that promote a faster metabolism and can eat more than others without gaining weight. Others are not so lucky and end up with a slow metabolism.

One way to think about metabolism is to view your body as a car engine that is always running. When you're sitting still or sleeping, you're engine is idling like a car at a stop light. A certain amount of energy is being burned just to keep the engine running. Of course, for humans, the fuel source is not gasoline. It's the calories found in foods we eat and beverages we drink energy that may be used right away or stored (especially in the form of fat) for use later.

How fast your body's "engine" runs on average, over time, determines how many calories you burn. If your metabolism is high (or fast), you will burn more calories at rest and during activity. A high metabolism means you'll need to take in more calories to maintain your weight. That's one reason why some people can eat more than others without gaining weight. A person with a low (or slow) metabolism will burn fewer calories at rest and during activity, and therefore has to eat less to avoid becoming overweight.

Did you know...? Lean people tend to be more active during everyday activities than people who are overweight. How? They may "fidget" more that is, they tend to be in motion even when engaged in non-exercise activities. Whether this tendency to move more or less is genetically programmed or learned remains uncertain. But it can add or subtract hundreds of calories each day. Obese people expend more calories, on average, than lean people during most activities, in part because it takes more effort to move around. But they tend to be more sedentary, which makes it harder to get rid of body fat.

Weight control: Part nature, part nurture

It's part truth and part myth that metabolism is the key to weight. The rising tide of obesity in this country cannot be blamed entirely on an inherited tendency to have a slow metabolism. Genes do not change that quickly. Something environmental particularly changes in diet and exercising too little is a much more likely culprit. Age can be a factor, too, although new evidence suggests metabolism reaches a peak earlier in life and slows down much later than previously thought.

The reality is that for most people, excess weight is not all due to bad luck, thyroid trouble, or some other unexplained, uncontrollable external factor. For most of us, calories in/calories out has a strong influence on changes in weight over a lifetime.

Regardless of whether your metabolism is fast or slow, our bodies are designed to store excess energy in fat cells. So, if you eat and drink more calories (energy intake) than your body expends (energy output), you will gain weight. On the other hand, if you eat and drink fewer calories than are burned through everyday activities (including exercise, rest, and sleep), you'll lose weight. Our bodies are also programmed to sense a lack of food as starvation. In response, our BMR slows down, which means fewer calories burned over time. That's one reason why losing weight is often difficult.

Perhaps the most remarkable thing about all of this is how little our weight tends to change from day to day. In fact, only a few excess calories each day could lead to significant weight gain at the end of a year. For example, eating an extra apple each day could lead to a weight gain of nearly 9 pounds by the end of one year! Similarly, even a small reduction in calories each day could lead to remarkable weight loss. Eliminating dessert one day a week would lead to weight loss of nearly six pounds in a year.

Many theories exist to explain what controls the amount of food a person eats, when they feel full, and why they eat past the point of feeling full. These factors also play a role in determining one's ultimate weight. One theory is that each of us has a set point a weight at which the body is "happy." If you lose weight, you'll feel hungry until you get back to your set point weight. That may be another reason it is so hard to lose excess weight. But how that set point is determined and whether there truly is such a mechanism remain uncertain.

The bottom line

When it comes to weight, metabolismisimportant and does have a genetic component. Whether you can change your metabolic rate, however, is a matter of considerable debate. Clearly, youcanchange how you balance the calories you take in against the calories you burn up through activity, which can change your weight.

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