Metabolism Myths and Facts, Checked Against the Research

Metabolism gets blamed for a lot: weight gain in middle age, why some people can eat anything, and why diets stall. It is also the target of countless products promising to "boost" it. Some of these beliefs hold up, and many don't. Here is what research actually shows about how the body uses energy, sorted into myths and facts.

What metabolism is

Metabolism is the set of chemical processes that keep the body alive, including turning food into energy. In everyday conversation, it usually means how many calories the body burns. Total daily energy use has three main parts:

Basal or resting metabolic rate: the energy needed to keep the body running at rest, including breathing, circulation, brain function and maintaining body temperature. For most people this is the largest share, often around 60 to 70 percent of daily energy use.
Physical activity: both planned exercise and everyday movement, such as walking, standing, fidgeting and housework, known as non-exercise activity thermogenesis, or NEAT. This is the most variable part between people.
The thermic effect of food: the energy used to digest and process food, roughly 10 percent of daily calories.

Myth: metabolism slows down steadily from your 20s

Mostly false. A large 2021 study published in Science, which analyzed energy expenditure data from more than 6,000 people aged from eight days to 95 years, found that once adjusted for body size and composition, metabolism stays remarkably stable from about age 20 to 60. It declines slowly after 60, by about 0.7 percent a year. Weight gain in midlife is more likely linked to changes in activity, muscle mass, sleep, stress and eating habits than to a sudden metabolic slowdown.

Fact: larger bodies and more muscle burn more energy

People with more total body mass burn more calories, including at rest. Muscle tissue burns more energy than fat tissue, though the difference is smaller than often claimed: roughly 13 calories per kilogram of muscle per day at rest compared with about 4.5 per kilogram of fat, according to commonly cited estimates. Building muscle through strength training helps, but the main benefits are for strength, function and health rather than a dramatic metabolic increase.

Myth: some foods "boost" metabolism enough to matter

Largely false. Caffeine, green tea and chili peppers can slightly increase energy use for a short time, but the effect is small and unlikely to make a noticeable difference to weight on its own. Protein does have a higher thermic effect than carbohydrates or fat, and it also helps with fullness and preserving muscle, which is one reason higher-protein diets can help with weight management.

Myth: eating small meals often stokes the metabolism

False. The thermic effect of food depends on how much you eat in total, not how often. Studies comparing frequent small meals with fewer larger meals of the same total calories have found little difference in energy use. Meal timing can be chosen based on hunger, convenience and preference.

Fact: dieting can slow metabolism somewhat

When people lose weight, they burn fewer calories, partly because a smaller body needs less energy and partly because of metabolic adaptation, a reduction beyond what body size alone predicts. A widely discussed 2016 study of contestants from the television show The Biggest Loser found that years after rapid weight loss, their resting metabolism remained lower than expected for their size. Slower, more moderate weight loss, adequate protein and strength training may help reduce muscle loss, though adaptation is a normal biological response.

Fact: some medical conditions affect metabolism

An underactive thyroid can slow metabolism and cause tiredness, weight gain and feeling cold, while an overactive thyroid can speed it up. Some medications, such as certain antidepressants, antipsychotics and steroids, can affect weight. If you notice unexplained weight changes with other symptoms, a doctor can check thyroid function and other causes.

Myth: thin people have fast metabolisms

Partly false. There is some natural variation in metabolic rate between individuals, but differences in appetite, everyday movement and eating patterns often explain more. People who seem to eat a lot without gaining weight may also move more throughout the day or eat less at other times.

What actually helps

Move more through the day. Walking, taking stairs and standing breaks add up through NEAT.
Do strength training two or more days a week to maintain muscle, especially with age.
Eat enough protein, spread through meals.
Sleep well, since short sleep affects appetite hormones and food choices.
Avoid extreme diets, which can increase muscle loss and are hard to sustain.

Be skeptical of supplements marketed as "metabolism boosters." Many have little evidence, and some contain stimulants that can be harmful for people with heart conditions or high blood pressure.