How Human Metabolism Works: Why Rates Vary and What It Means for Health

Metabolism refers to every chemical process a cell carries out to keep the body running, from breathing and heartbeat to digesting a meal, according to the Cleveland Clinic. Two opposite processes make this possible: catabolism, which breaks down carbohydrates, proteins and fats into usable pieces such as glucose, and anabolism, which uses those pieces to build and repair tissue, including muscle and bone.

How the body turns food into energy

Not all of the energy the body uses each day goes to the same place. The Cleveland Clinic breaks total daily energy use into three parts: basal metabolic rate (BMR), the energy needed just to keep organs and cells functioning at rest; the energy spent digesting and processing food; and the energy used for physical movement.

• Basal metabolic rate (BMR): covers 60% to 70% of total daily energy use, according to the Cleveland Clinic
• Processing food (thermic effect of food): uses about 10% of daily energy, per the same source
• Physical activity: accounts for the remaining energy, and is the portion that varies most between individuals

Why metabolic rates differ from person to person

BMR is not fixed, and it is not the same for everyone. The Cleveland Clinic lists body size and lean muscle mass as the biggest long-term drivers, because muscle tissue demands more energy to maintain than fat tissue does. Sex plays a role too: on average, men have a higher BMR than women, which the Cleveland Clinic attributes to typically larger body size and greater lean muscle mass linked to testosterone.

Age is another major factor. BMR tends to fall as people get older, driven largely by the gradual loss of muscle mass along with hormonal and neurological changes, the Cleveland Clinic reports. Genetics and race or ethnicity may also influence BMR, according to the Cleveland Clinic.

Several factors change metabolic rate only temporarily, the Cleveland Clinic notes: extreme heat or cold, illness or injury, stimulants such as caffeine and nicotine, skipping meals, and shifts in thyroid hormone levels. Pregnancy raises energy needs due to increased body mass and fetal development, while breastfeeding raises energy needs by roughly 15% to 25% to support milk production, and menopause brings its own hormonal changes, including reduced lean muscle mass, that affect metabolic rate, according to the Cleveland Clinic.

Beyond the metabolic machinery itself, the U.S. National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) points to broader influences on body weight: family history and genetics, health conditions such as hypothyroidism, polycystic ovary syndrome and Cushing's syndrome, certain medications (including some used for epilepsy, depression, diabetes and high blood pressure), sleep duration, and access to healthy food and safe places to be active.

How metabolism changes over a lifetime

A 2021 study published in the journal Science, led by Herman Pontzer and summarized by Duke University, measured daily energy expenditure in more than 6,600 people across 29 countries, ranging in age from about one week to 95 years old, using the doubly labeled water method, a technique that tracks isotopes in the body to measure total energy use in daily life rather than only at rest. The researchers found that, once body composition is accounted for, metabolism moves through four distinct stages rather than falling in a straight line from adulthood onward.

What this means for weight and health

The Cleveland Clinic cautions against a common assumption: that body weight is mainly a product of having a 'fast' or 'slow' metabolism. Metabolic rate, the clinic states, is 'rarely the sole cause of weight gain or loss,' and metabolic rate on its own does not determine body size, nor does body size on its own determine metabolic rate.

NIDDK frames body weight as the outcome of many interacting factors, not metabolism alone, listing genetics, underlying health conditions, medications, sleep and environment alongside diet and activity as elements that affect weight over time. Because BMR already accounts for most of a person's daily energy use, and because factors such as age, sex, muscle mass and underlying health conditions shape it in ways that are mostly outside a person's immediate control, researchers describe metabolic rate as one piece of a larger, individualized picture rather than a single lever that explains weight on its own.