Factors That Influence Metabolic Rate: A Closer Look

Larger bodies require more energy to sustain basic functions — maintaining cell membranes, circulating blood, regulating temperature — simply because there is more tissue to support. This is why body size is one of the strongest predictors of RMR across populations.

Equally important is what your body is made of. Lean tissue (primarily muscle) burns significantly more calories at rest than fat tissue. This distinction matters: two people of the same total weight can have meaningfully different metabolic rates if one carries substantially more muscle mass. To understand exactly how this works at the tissue level, see how muscle and fat tissue affect your resting energy burn.

On average, males tend to have higher resting metabolic rates than females of comparable size, largely because men typically carry a greater proportion of lean muscle mass and less body fat. Hormonal differences — particularly the role of testosterone in supporting muscle tissue — contribute to this disparity.

It's important to note that this represents a population-level average, not a fixed rule for any individual. Body composition varies considerably within sexes, and lifestyle factors can close much of the metabolic gap observed at a group level.

Metabolic rate generally declines with age, though the timeline and causes are more nuanced than a simple linear decline. Research suggests that major metabolic slowdowns are less dramatic in early and middle adulthood than once believed, with more significant shifts occurring later in life — partly due to hormonal changes and partly due to age-related muscle loss (sarcopenia).

Reduced physical activity levels as people age also contribute to the perceived slowdown, since less activity means less muscle maintained over time. For a full exploration of the biology behind this, see how age changes the way your body burns energy.

Several hormones act as metabolic regulators, adjusting how quickly or slowly the body processes energy. Thyroid hormones — particularly triiodothyronine (T3) — are among the most powerful: an underactive thyroid can significantly suppress metabolic rate, while an overactive thyroid can elevate it.

Insulin governs how cells take up glucose; cortisol (the stress hormone) influences fat storage and glucose availability; and leptin signals energy status to the brain, affecting both appetite and energy expenditure. When any of these systems are disrupted — whether by disease, chronic stress, or other factors — metabolic function can shift in measurable ways. Learn more in our companion piece on hormones that regulate metabolism.

Heritability studies suggest that a meaningful portion of metabolic rate variation between individuals has a genetic basis. Some people are born with a higher baseline tendency to expend energy; others are naturally more efficient (i.e., burn fewer calories for the same physiological output). This partly explains why family history can influence body weight patterns even when lifestyle habits appear similar.

However, genetics set a range of possibility rather than a fixed destiny. Environmental and behavioral factors — particularly physical activity and muscle maintenance — can meaningfully shift outcomes within that genetic range. For a research-grounded perspective on individual variation, see fast metabolism vs. slow metabolism: what the evidence really shows.

Exercise influences metabolism in two distinct ways. First, physical activity directly increases total calorie expenditure during the activity itself. Second — and more durably — resistance training builds and preserves lean muscle mass, which raises resting metabolic rate over time.

Non-exercise activity thermogenesis (NEAT) — the energy used in everyday movement such as walking, fidgeting, and standing — also varies significantly between individuals and can account for hundreds of calories of daily difference. This underscores why total lifestyle movement, not just formal workouts, matters for metabolic health. Body composition factors related to activity are also explored in factors that influence body composition beyond diet and exercise.