Does metabolism really slow after 40?

Not in the way most people think. The largest study of human energy use found that, once you account for body size and composition, the calories people burn each day stay remarkably stable from age 20 to 60. What does change after 40 is how much muscle you carry, how much you move, and where fat goes, and those are things you can work on.

What the 2021 lifespan study found

In 2021, Herman Pontzer and an international team published an analysis in Science of 6,421 people aged 8 days to 95 years from 29 countries, 64 percent of them female. Total daily energy expenditure was measured with doubly labeled water, a well-validated method that captures the calories people burn during normal daily life, outside a laboratory.

Body size, especially fat-free mass such as muscle and organs, explained 83 percent of the differences in daily energy use. So the researchers adjusted for fat-free mass and fat mass before comparing ages. They found four phases:

Life stage	Energy expenditure, adjusted for body size and composition
Birth to about 1 year	Rises quickly to about 50 percent above adult levels
1 to 20 years	Declines slowly to adult levels
20 to 60 years	Stable, including during pregnancy
After about 60	Declines about 0.7 percent a year; about 26 percent lower in the 90s

Three details matter for women in midlife:

• Sex made no difference once fat-free mass and fat mass were accounted for. Women burn fewer calories than men on average because they tend to be smaller and carry less muscle, not because their metabolism runs slower.
• Resting metabolism may turn earlier than total expenditure. For basal (resting) expenditure alone, the analysis placed the turning point around age 46, but the authors noted few resting measurements between 45 and 65, so that estimate is imprecise.
• Individuals vary. Even after accounting for size, composition, sex and age, people differed by more than 20 percent in either direction.

The study compared people of different ages rather than following the same people for decades, and it did not examine menopause status.

What other studies add

A 2023 study measured resting metabolism in 120 Finnish women with indirect calorimetry and DXA scans. After adjusting for body composition, women aged 41 to 58 burned about 88 fewer calories a day at rest than women aged 22 to 38, and 16 daughters burned about 100 calories a day more than their own midlife mothers. Within the midlife group, resting metabolism was not lower in postmenopausal women and did not vary with estradiol or FSH levels. The authors concluded that resting metabolism declines with age, and that menopause may not contribute.

Together, these studies suggest a small age-related dip in resting metabolism, on the order of 100 calories a day by midlife, without a menopause-driven crash.

What does change after 40

Body composition. The Study of Women’s Health Across the Nation (SWAN) followed US women through the transition with repeated DXA scans. At the start of the transition, the rate of fat gain doubled and lean mass began to fall, trends that continued until about two years after the final period. Because fat-free mass is the main driver of how many calories you burn, losing muscle slowly lowers your daily needs. See muscle loss in midlife.

Activity. A Canadian study followed 102 women, average age 50, for five years with yearly measurements. In women who became postmenopausal, total daily energy expenditure fell, mostly because of less physical activity. Resting expenditure stayed stable. Time spent in moderate activity fell and sedentary time rose during the transition.

Everything around it. Night sweats, broken sleep, joint pain and busy midlife schedules all make it harder to stay active. Our weight gain guide covers how these pieces fit together.

What helps

Lever	What it does	Evidence
Strength training	Preserves or builds the fat-free mass that drives daily energy use	Resistance training raised resting metabolic rate by about 96 calories a day versus controls in a 2020 meta-analysis (postmenopausal women were excluded)
Aerobic activity and daily movement	Raises total expenditure through activity	Did not change resting metabolic rate in the same meta-analysis, but activity is the part of expenditure that fell in midlife women
Sitting less	Adds up across the day	Federal guidelines: move more and sit less; any amount of moderate-to-vigorous activity brings benefit
Protein and overall diet	Supports muscle and helps manage intake	See perimenopause diet basics

A practical week, following the Physical Activity Guidelines for Americans:

• Two or three strength sessions working all major muscle groups, each set taken close to the point where another repetition would be hard.
• 150 to 300 minutes of moderate aerobic activity, such as brisk walking, or 75 to 150 minutes of vigorous activity, spread across the week.
• More everyday movement: walking meetings, stairs, short walks after meals, standing breaks.

Be wary of products or programs promising to “reset” or “boost” metabolism. Claims that a supplement can meaningfully raise metabolism are not backed by good evidence.

Who should check with a clinician first

Most healthy women can increase activity gradually without a medical visit. Check first if you have heart disease, diabetes, high blood pressure that is not controlled or another chronic condition, or symptoms such as chest pain, dizziness or joint pain.

When to see someone

A slower-feeling metabolism is often blamed for changes that have other causes. See a clinician if you have:

• weight gain of more than a few pounds over a few months without a clear reason
• fatigue, feeling cold, constipation, dry skin or heavier periods, which can point to an underactive thyroid
• weight change after starting a new medication
• loud snoring, gasping at night or daytime sleepiness
• rising blood pressure, blood sugar or waist size despite stable weight

An endocrinologist or a menopause-informed clinician can check thyroid function, blood sugar and other causes. You can find one near you.