Does metabolism slow with age?

Not until about 60. In a 2021 study of 6,421 people, the calories burned each day, adjusted for body size, held steady from age 20 to 60 and then fell about 0.7% a year. What changes before 60 is usually how much muscle you carry and how much you move, and you can change both.

Why does it feel like it slowed down?

At 25 you ate what you liked. At 40 you eat the same and the waistband disagrees. The obvious suspect is a metabolism that wound down somewhere in your 30s, and everyone around you says the same thing.

Think of a car that gets the same miles per gallon at year two and at year eight. The engine didn’t change. What changed was the driving: shorter trips, more time parked, a heavier load in the trunk. For most people the engine of their metabolism holds steady through middle age, and the driving changes.

The study that settled it

Strong Thousands of people, measured with the most accurate method there is for daily burn, across 29 countries.

In 2021, Herman Pontzer and dozens of colleagues pooled doubly labeled water measurements of 6,421 people aged 8 days to 95 years. That method counts the calories a person really burns over days to weeks of normal life. They found four stages.

The break in adult life came at about 63. Before that, a 50 year old burned what a 25 year old of the same size and build burned. After 60, the drop wasn’t only from smaller muscles. The organs themselves seemed to slow, burning about 20% less than their size predicted in people over 60.

So what does change in your 30s, 40s and 50s?

Muscle, slowly. In an MRI study of 468 adults aged 18 to 88, muscle as a share of body weight started falling from early adulthood, but the amount of muscle didn’t drop noticeably until the end of the 40s, mostly in the legs. A 2012 review put the loss at 3 to 8% a decade in adults who don’t train.

Muscle matters less to your calories than people think. A pound of muscle burns about 6 calories a day at rest, by the tissue rates confirmed in a 2010 study. Losing 5 pounds of it over a decade costs about 30 calories a day. Its bigger price is strength and how much you move. More in does a pound of muscle burn 50 calories?

Movement, a lot. Across doubly labeled water studies, the ratio of total burn to resting burn averages 1.7 to 1.8 in adults of working age and falls to about 1.4 by age 90. On a resting burn of 1,500 calories, the gap between 1.75 and 1.4 is about 525 calories a day. Much of it comes from moving less, not from a slower engine.

For women, the menopause years. In the SWAN study, which followed women through menopause with body scans, the rate of fat gain doubled at the start of the transition and lean mass began to fall, for about two years after the final period. Weight kept climbing at the same steady rate as before, so the scale didn’t show the change. The shape of the body did.

The case on each side

Both describe real people. The tables and formulas describe what typically happens with age, which includes losing muscle and moving less. The 2021 study shows that once body size and build are counted, the burn holds until about 60. Keep the body and you keep the burn.

What keeps your burn up

1. Lift two or three days a week. In a 2020 review that pooled 18 studies of adults under 65, lifting raised resting burn by about 96 calories a day compared with no exercise, with a likely range of about 45 to 147. Aerobic exercise alone didn’t show a clear rise.
2. Keep your steps where they were. Everyday movement is the part that falls most with age, and the part a step count shows you.
3. Eat protein at every meal. The 2025 to 2030 US Dietary Guidelines say 1.2 to 1.6 g per kg of body weight a day. The Gyroscope app personalizes yours, and the protein calculator shows your range.
4. Re-check your number from your scale. If your weight drifts up over a few months, measure your real maintenance instead of cutting blind.