What did the 2021 Science study find?
That adult metabolism is remarkably steady through midlife. Researchers pooled measurements of total daily energy use, taken with the doubly labeled water method, from 6,421 people aged 8 days to 95 years in 29 countries. Their findings in Science fell into four life stages:
| Age range | What happens to energy use, adjusted for body size |
|---|---|
| Birth to about 1 year | Rises to roughly 50% above adult levels |
| About 1 to 20 years | Declines slowly toward adult levels |
| About 20 to 60 years | Stays stable |
| After about 60 | Declines by about 0.7% a year |
The adjustment matters. A bigger body burns more in total, so the researchers compared people after accounting for their fat-free mass and fat mass. Measured that way, a 45-year-old man’s tissues are not burning calories much more slowly than they did at 25.
Then why do so many men gain weight after 40?
Because total burn falls for other reasons. Three show up again and again in men: less muscle, less movement and lower testosterone. None of them is a fixed fate, which is the useful part of the answer.
How much does muscle loss matter?
A lot, because muscle is a large share of the tissue you are paying to maintain. MedlinePlus notes that people tend to lose lean tissue after age 30, that body fat rises steadily after 30, and that fat builds up toward the center of the body, including around the internal organs. Older people may carry almost a third more fat than when they were younger.
That shift explains why a man can weigh the same at 45 as at 30 and still look and feel different. The scale holds steady while muscle turns into fat. Our page on muscle loss in men after 40 covers what speeds it up and how to slow it.
Does moving less explain the rest?
Much of it. Desk jobs, commutes and family schedules quietly cut daily movement and hard physical work out of the week. The Science study also tied the decline in older adults partly to lower fat-free mass and lower physical activity levels.
The CDC’s guidelines for adults call for at least 150 minutes of moderate activity each week plus muscle-strengthening work on two or more days. For men over 40, the strength days matter most, because they rebuild the muscle that keeps total burn up.
Where testosterone fits in
Testosterone falls gradually with age and is tied to muscle and fat. In the Massachusetts Male Aging Study, which followed men aged 40 to 70, total testosterone fell by about 1.6% a year within the same men over time, and the more usable bioavailable fraction fell by 2% to 3% a year. Men in good health, without chronic illness, obesity or heavy drinking, had androgen levels 10% to 15% higher.
That link runs both ways. Lower testosterone makes it harder to keep muscle and easier to gain fat, and extra fat tends to lower testosterone further. Our page on whether low testosterone makes it harder to lose weight explains that cycle, and weight gain after 40 in men pulls the causes together.
Related question: does losing weight raise testosterone?
Often, yes, in men whose low testosterone is tied to excess weight. The size of the change depends on how much weight comes off. Our article on whether weight loss raises testosterone covers the research.
How Ultimate Male looks at midlife weight gain
At Ultimate Male, a man who says his metabolism has slowed gets measured rather than reassured. At the San Gabriel or Downey clinic, labs include morning testosterone, thyroid and blood sugar markers, and a body composition analysis shows how much of your weight is muscle versus fat.
From there, a PA-C or MD talks through the options that fit what the numbers show. That might be a strength and protein plan, medical weight loss with semaglutide or tirzepatide, or, if two morning tests confirm low testosterone with symptoms, a discussion of testosterone therapy. Start with the free online assessment or a 10-minute call.
