Does your metabolism crash at 40? No, but your body composition shifts.

The 2021 study, published in Science, pooled doubly labeled water measurements, a lab method that tracks calories burned in everyday life, from 6,421 people aged 8 days to 95 years in 29 countries. Sixty-four percent were female. After adjusting for body size and composition, daily energy use was stable from about 20 to 60, with no difference between women and men. It started falling at about 60, by 0.7% a year, and people in their 90s burned about 26% less than middle-aged adults.¹

What does change is the mix. SWAN tracked 1,246 women with body-composition scans for years around their final period. Before the transition, fat mass grew by about 0.25 kg (half a pound) a year. During it, that rose to 0.45 kg (about a pound) a year, and lean mass, mostly muscle, began shrinking by 0.2% a year. Those shifts continued until about two years after the final period, then leveled off. Weight didn’t speed up at the transition, so the scale may not show it.²

This week: count the days you did something muscle-strengthening, such as bands, weights or squats. US guidelines recommend muscle-strengthening activities that work all major muscle groups on 2 or more days a week.³

Ask your clinician: “Since the scale may not show it, should we track my waist or body composition, and is a strength program safe for me?”

The limitation: the energy study compared different people at different ages and wasn’t built around menopause. Its estimate for resting energy use, one part of the total, hinted at an earlier decline around 46, but the authors note there were few measurements between 45 and 65, so that figure is imprecise. SWAN is observational, and this analysis didn’t include Hispanic women.¹²

Go deeper: Weight and metabolism in midlife: what’s really changing ↗