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Many people gain weight as they age, and they often attribute this to an unavoidable decline in energy expenditure. However, there is little research assessing the effects of age alone on TDEE or BMR.

The study

This retrospective cohort study assessed the effects of age, sex, and body composition on TDEE, using two sets of data:

  • Data on 6,421 people from 29 countries, aged eight days to 95 years, whose TDEE had been assessed via doubly labeled water.
  • Data on 136 neonates and infants and 141 women before, during, and after pregnancy.

The results

After adjusting for the participants’ fat mass and fat-free mass, the authors calculated their size-adjusted TDEE and BMR. This allowed them to distinguish four phases of life with differing levels of TDEE:

  • Neonates, 0–12 months of age: In their first month of life, the neonates had a size-adjusted TDEE similar to adults’ and a size-adjusted BMR 22% lower than adults’. Then their TDEE and BMR both increased to ≈50% higher than adults’ for the rest of their first year of life.

  • Juveniles, 1–20 years of age: Throughout childhood and adolescence, TDEE and BMR increased along with fat-free mass, while size-adjusted TDEE and BMR declined at a rate of 2.8% per year. Although males had a higher TDEE and size-adjusted TDEE, sex didn’t affect the rate of decline in size-adjusted TDEE.

  • Adults, 20–60 years of age: TDEE, BMR, size-adjusted TDEE, and size-adjusted BMR all remained stable throughout this period, including during pregnancy, with no differences between sexes.

  • Older adults, >60 years of age: TDEE, BMR, size-adjusted TDEE, and size-adjusted BMR all declined during this period, with TDEE declining at a rate of 0.7% per year. Size-adjusted TDEE was 26% lower in adults aged >90 than in adults aged 20–60.


The results suggest that, contrary to popular belief, the body’s metabolism (BMR) does not “slow down” before old age, for either sex. Instead, most individuals who gain weight during their middle years do so because they eat too much, don’t move enough, or both.

Some people move less as they grow older and new professional and familial responsibilities eat away at the time they used to spend exercising; but most people, according to the findings of this study, don’t actually burn fewer calories in their 50s than they did in their 20s, so they don’t exercise less in their 50s than they did in their 20s (which often means: not at all).

In other words, people who overeat (i.e., eat more than they burn) in their 50s were already overeating in their 20s, but the caloric excess is small enough that the year-to-year weight gain isn’t apparent. For instance, if you gain just one pound a year, you won’t be able to see a difference between one year and the next, but your body at 50 will have visibly more fat than your body at 20.

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This Study Summary was published on September 1 2021.