On Nutrition

I get a lot of questions about metabolism. What is it? Does slow metabolism cause weight gain? Does metabolism slow down at menopause? How can I boost my metabolism? I thought I knew the answers — until I read two seriously good research papers published in the past year on this topic.

First, a brief, not-too-sciencey definition. Metabolism refers to the processes that take place as your body converts food into the energy that fuels its functions. The “faster” your metabolism, the more energy your body uses. Your basal energy expenditure is the energy your body uses just to stay alive when you are at rest. You breathe, digest, circulate blood, grow and repair cells, regulate body temperature and manage hormone levels 24/7, so basal expenditure accounts for between 50% and 75% of your total energy expenditure, with the rest coming from energy you expend through activity.

A study published last August in the journal Science kind of debunked the idea that our metabolism slows when we hit middle age. Researchers obtained measurements of daily energy expenditure from a diverse sample of 6,421 people from 29 countries spanning ages 8 days to 95 years. They found that total energy use:

  • Rises rapidly after birth, peaking at about 1 year of age — which makes sense given how much growth and development happens in the first year of life
  • Declines slowly through childhood and adolescence
  • Plateaus between about ages 20 and 60
  • Starts to decline again — by age 90, total energy expenditure is about 26% less than that of middle age adults

Interestingly, the researchers did not find that metabolism increases during puberty or pregnancy — or slows around menopause. They did find that once differences in fat-free mass (including muscle and bone) and body fat were accounted for, metabolism was the same in both males and females.

OK … well does slow metabolism lead to weight gain? Can we alter our metabolisms? A paper published in January in the journal Nature by the same team of researchers again looked at measures of total energy expenditure — this time in 347 adults and 47 children — to determine whether slow or fast metabolism is an individual characteristic that persists over time. Not only did they find that it is in adults, but they found no evidence that adults with slow metabolism are likely to have more body fat, or that adults with fast metabolism are likely to have less body fat. Again, debunking both “common wisdom” and previous research.

Using fat-free mass as a proxy for physical activity — assuming that people with more muscle exercise more — the study’s findings also suggest that exercise have only a modest effect on total energy expenditure. That’s in line with previous research suggesting that when we increase physical activity, our bodies reduce basal energy expenditure to keep total expenditure within a relatively narrow range. In other words, while exercise is great for overall health, it may not contribute much to higher metabolism, or to weight loss, over time.

So the next time you see one of those ubiquitous “5 easy ways to boost your metabolism” articles, look away. And if you judge your exercise based on how many calories it burns or what it might do for your metabolism, think instead about how exercise might make you stronger, build your bones, enhance your mobility, help you shake off stress and generally make you feel better.