Will My Metabolic Rate Slow Down As I Age?

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Looks like we have been very wrong about our metabolic rates. Whether our metabolic rate is fast or slow, it cannot predict weight gain. 

Metabolisms varies by 20 percent or more from person to person regardless of activity level or body composition (Rimbach et al, 2022). So for those of us who run or exercise to manage our weight, yes it does burn calories, but it does not boost our metabolism, at least not in the long term. 

The researchers also found that if you happen to be on the skinnier side, it is not that you have a faster metabolism (Rimbach et al, 2022).

The researchers measured metabolism using doubly labeled water. Participants in their studies drank water containing hydrogen and oxygen isotopes, which researchers can track in the body. How quickly they lost the isotopes through peeing, sweating and even breathing were monitored. These enabled them to ascertain how many calories are burned in that time, or what is known as total energy expenditure i.e. metabolism.

The total energy expenditures studied were measured at 2 time points, ranging from 2 weeks to more than 8 years. Body size and composition were adjusted for since more cells burned more calories and fat and muscle burned calories at different rates. 

The researchers found that the subjects' daily energy expenditures did not change over time. A person with high metabolism today will have it high in a few months or even a few years. Likewise, a person with slow metabolism today will still have it slow in a few months or a few years.

Here's the good news. The researchers found that having a slow metabolism did not make people more likely to gain weight. Nor did having a fast metabolism less likely to gain weight.This is attributed to the human brain's outstanding ability to match calories consumed to calories burned in the long run.

So, running or any exercise is not a magic metabolism booster. Exercise is complex and it affects how our body regulates all its cells, which in turn affects hunger, fullness, immune function and even behaviour. 

Those who exercise will obviously see day to day fluctations in the calories burned. More on long runs/ hard days compared to recovery days. Our bodies adjust how much energy we use during our non exercise hours to keep the total caloric burn within a fairly narrow range.

So, you do not gain weight as you age because your metabolism slows down (Pontzer et al, 2021). We have all been mislead by this myth. We should not use it as as excuse. You gained weight because we do not move and/ or exercise as much and because we may have lost some muscle mass.

Reference

Pontzer H, Yamada Y, Sagayama H et al (2021). IAEA DLW Database Consortium. Daily Expenditure Through The Human Life Course. Science. 373(6556): 808-812. DOI: 10.1126/ science.abe5017

Rimbach R, Yamada Y, Sagayama H et al (2022). Total Energy Expenditure Is Repeatable But Not Associated With Short-term Changes In Body Composition. Nat Commun 13,9. DOI: 10.1038/s41467-021-27246-z