Why a Slowing Metabolism Probably Isn’t Causing Your Weight Gain

Many of us are carrying a little bit more weight now than when we were in our 20s. However, as tempting as it is to blame this weight gain on a slowing metabolism, that may not actually be the case, according to a study released last week in the journal Science.

In this study, scientists found that once you adjust for weight and body fat percentage, metabolic rates remain stable from ages 20 to 60. What this means is that during that time, your body doesn’t change how many calories it burns just because of your age — instead, your metabolism is tied to your body size and the amount of body fat versus lean mass that you carry.

Why we are still learning how our metabolism works

If you are wondering how a single study can upend everything we thought we knew about our metabolism, that’s because most of our conventional wisdom isn’t based on solid, substantive evidence. The reason for this is because the studies that can answer these questions are expensive, which made it impossible to get a big enough sample size that would offer a rigorous, evidence-based answer of how our metabolism changes with age.

“It’s hard to believe that in 2021, a paper that says, ‘This is how metabolism changes with body size and over the lifespan,’ is news,” said Herman Pontzer, an associate professor of evolutionary biology at Duke University, who was one of the lead authors for the study. “We haven’t been able to get the sample sizes. Getting even 100 people in a metabolism study is usually considered a pretty big number.”

In order to get these numbers, Pontzer, along with about 80 collaborators, combined data collected from multiple labs using the “doubly labelled water” method, which is considered the gold standard in the field of metabolism studies. They compiled a database containing the metabolism data of more than 6,400 study participants, ranging in ages from 8 days to 95 years.

“We haven’t had those numbers to play with before,” Pontzer said.

This gave them a big enough sample size to finally answer the question of how our metabolism changes over the course of our lifetime.

How your metabolism changes over your lifetime

What they found was that, once you adjust for weight and body fat percentage, our metabolism peaks at 1 year old, declines by about 3% per year until you reach 20, then stays stable for the next 40 years, after which it will decline by about 1% per year.

In terms of our own metabolism, if your weight and body fat percentage stay the same during that time, your daily energy needs will not vary. However, if you remain the same weight but some of your muscle mass is replaced with fat, then your body will need more energy each day. However, this change would be a factor of weight and body fat percentage, rather than a fundamental change in your metabolism.

On a cellular level, “your cells are just as busy when you are in your 50s as they were when you were in your 20s,” Pontzer said. However, once you hit 60, your metabolism does change on a fundamental level, with your cells using a little bit less energy as time goes on.

The good news is that weight and body fat percentage are factors you can change. For example, if you have more muscle mass now than you did in your 20s, this would mean your body would have a higher daily energy requirement now than it did then.

They also found that, once you adjust for weight and body fat percentage, men and women burn calories at the same rate. Granted, women tend to have a higher body fat percentage than men, but if you compare men and women who are of the same weight and same body fat percentage, on average, “they would have the same energy expenditure,” Pontzer said.

What to focus on if you are gaining weight

Practically speaking, this means that if you are gaining weight, it’s not because your metabolism is slowing down. Instead, there are probably other factors at play, such as diet and exercise, which are all too easy to let slip in face of all the adult pressures that tend to hit when you are in your 30s and 40s.

This study doesn’t get into why we often gain weight as we get older, but it provides reassurance that a slowing metabolism is not the reason. It also gives us a roadmap for adjusting our weight that is simple (if not easy): Losing fat and/or increasing muscle mass will keep our calorie burn high. That means healthy eating and strength training are especially important as we get older.

When you are faced with a stocked pantry of highly processed food after a long, exhausting day from juggling work, family, and other responsibilities, it’s easy to eat just a little bit more, while also struggling to find the time to get in the activity your body needs to maintain your muscle mass. Over time, this adds up.

The good news is that the database that allowed Pontzer and his collaborators to finally answer these questions about our metabolism is now public access, which means any scientist who would like to use the data can, as long as their proposed research adheres to all ethical guidelines for using patient data. What this means is that we will be able to learn even more about how our metabolism actually works in the years ahead.

“The surprise from this work isn’t that it contradicts previous data, because there just wasn’t any data,” Pontzer said. “The surprise is that when you go and measure this, the biology is surprising in ways you don’t expect. I think there is going to be a lot here to learn.”