Buzz
Many of us have noticed an increase in weight compared to our 20s. Although it's easy to attribute this to a slower metabolism, recent research suggests that's not necessarily the cause, as revealed in a study published last week in the journal Science.
The study indicates that when adjusting for body weight and fat percentage, metabolism rates remain consistent from ages 20 to 60. This suggests that your body doesn't naturally burn fewer calories as you age. Instead, the key factors are your body size and the balance between fat and lean mass.
Why we're still uncovering the mysteries of metabolism
You might be wondering how a single study can challenge everything we thought we knew about metabolism. The reason is that much of our common understanding lacks solid evidence. This is due to the high cost of the studies that could provide answers, making it difficult to gather large enough sample sizes for definitive, evidence-based conclusions about how metabolism changes with age.
“It’s hard to believe that in 2021, a paper stating, ‘This is how metabolism changes with body size and over the lifespan,’ is considered groundbreaking,” said Herman Pontzer, associate professor of evolutionary biology at Duke University and one of the lead authors of the study. “We’ve struggled to get large enough sample sizes. Even having 100 participants in a metabolism study is usually seen as a substantial number.”
To gather these data, Pontzer, along with 80 collaborators, pooled information from multiple labs using the ‘doubly labeled water’ technique, which is considered the gold standard in metabolism research. The result was a database of metabolism data from more than 6,400 participants, spanning ages from 8 days to 95 years.
“We’ve never had access to numbers like these before,” Pontzer remarked.
This substantial sample size finally allowed researchers to provide a clear answer to the question of how metabolism changes throughout a lifetime.
The way metabolism evolves throughout your life
Research indicates that after adjusting for body weight and fat percentage, metabolism reaches its peak at one year old, then declines by approximately 3% annually until the age of 20. From there, it remains steady for the next four decades, before decreasing by about 1% each year after turning 60.
Regarding your metabolism, as long as your weight and body fat percentage remain constant, your daily energy requirements will stay the same. However, if your weight stays the same but muscle mass is replaced by fat, your body will need more energy each day. Yet, this change relates to weight and fat percentage, not a fundamental shift 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,' says Pontzer. However, once you reach 60, your metabolism does undergo a fundamental change, with cells using slightly less energy over time.
The encouraging news is that weight and body fat percentage are modifiable factors. For instance, if you have more muscle mass now than in your 20s, your body will now require more daily energy compared to then.
The study also revealed that, when weight and body fat percentage are taken into account, both men and women burn calories at an equal rate. While women generally have a higher body fat percentage than men, those with the same weight and body fat percentage will, on average, have identical energy expenditure, according to Pontzer.
What to consider if you're gaining weight
In practical terms, if you're gaining weight, it's not due to a slowing metabolism. More likely, other factors like diet and exercise are influencing this change, which can easily be overlooked as you navigate the pressures of adulthood in your 30s and 40s.
While this study doesn’t explore why we tend to gain weight as we age, it offers reassurance that a slowing metabolism isn’t the cause. It also provides a clear guide for managing weight: losing fat and/or building muscle keeps calorie burn high. Thus, maintaining a healthy diet and incorporating strength training become especially crucial as we get older.
After a long, exhausting day balancing work, family, and other responsibilities, it’s tempting to overeat from a well-stocked pantry of highly processed foods, while also struggling to find time for the physical activity your body needs to preserve muscle mass. Over time, these small indulgences accumulate.
The exciting news is that the database which enabled Pontzer and his team to address fundamental questions about metabolism is now available to the public. This means that any researcher interested in using this data can access it, as long as their research follows all ethical guidelines for using patient data. This opens up the possibility of uncovering even more insights into the functioning of our metabolism in the coming years.
"The surprise in this study isn’t that it contradicts earlier data, because there was a lack of data to begin with," Pontzer said. "The real surprise is that when you go ahead and measure it, the biological findings turn out to be unexpected in ways you wouldn’t anticipate. I believe there’s still a lot more to uncover here."
