Think Your Metabolism Slows at Middle Age? Think Again: New Study Shows Metabolism Changes With Age, But Not Until Later Years

From swallowing food to supporting cell growth to supplying oxygen to the brain, all of life’s essential movements and mechanisms — no matter how small, automatic, or unnoticed — require energy. And that energy is churned out by our metabolism, the complex series of reactions that turn the food we eat into fuel our cells can use.   

Many people have the preconceived notion that our metabolic function slows down with age. “I used to be able to eat anything and stay stick thin, but not anymore,” you might say. Or, “I can’t even look at a donut without gaining weight,” your best friend bemoans. But, while a slowing mid-life metabolism might be partially true for some people, a recent study published in the journal Science finds that this is not the case for most. Authored by Pontzer and colleagues, this large international study shows that metabolic function doesn’t decline until much later than previously thought — typically not until our 60s, well past the commonly touted middle-aged metabolic slump.

Feeling the Caloric Burn

In this study of almost 6,500 participants with ages ranging from 1 week to 95 years, a collaborative team primarily from Pennington Biomedical Research Center in Louisiana and Duke University in North Carolina used a method called “doubly labeled water” to track how many calories each person burned. Essentially, this test measures how quickly we break down carbon-containing molecules like glucose (sugar) to release energy in our bodies.

While doubly labeled water is the gold standard for calculating energy expenditure, it is an expensive and difficult test to implement on large groups of people, leading to many metabolism studies being contained to small, undiverse populations. To get around this, Pontzer and colleagues collaborated with multiple labs around the world to share data from their experiments, making this study the largest of its kind to assess total energy expenditure. 

As opposed to basal energy expenditure, which measures how many calories our bodies burn at rest while maintaining basic functions like breathing, total energy expenditure (TEE) measures, well, the total. Specifically, TEE is comprised of basal energy expenditure (accounting for 50-70% of TEE), the energy expended during any physical activities (typically 20-30% of TEE), and the energy it takes to digest, absorb, and convert food into energy (about 10%.) 

Metabolic Baby Boom

After pooling the data from this group spanning 29 countries, Pontzer and colleagues uncovered some surprising results. One such finding was that newborns and infants had the highest metabolic rates of the entire lifespan, with one-year-olds burning calories at a 50% faster rate than adults. (This was size-adjusted, of course — while adults burn more total calories, the babies burned more calories at a rate adjusted to their small size.)  

As Peter Katzmarzyk, Ph.D., Associate Executive Director for Population and Public Health Science at the Pennington Biomedical Research Center, states of these findings, “Some people think of their teens and 20s as the age when their calorie-burning potential hits its peak. But the study shows that, pound for pound, infants had the highest metabolic rates of all.”

Although we know that babies grow quickly — they can triple their weight in their first year of life, after all — their total energy expenditures were much higher than what the researchers expected for their body sizes. But they aren’t entirely sure why; as co-author Corby Martin, Ph.D., states, “More research is needed to better understand the metabolism of babies. We need to know what is driving higher energy expenditures.”

Older Adults Show Slower Metabolic Rates 

After infancy, the metabolic rates of children and teens slows by about 3% per year until they reach their early 20s. This was also unexpected — as many parents can attest, their growing adolescents can seem to eat them out of house and home. Yet, the teens’ overall metabolic rates were actually slowing down. Once reaching age 20, metabolism tended to stay relatively stable, with most people maintaining an expenditure plateau until their early 60s — some 20 years after many blame a slowing metabolism for their middle-aged weight gain.

Reflecting on the surprising nature of these results, study co-author Jennifer Rood, Ph.D., states, “As we age, there are a lot of physiological changes that occur in the phases of our life such as during puberty and in menopause. What’s odd is that the timing of our ‘metabolic life stages’ doesn’t appear to match the markers we associate with growing up and getting older.” 

However, what was expected was that older adults had lower energy expenditures, dropping by 0.7% per year after age 60. Upon reaching age 90, the older adults experienced caloric burns 26% lower than middle-aged adults, with subsequent declines in both muscle mass and organ-specific metabolism. These metabolic alterations could be significant contributors to the loss of muscle mass known as sarcopenia, as well as the commonly seen organ dysfunction that leads to disease with advancing age.

Metabolism Exits the Middle-Aged Weight Gain Blame Game

So, what do these results mean for us? Co-author Dr. John Speakman may have said it best: ”This suggests that if you are experiencing middle-age spread, it’s more likely to be because you are eating more rather than expending less.” In other words, you can’t blame weight gain on your malfunctioning metabolism anymore — until you reach your 60s, at least. 

One possible way to slow down this metabolic aging may be through maintaining adequate levels of the compound NAD+ (nicotinamide adenine dinucleotide), a coenzyme that is required by all of our cells but declines with age. As NAD+ is a crucial component of regulating cellular metabolism, it’s feasible that its declining levels in older age contribute to diminished energy expenditure. Although research is still in infancy, one recent study of women aged 55 to 75 found that taking supplemental NMN — a precursor to NAD+ — boosted sugar utilization in the muscles by 25%, suggesting that increasing NAD+ improves metabolic abilities in muscle.  

When it comes to clinical applications, this large international study sheds light on how human metabolic rates change over the entire lifespan, allowing for more accurate calculations of medication dosages, disease progression, and projected rates of healing. As the authors conclude in their paper, “Elucidating the processes underlying metabolic changes across the life course and variation among individuals may help reveal the roles of metabolic variation in health and disease.” 

References:

Okabe K, Yaku K, Tobe K, Nakagawa T. Implications of altered NAD metabolism in metabolic disorders. J Biomed Sci. 2019;26(1):34. Published 2019 May 11. doi:10.1186/s12929-019-0527-8

Pontzer H, Yamada Y, Sagayama H, et al. Daily energy expenditure through the human life course. Science. 2021;373(6556):808-812. doi:10.1126/science.abe5017

Yoshino M, Yoshino J, Kayser BD, et al. Science. 2021;372(6547):1224-1229. doi:10.1126/science.abe9985