As we age, our bodies undergo metabolic changes, but the rate at which that happens may surprise you. Many people look back on their teenage years with fond memories of eating whatever they wanted without consequence. Back then, you could probably eat a few pounds of junk food and not gain any weight. Previously, scientists thought that a faster metabolism during the teenage years explained this minor miracle.

However, a new study reveals that your metabolism peaks much earlier than in adolescence. And, it starts to slow down later than you’d think. The findings have been published in the journal Science.

“As we age, there are a lot of physiological changes that occur in the phases of our life such as during puberty and 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,” said study co-author Jennifer Rood, Ph.D., Associate Executive Director for Cores and Resources at Pennington Biomedical Research Center.

An international team of scientists, including four Pennington Biomedical researchers, analyzed the average calories burned by over 6,600 people. The participants spanned 29 different countries and ranged in age from one week old to 95 years. The vast age range allowed scientists to measure metabolic changes over an entire lifespan.

The study reveals surprising age-related metabolic changes.

metabolic changes
Most prior large-scale studies only measured basal metabolic rate (BMR) – the energy your body needs for life-sustaining functions. These vital functions include breathing, digesting food, pumping blood, circulation, and cell production. However, essential functions only account for 50-70% of the calories we burn every day. This doesn’t include the energy required for other activities like household chores, exercise, or even thinking and fidgeting.

To measure total daily energy expenditure, the team used the “doubly labeled water” method. This involves having a person drink water in which the hydrogen and oxygen have been replaced with naturally occurring “heavy” forms. It then measures how quickly the molecules get flushed out in the urine.

This method is considered the gold standard for measuring daily energy expenditure during everyday life. Scientists have utilized this technique to measure metabolism in humans since the 1980s. However, due to cost constraints, prior studies were too small to gather sufficient data. For this study, scientists obtained data from multiple labs to attempt to validate previous findings.

After gathering data and analyzing metabolism throughout a person’s life, scientists found surprising age-related metabolic changes.

Dr. Katzmarzyk said the following:

“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.”

Babies grow rapidly during their first year of life, and this requires a lot of energy. By the time they turn one, babies burn calories 50% faster for their body size than adults. And, scientists discovered that’s not just because of the explosive growth spurts babies go through.

“The babies grow rapidly, which accounts for much of the effect. However, after you control for this, their energy expenditures tend to be higher than what you would expect for their body size,” Dr. Martin said.

A growing infant’s fast metabolism may explain the effects of malnourishment during this developmental stage. Childhood hunger leads to a lower chance of survival and can affect health into adulthood.

“More research is needed to understand the metabolism of babies better. We need to know what is driving higher energy expenditures,” Dr. Martin said.

Metabolism levels out after the teenage years.

After a peak in infancy, a person’s metabolism declines by nearly 3% annually until their 20s. At this phase of life, metabolism becomes more stable. Interestingly, researchers found that the growing pains of adolescence didn’t result in increased calorie needs after taking body size into account. Even more surprisingly, they discovered that metabolism remained relatively stable from a person’s 20s through their 50s.

The team observed calorie needs during pregnancy increased, as expected. These findings imply that other factors may lead to metabolic changes in middle age. Furthermore, the data shows that metabolism doesn’t begin to decline again until after age 60. At this age, metabolism slows gradually by only 0.7% per year.

However, a person in their 90s requires 26% fewer daily calories than a middle-aged individual. This means there’s more at play than just metabolic changes. Researchers believe that lost muscle mass as we age may explain the decrease in calorie needs. Muscle burns more calories than fat, and older adults typically have a higher body fat percentage.

This still doesn’t explain all the metabolic changes that occur in older people, however.

Dr. Ravussin said:

“We took dwindling muscle mass into account. After 60, a person’s cells slow down.”

Researchers also considered varying activity levels among participants, but the patterns remained. Since both the body and brain transform aging, it’s often difficult to pinpoint what causes metabolic changes. However, this new research suggests that there’s more to it than just age-related changes in lifestyle or body composition.

“This study shows that the work cells do changes over the course of the lifespan in ways we couldn’t fully appreciate before. But massive data sets like the one we collaborated on allow us to answer questions we couldn’t address,” Dr. Ravussin said.

metabolic changes
Final thoughts on a study revealing unexpected metabolic changes throughout a lifespan

Most people probably believe that teenagers have the fastest metabolism. It seems that they can eat whatever they want and get away with it. However, a new study reveals that babies actually have a higher energy expenditure relative to their body size. It’s still a mystery why teens feel so ravenously hungry because the research found that their calorie needs don’t increase with body size.

The study did reveal that metabolism levels out in a person’s 20s through their 50s. It then begins to decline once a person turns 60. This shouldn’t come as a surprise since this stage of life usually requires less energy. However, it’s interesting to learn that babies need more energy than teenagers for their body size.

Understanding the metabolic changes that occur over a lifespan can potentially guide nutrition and health strategies.