A Study Uncovers When Metabolism May Really Start To Peak & Decline

I've heard many a middle-aged family member tell me their metabolism "just isn't what it was in their 20s." And while aging does usher in several changes, a study published in the journal Science says changes in your energy expenditure (i.e., metabolism) may not happen as early as you'd think. 

To get the most accurate and comprehensive data on metabolism's average peak and decline, researchers gathered various studies into one single database, called the IAEA Doubly Labelled Water (DLW) Database. Together, the researchers analyzed the daily energy expenditure of more than 6,600 people between 8 days and 95 years old. 

Much to the authors' surprise, growth spurts in puberty did not play a role in a speedy metabolism. The actual peak happens much, much earlier in life—as in, the first 12 months. 

At 1 year old, babies are burning calories 50% faster for their size than an adult does. "Something is happening inside a baby's cells to make them more active, and we don't know what those processes are yet," study co-author Herman Pontzer, Ph.D., said in a news release. 

Following that first year of life, the research shows that metabolism slows down by about 3% every year until it levels to a new normal in the 20s.

Another surprise to the authors? Midlife (and menopause, for that matter) did not play a role in slowing metabolism.

Between participants' 20s, 30s, 40s, and 50s, their energy expenditures remained stable.

Loss of muscle mass and the slowing down of cells are two possible explanations for this change later in life, Pontzer explained. "All of this points to the conclusion that tissue metabolism—the work that the cells are doing—is changing over the course of the lifespan in ways we haven't fully appreciated before," he said.