01/19/2022
We’ve all heard various versions of the same warning as we’ve gotten older:
“Just wait until you reach your 30s. You won’t be able to eat like that anymore!”
“Once you hit 40, exercise gets so much harder!”
“Once you reach your 50s, you’ll gain weight and everything will just start to hurt!”
I’m 48 years old and, to be fair, I can’t eat like that anymore, exercise is harder, I have gained weight and just about everything does hurt—and two years ahead of schedule!
However, the underlying implication of those warnings is that our metabolism is to blame and that the downward trends are all but inevitable. According to a new study, however, that does not appear to be the case.
Research recently published in the journal Science investigated daily energy expenditure throughout the human lifespan. The study was conducted by an international team of researchers, who analyzed the daily caloric expenditure of more than 6,400 people from around the world. The participants’ ages ranged from eight days to 95 years old.
While most large studies have measured basal metabolism—that is, how much energy the body uses to perform vital life-sustaining functions such as respiration and digestion—this accounts for only 50 to 70% of the calories people burn each day. This research also analyzed the energy burned to fuel all other activity, including both planned exercise and activities of daily living. Every calorie expended was accounted for, even those burned while fidgeting or thinking.
What the research team learned runs counter to a lot of the “common knowledge” around the way metabolism changes as we age.
As you might expect, energy needs skyrocket during the first year of life, as babies burn calories approximately 50% faster for their body size than adults. Interestingly, while rapid growth during this period accounts for some of that increased metabolism, the rate of calorie burn is not fully explained by the increase in weight. More research is needed to understand what else is driving the high metabolic rate during that first phase of life. After age 1, a person’s metabolism slows by about 3% per year until they reach their 20s.
Then, and here’s where things become counterintuitive, metabolism levels off and stays relatively consistent until around age 60. When body size was considered, even the growth spurt of adolescence and the changing physiology of pregnancy failed to drive an increase in metabolism. Even after age 60, metabolism declines fairly slowly, by only 0.7% per year.
Stated simply, when body mass is accounted for, metabolism remains virtually unchanged from around age 20 to age 60.
So, what’s going on here? A primary reason for the decline in metabolism is a reduction in muscle mass since muscle burns more calories than fat does. It appears that those physiological changes that take place as we pass from one decade into the next have more to do with lifestyle factors and body-composition changes than they do with a naturally slowing metabolism.
The findings of a new study on daily expenditure across the lifespan run counter to a lot of the beliefs many of us have about the way metabolism changes as we age. As the researchers discovered, the physiological changes that take place as we pass from one decade into the next likely have more to d...
