Putting a little bit of weight on in middle age has often been waved away as a result of slowing metabolism. "I’m not burning those calories as quickly as I did when I was younger," people often say. But a landmark new study tracking energy expenditure in a large cohort aged from one week to 95 years may put an end to that popular reasoning.
The research found, among a variety of unexpected results, energy expenditure is remarkably stable between our 20s and late 50s, suggesting those extra pounds gained in middle age may be due to people simply eating more and moving less.
“There are lots of physiological changes that come with growing up and getting older,” says Herman Pontzer, co-author on the new study. “Think puberty, menopause, other phases of life. What's weird is that the timing of our ‘metabolic life stages’ doesn't seem to match those typical milestones.”
To calculate an individual’s energy expenditure the researchers looked at data gathered using a method called "doubly labeled water." The method involves subjects drinking a cup of water in which the oxygen and hydrogen molecules have been replaced with uncommon, heavy, non-radioactive isotopes. Tracking how quickly these isotopes are flushed out in urine allows researchers to calculate a person’s base rate of energy expenditure.
This method is not new. It has been used by researchers in humans since the early 1980s, however, this kind of isotope dosing is not a cheap or simple process, which has limited its use to small, more targeted studies.
This new study is the result of a massive international collaborative data-sharing effort. Dozens of scientists from all over the world shared 'doubly labeled water" metabolic data from individual small studies, resulting in the first broad investigation of metabolism across a human lifespan.
Data was studied from 6,421 subjects spanning 29 countries. The oldest subject in the study was aged 95, while the youngest was just eight days old.
The researchers found energy expenditure rapidly increases from birth for the first 12 months of life, before peaking at the age of one year, with infants burning calories around 50 percent faster than adults.
“Of course they're growing, but even once you control for that, their energy expenditures are rocketing up higher than you'd expect for their body size and composition,” adds Pontzer.
From this point metabolism slows consistently at a rate of around 3 percent per year until a person reaches their early 20s. Pontzer notes this was one of the surprising findings from the research as it was hypothesized puberty would cause some kind of change to metabolic activity. But it didn’t, regardless of individual growth spurts, teenagers did not suddenly burn calories more rapidly during that period of adolescence.
The next surprise was how stable energy expenditure levels were for the next few decades of life. Once metabolic activity stabilized in a person’s 20s it remained relatively constant until around the age of 60.
John Speakman, co-author on the research, says this finding pushes back against the anecdotal idea that our metabolism slows down over our 30s and 40s. Speaking to The Guardian, he frankly suggests we have few excuses for midlife weight gain other than we are simply eating more and exercising less.
“Previously there was a suggestion that metabolism might slow in your 30s and that was then thought to [cause] susceptibility to middle age spread,” explains Speakman. “We found no evidence to support that. So if you are piling the weight [on] and your waistline is expanding during your 30s and 40s, it’s probably because you are eating more food, then expending less energy.”
After the age of 60 metabolism begins to gradually slow down. The researchers calculate energy expenditure drops at a rate of around 0.7 percent per year.
This metabolic shift detected as beginning around the age of 60 aligns with the general increase in non-communicable age-related diseases that tend to arise around that time. It is hypothesized that these changes in metabolism could be playing a role in driving general age-related health decline.
Pontzer is cautious to note there are a huge array of factors that influence a person’s health as they age, so these new findings are not presented as a singular reason for all age-related disease. Rather, these extraordinarily novel metabolic findings affirm we still have plenty to learn about how our bodies change over the course of our life.
“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,” concludes Pontzer. “You really need a big data set like this to get at those questions.”
The new research was published in the journal Science.
Source: Duke University
