Metabolism gradually declines after peaking in infancy, finds large-scale study

16 Aug 2021 --- A new, large-scale study suggests the body’s metabolism – the rate at which it burns calories – peaks earlier and declines later than previously believed. The findings underscore the vital importance of nutrition during infancy – the time when the body’s metabolism is at its highest. 

“Some people think of their teens and 20s as the age when their calorie-burning potential hits its peak,” says Dr. Peter Katzmarzyk, who collaborated on the international study. He is the associate executive director for population and public health sciences at Pennington Biomedical Research Center in the US.

“But the study shows that, pound for pound, infants had the highest metabolic rates of all.”

The study is notable for its scale, analyzing the metabolisms of more than 6,600 participants across all ages. Previous studies were limited in size and scope due to cost. The team bypassed this limitation by sharing data from multiple labs in a single database. 

Increased needs in early life
Energy needs shoot up during the first 12 months of life. By their first birthdays, babies burn calories 50 percent faster for their body size than adults.

Adjusting for weight, infants had the highest metabolic rates of all.That’s not just because infants are busy tripling their birth weight in their first year.

“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,” affirms Dr. Corby Martin, professor and director of ingestive behavior laboratory at Pennington. 

An infant’s explosive metabolism may help explain why children who don’t get enough to eat during this developmental stage are less likely to survive and grow up to be healthy adults.

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

No burst in adolescence 
After the initial surge in infancy, a person’s metabolism slows by about 3 percent each year until our 20s, when it levels off into a new normal.

Surprisingly, the growth spurts of adolescence didn’t generate an increase in daily calorie needs after researchers took body size into account. 

“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,” explains study co-author Dr. Jennifer Rood, associate executive director for cores and resources at Pennington. 

People’s metabolisms were most stable from their 20s through their 50s. Moreover, calorie needs during pregnancy grew no more than expected.

People’s metabolisms are most stable from their 20s through their 50s.Gradual decline
The data also suggests that the metabolism doesn’t start to decline again until after the age of 60. The slowdown is gradual, only 0.7 percent a year. 

A person in their 90s needs 26 percent fewer calories each day than someone in midlife.

The researchers say that lost muscle mass as we get older may be partly to blame since muscle burns more calories than fat.

“We took dwindling muscle mass into account. After 60, a person’s cells slow down,” says Dr. Eric Ravussin, associate executive director for clinical science at Pennington. 

The patterns held even when differing activity levels were taken into account.

Decoding aging
Aging goes hand in hand with so many other physiological changes that it has been difficult to parse what drives the shifts in energy expenditure. 

However, the new research supports the idea that it’s more than 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,” says Ravussin. 

Employing “the gold standard”
To come up with a number for total daily energy expenditure, the researchers turned to the “doubly labeled water” method. It’s a urine test that involves having a person drink water in which the hydrogen and oxygen in the water molecules have been replaced with naturally occurring “heavy” forms and then measures how quickly they’re flushed out.

Scientists have used the technique – considered the gold standard for measuring daily energy expenditure during everyday life outside of the lab – to measure energy expenditure in humans since the 1980s. 

The results were published in the journal Science and supported by the United States National Science Foundation, the International Atomic Energy Agency, Taiyo Nippon Sanso and SERCON.

By Missy Green