Infants have higher metabolism than adolescents or adults
New research related to metabolism and energy expenditure carried out by an international team of scientists have some surprising revelations: infants – not adolescents – have the highest metabolic rates across the entire lifespan; after the initial surge in infancy, a person’s metabolism slows by about 3% each year until our 20s, when it levels off into a new normal; and metabolic rates only decline by 0.7% after age 60.
“As we age, there are a lot of physiological changes that occur in the phases of our life such as during puberty and in menopause [but] the timing of our ‘metabolic life stages’ doesn’t appear to match the markers we associate with growing up and getting older,” pointed out Dr. Jennifer Rood, Associate Executive Director for Cores and Resources at Pennington Biomedical Research Center (PBRC) in Louisiana, US, who was part of the research team.
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The team had studied the average calories burned by more than 6,600 people of varying ages as they went about their daily lives, using the “doubly labeled water” method – it has been used to measure energy expenditure in humans since the 1980s and is considered the gold standard for measuring daily energy expenditure during normal daily life, outside of the lab.
The method is essentially 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. In this research, multiple labs fed their data into a single database to better analyse energy expenditures.
According to Dr. Peter Katzmarzyk, Associate Executive Director for Population and Public Health Sciences at PBRC, infants had the highest metabolic rates of all – by their first birthdays, babies burn calories 50% faster for their body size than adults.
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.
Meanwhile, the growth spurts of adolescence didn’t generate an increase in daily calorie needs even after taking body size into account. People’s metabolisms were most stable from their 20s through their 50s. Calorie needs during pregnancy grew no more than expected. These findings suggest that other factors lie behind the so-called “middle-age spread.”
The data also revealed that our metabolisms don’t really start to decline again until after age 60, a gradual slowdown at only 0.7% a year – a person in their 90s needs 26% fewer calories each day than someone in midlife, for example.
The new research ultimately supports the idea that energy expenditure relies on more than age-related changes in lifestyle or body composition. More research is needed to understand the molecular mechanism changes over the course of the lifespan.
