As any parent knows, toddlers are bundles of energy. Now, the first comprehensive study of energy use over the human life span has quantified their burn rate: Infants between the ages of 9 and 15 months expend a stunning 50% more energy in 1 day than adults do, adjusted for body size. These wee dynamos consume and use up energy even faster than pregnant women and teenage boys, most likely to fuel their energetically expensive brains and organs.
“Little people are not burning energy like small adults,” says Duke University evolutionary biologist Herman Pontzer, who led the new analysis of data from around the world. “They are burning energy superfast … like a different species.”
But children also burn out fast. Their high metabolisms make them particularly vulnerable to stunted growth and disease if they don’t get the calories they need. Their cells may also metabolize drugs faster than those of adults, which means they may need more frequent doses. On the flip side, adults older than 60 begin to use less energy daily than younger people, and they may require less food or lower doses of medications, especially after age 90 when they use 26% less energy than middle-age people.
Scientists know surprisingly little about how much energy we burn throughout our lives. That’s because such data require so-called doubly labeled water studies, an expensive test in which people drink “heavy” water with unusual versions of hydrogen and oxygen that can be chemically traced. Scientists measure the amount of these “isotopes” excreted in urine, blood, or saliva over 24 hours for 1 week or more to calculate how much energy individuals use on average in a day.
In the new study, Pontzer and his colleagues created a large database that pooled the results of existing, high-quality doubly labeled water studies of 6421 people from 29 nations, between the ages of 8 days and 95 years. The team calculated the daily metabolic rates for each individual by taking their total daily energy rate from the doubly labeled water studies and adjusting it for body size and mass, as well as organ size. Lean tissue in organs uses more energy than fat, and children’s energetically expensive organs take up more of their body mass than in adults.
When the scientists plotted metabolic rates across life span, they found infants are born with the same metabolic rates as their mothers, when adjusted for their smaller body size. But between 9 and 15 months, they rev up their cells to burn energy faster, the team reports today in Science.
Children’s metabolic rates stay high until age 5, but the rate slowly begins to glide down until it plateaus around age 20. Interestingly, adult rates are stable until age 60, when they begin to decline. After age 90, humans use about 26% less energy daily, Pontzer says.
The study also found that pregnant women don’t have higher metabolic rates than other adults; their energy use and calorie consumption scales up with body size. “We know that pregnant women burn more calories, but they’re burning more energy just by virtue of being bigger,” Pontzer says.
The metabolic rate didn’t zoom up in hungry teenagers either, which also makes the findings seem counterintuitive. “When kids hit puberty, there seems to be a big spike in how many calories they’re consuming,” Pontzer says. “In your 30s and 40s, people often feel like they slow down; when menopause hits, you slow down more.” But metabolic rate doesn’t change at those times. Hormonal changes, stress, disease, growth, and activity levels influence appetite, energy, and body weight, he says.
Pontzer speculates that the metabolic rate speeds up in toddlers because developmental changes in the brain, other organs, or the immune system consume lots of energy. And it slows in older people as their organs shrink and they lose gray matter in their brains.
The growing brain is likely the key energy sucker in little kids, says biological anthropologist Chris Kuzawa of Northwestern University. Kuzawa did not participate in this study, but in 2014 his team found that the brains of young children consume a stunning 43% of all energy used by the body.
“This is interesting from a public health point of view,” adds Grazyna Jasienska, a biological anthropologist at Jagiellonian University in Poland who was not part of this study. She says these data should be factored into how many calories babies, pregnant women, and older adults need to consume, as well as doses of medicine. “If you think about undernourished children in many parts of the world, the may need more food than we previously thought.”