The new study, published last month in the Proceedings of the National Academy of Science, was led by radiologist Manu Goyal and neurologist Marcus Raichle, both at the Washington University School of Medicine. It shows that, when measuring brain metabolism—that is, the rate at which it uses glucose and oxygen to power its energy-hungry activities—the brains of adult women consistently appear about three years younger than men’s brains do. This brain-age gender gap mirrors the difference in longevity, and it may tell us something important about how sex differences in neural development affect how long we keep our marbles—and, ultimately, how long we live.

In the early 1970s, Dr. Raichle was one of the first neuroscientists to use PET scans to look at cognitive function in a living person’s brain. Now 82, he is still at it. In this study, the Goyal-Raichle research team deployed PET scans to assess how much energy is consumed by an adult’s brain and exactly where in the brain the demand is greatest. “Glucose is like coal. It burns up in the brain and produces energy,” Dr. Raichle said. “But when you burn up things in the brain you produce byproducts that the brain doesn’t want hanging around,” he explained. Glucose makes energy and also does the mopping up afterward. Thus, how much glucose is used to power the brain’s daily activities, including its cleanup functions, will tell you something about how vigorous and youthful that brain is.

In the new study, 205 healthy adults between the ages of 20 and 82 underwent PET scans while lying quietly in the scanner with their eyes closed. The researchers used the scans to assess blood flow to various neural regions, and also to track two forms of glucose uptake: one that burns oxygen (oxydative) and one that doesn’t (non-oxydative). Based on a study they published in 2017, the researchers already knew that our brains use glucose in both ways when we’re young, but that non-oxydative glucose consumption takes a nose dive as we age. They also knew that, after puberty, blood flow in the brain declines less in women than it does in men.

These persisting gender differences in brain metabolism meant that women’s brains often looked younger than those of men their age. The researchers used machine learning to detect distinctive patterns in the brains they studied. “When we trained it on males and tested it on females, then it guessed the female’s brain age to be three to four years younger than the women’s chronological age,” said Dr. Goyal. Conversely, when the machine was trained to see female metabolic patterns as the standard, it guessed men’s brains to be two to three years older than they actually were. That difference in metabolic brain age added up to approximately a three year advantage for women.

These brain age differences persisted across the adult lifespan and were visible even when people’s brains showed the harbingers of Alzheimer’s disease. “These new findings provide yet more evidence, as if more were needed, of just how ubiquitous sex influences on brain function have proven to be, often showing up in places we least expect them,” said Larry Cahill, a neuroscientist who studies sex differences in the brain at University of California, Irvine. “The fact that we often struggle to understand what they mean—as happens in the rest of neuroscience—does not make them less important.”