Women’s brains are nearly four years younger than men’s, at least in how they burn fuel, according to scans performed by US researchers.

Scientists found that healthy women have a “metabolic brain age” that is persistently younger than men’s of the same chronological age. The difference is apparent from early adulthood and remains into old age.

The finding suggests that changes in how the brain uses energy over a person’s lifetime proceed more gradually in women than they do in men. While researchers are unsure of the medical consequences, it may help explain why women tend to stay mentally sharp for longer.

“Brain metabolism changes with age but what we noticed is that a good deal of the variation we see is down to sex differences,” said Marcus Raichle, a neurobiologist at Washington University school of medicine in St Louis. “If you look at how brain metabolism predicts a person’s age, women come out looking about four years younger than they are.”

The scientists used a brain scanning technique called positron emission tomography to measure the flow of oxygen and glucose in the brains of 121 women and 84 men aged 20 to 82. The scans revealed how sugar was being turned into energy in different parts of the volunteers’ brains.

In babies and young children, a process called aerobic glycolysis is increased to grow and mature the developing brain. It is scaled down in adolescents and young adults, then drops steadily in older people until it reaches a very low level by the time people reach their 60s.

To see how brain metabolism differed between the sexes, the researchers used a computer algorithm to predict people’s ages based on brain metabolism as measured by the scans. First, the scientists taught it to predict men’s ages from metabolism data gleaned from the male brain scans.

The striking result came when the scientists fed metabolism data from the women into the same program. While the program estimated male ages accurately, it judged the women’s brains to be, on average, 3.8 years younger than their real ages.

The scientists then flipped the analysis around. They trained the algorithm to predict women’s ages from data garnered from their brain scans. This time, when they fed metabolism data from the men into the computer, it estimated them to be 2.4 years older than they were. The way male brains burned sugar made them seem older than female ones of the same age.

“The great mystery is why,” said Raichle. The researchers suspect something other than hormonal differences are at work because the difference in metabolism stays the same when women enter the menopause.

“I refer to things like this as the curve balls of Mother Nature,” said Raichle. “Maybe women start off with this difference and it’s perpetuated throughout life.”

It is not clear what the difference means. The scientists are keen to investigate whether people with low glucose metabolism in particular parts of the brain are more prone to memory loss, learning problems and neurodegenerative diseases as they age.

“Is lower metabolism in such and such an area predictive of a particular event down the road? We don’t know,” said Raichle. “But if aerobic glycolysis is protective in some way, and the brain loses some element of it, that could be a problem.” Details of the study are reported in Proceedings of the National Academy of Sciences.

Mani Goyal at the university’s Mallinckrodt Institute of Radiology and first author on the paper, said men’s brains were not ageing faster than women’s. “They start adulthood about three years older than women and that persists throughout life,” he said. “What we don’t know is what it means. I think this could mean that the reason women don’t experience as much cognitive decline in later years is because their brains are effectively younger.”