The lifetime risk for Alzheimer's disease at age 45 is one in five for women compared to one in 10 for men, according to the Alzheimer's Association

Men's brains shrink faster than women's, despite Alzheimer's disease being diagnosed more frequently in women, new research shows.

More than seven million Americans have Alzheimer's, and by 2050, that number is expected to increase to nearly 13 million, according to the Alzheimer's Association. The lifetime risk for Alzheimer's at age 45 is one in five for women compared to one in 10 for men, the association says.

A study recently published in Proceedings of the National Academy of Sciences set out to determine whether sex differences in healthy brain aging explain why women are more often diagnosed with Alzheimer's.

Researchers studied more than 12,000 brain MRIs from nearly 5,000 participants aged 17 to 95, and found men's brains shrank faster across more regions than women's brains. Notably, areas related to memory, emotion and sensory processing were affected, according to the study.

Women's brains showed slightly more expansion in their ventricles, which are fluid-filled spaces inside the brain, the study found.

"Our findings show that men experience greater structural brain decline across more regions, meaning that normal brain aging doesn't explain the sex difference in Alzheimer's rates," the lead author of the study, Anne Ravndal of the University of Oslo in Norway, told Fox News Digital.

Ravndal said the study's results "instead point toward other possible explanations, such as differences in longevity, diagnostic patterns or biological factors."

One theory that has been floated about the prevalence of Alzheimer's in women stems from the facts that aging is the primary risk factor for the disease and women live longer than men.

"It's unclear why this imbalance exists. Longevity has been an explanation, because age is such a strong risk factor for dementia," Paola Gilsanz of the Kaiser Permanente Northern California Division of Research said in a 2020 article from the Alzheimer's Association.

"But now we see that doesn't explain everything. So we're looking at a variety of factors, including those that are biological and those that are social or cultural."

New Alzheimer's research shows male brains shrink faster than female's