Humans continue producing new brain cells at least into their 90s, according to a new study that is the latest contribution to a longstanding debate among neuroscientists. The findings could also help doctors diagnose Alzheimer’s earlier, and recommend measures like exercise to stimulate the production of new brain cells.

Earlier studies in other mammals had suggested brain cells are produced later in life, but the new study reveals the degree to which cell production, or neurogenesis, continues into old age in humans. It also showed how precipitously it declines in the first stages of Alzheimer’s disease, according to BBC News.

The study examined the brains of 58 individuals that were neurologically healthy when they died, all between the ages of 43 and 97. It was published in the journal Nature Medicine on Monday.

Since neurons go through a development process rather than starting out fully formed, researchers were able to discern newly formed neurons. They found that neurogenesis saw a “slight decrease” with the aging process, of about 300 fewer neurons per cubic millimeter in the dentate gyrus, a part of the hippocampus crucial for learning and memory, with each passing year of middle age.

In Alzheimer’s patients, there was a much more dramatic reduction in neurogenesis, from 30,000 per millimeter to 20,000 per millimeter at the onset of the disease.

“That’s a 30% reduction in the very first stage of the disease,” the study’s senior researcher, Dr. Maria Llorens-Martin, told BBC News. “It’s very surprising for us, it’s even before the accumulation of amyloid beta and probably before symptoms, it’s very early.”

Amyloid beta is a protein fragment that’s a marker of Alzheimer’s in the brain. Targeting the buildup of amyloid beta has been the main focus of research on treating the disease. But as recently as last week, trials on this method have failed, suggesting there is a more fundamental cause that needs to be addressed.

According to Llorens-Martin, understanding the causes of the decline of neurogenesis could help pave the way toward new treatments for both Alzheimer’s and normal cognitive decline.

“This is very important for the Alzheimer’s disease field because the number of cells you detect in healthy subjects is always higher than the number detected in Alzheimer’s disease patients, regardless of their age. It suggests that some independent mechanism, different from physiological ageing, might drive this decreasing number of new neurons.”

She says the next step will likely involve examining the brains of living individuals, to observe the change over time.

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