NEW YORK — Walking three times a week may improve memory in older adults and help prevent mental decline as people age, a study by U.S. researchers found.

The study of adults ages 55 and older found a 2 percent expansion of the hippocampus, the brain’s memory processor that can shrink during middle age, in those who walked 40 minutes, three times a week, for a year.

The findings were reported in the Proceedings of the National Academy of Sciences.

Researchers said the study shows the typical, age-related shrinkage of the hippocampus isn’t “inevitable” and can be reversed with moderate exercise.

“You can think of this improvement as winding back the clock about two years in brain health,” said Arthur Kramer, a professor of neuroscience and psychology at the University of Illinois at Urbana-Champaign. “It really is surprising that such modest changes … can have such dramatic benefits.