In a previous study of ARIC participants, Gottesman's team found that heart risk factors in midlife were associated with higher levels of beta amyloid, a gummy protein that accumulates in the brains of Alzheimer's patients.
It's possible that strong blood flow is better at flushing beta amyloid out of brain cells, particularly in middle-aged people, Gottesman suggested.
Even if you failed to address these risk factors in middle age, you can still help your brain by adopting a heart-healthy lifestyle in old age, Gottesman and Fargo said.
"I would say it's never too late, but the earlier the better," Fargo said. "There are things you can do at any point in your life to reduce your risk."
The study also found other factors that appear to influence dementia risk:
Black people had a 36 percent increased risk compared with whites.
Those who did not graduate from high school had a 61 percent increased risk compared with high school graduates.
People carrying the APOE-e4 gene had a 98 percent increased risk compared with those who do not carry that gene.
The study results were published online Aug. 7 in JAMA Neurology.