Caregivers who are stressed from caring for their spouse are at nearly double the risk of suffering a stroke, a new study finds.

There are about 43.5 million adults in the U.S. who are unpaid caregivers. One in every 10 are those who care for their husband or wife, and they are also nearly four times more likely to put in the highest number of hours, according to AARP.

For this study, University of Alabama researchers followed 24,232 African-American or white men and women over age 45 who were free of stroke and contacted them every six months by phone and also by reviewing medical records of suspected strokes. They were enrolled in a study from 2003 to 2007.

The study found that spousal caregivers who reported being under moderate or severe strain had a 5.1 percent rate of stroke compared with 2.6 percent of individuals that were not caregivers. The subjects were matched according to age, race, sex, region, relationship status, education, income, and blood pressure, as well as other major stroke risk factors.

Caregivers under stress were also at a higher risk of stroke regardless of the family relationship but this finding fell just short of statistical significance, the researchers said of their study, which was presented at the Epidemiology and Prevention and Lifestyle and Cardiometabolic Health Scientific Sessions meeting this week in Phoenix.

Caregivers who reported being at no or low stress were found not to be at any increased risk, even if they cared for a spouse, the researchers found.