(CNN) Losing your spouse or life partner and gaining the designation "widow" or "widower" is one of life's cruelest blows.

Now science believes that widowhood may hasten the development of a type of cognitive decline associated with Alzheimer's.

Over a three-year period, cognitive abilities declined three times faster in widowed adults with high levels of beta-amyloid -- a key marker for Alzheimer's -- than in married people with equally high levels, according to a new study published Wednesday in the journal JAMA Network Open.

Even for those without beta-amyloid accumulation and no signs of cognitive decline, the risk for dementia was greater for men and women who were widowed.

"Cognitively unimpaired, widowed older adults were particularly susceptible to Alzheimer disease clinical progression," the study concluded.

The study called for "increased research attention and evidenced-based interventions for this high-risk group."

"This is a very important study," said Dr. Richard Isaacson, who directs the Alzheimer's Prevention Clinic at Weill Cornell Medicine in New York City.

"The loss of a life partner has to be among the most devastating and stressful life events, and it is thus not surprising of this impact on cognitive health," Isaacson said.

"Alzheimer's is not a disease with one single cause," he continued, "and it's progression and severity can be impacted by a combination of life stressors, cardiovascular disease, lifestyle behaviors [such as diet and exercise], and genetics, among many other risk factors."

Mental impact of widowhood

More than 34% of American women and 11% of men ages 65 years or older are widowed, according to a 2016 US Census Bureau report

Previous research has shown that becoming a widow, especially later in life, is associated with memory problems, cognitive decline and dementia.

meta-analysis published in 2018 looked at more than 800,000 people from 15 studies and found widowed men and women were 20% more likely to develop dementia during the next three to 15 years than married people. published in 2018looked at more than 800,000 people from 15 studies and found widowed men and women were 20% more likely to develop dementia during the next three to 15 years than married people.

Science isn't sure why. But there are hints.

Constant high levels of cortisol, the "stress hormone" have been linked to problems with memory. Cortisol is meant to rise and fall quickly in response to "fight or flight" situations. Chronic stress contributes to depression and anxiety, two more risks factors for dementia.

Stress also impacts the immune system. A study funded by the UK Alzheimer's Society found an association with immune disruption and mild cognitive impairment. In addition, the trauma of losing a spouse may trigger a sort of post-traumatic stress after the death. PTSD has been linked to dementia in studies. And research on animals links long-term stress to cell death in the hippocampus, the part of the brain primarily responsible for our memories.

'Underrecognized risk factor'

The study examined 257 older adults living in their own homes who showed no sign of cognitive issues at the start of the study. Only 35 were widowed: 31 women, four men. Of the rest, 145 were married and 77 were unmarried. All underwent a baseline test of beta-amyloid levels and a cognitive test at the start of the study. The groups underwent additional cognitive testing at one year intervals for three years.

There was no difference in brain functioning between married and unmarried groups. However, widowed adults showed more cognitive decline than the married/unmarried groups, regardless of age, sex, socioeconomic status or depression. The decline was three times as fast for those with existing beta-amyloid plaques.

Women are more likely to survive a spouse than men; in fact, US census data shows that 43% of widows between 75 and 84 years and 72% of widows 85 years or older are women, compared to 15% and 35% of men.

Alzheimer's strikes many more women than men, and the study authors suggest widowhood may explain some of that discrepancy between the sexes.

"Widowhood is an underrecognized risk factor associated with Alzheimer's-related cognitive decline and impairment," the study concluded.

"The next step toward improving cognitive outcomes in widowers at risk should be greater attention toward the types of interventions, support and resources that could maintain brain health," Isaacson said. "While different types of stress may not be entirely created equal, the significant impact of widowhood is one that requires further study."