New research finds that feeling lonely increases the rate of cognitive decline, independently of depression.

If you’re older and you’re lonely, chances are your mental faculties will decline at a quicker rate.

That’s the conclusion from new research presented today at the Alzheimer’s Association International Conference 2015 in Washington, D.C.

In their study, researchers said they discovered that loneliness can be a key predictor of the rapid progression of dementia-related diseases.

Learn More: Loneliness Is a Serious Health Risk for Seniors »

More than 45 million people worldwide are living with dementia today.

One of the earliest symptoms of dementia is cognitive decline, a gradual reduction in high-level functions like memory and problem-solving.

Although social isolation has long been linked with cognitive decline, it’s been unclear which causes which. Or if both are caused by a third factor, such as depression.

The study released today examined 8,300 adults age 65 or older over the course of 12 years.

Every two years, the participants came in for a memory test. The researchers also measured age, sex, race, wealth, income, health conditions, strength of social network, and levels of depression.

One in six of the participants reported feeling lonely much of the time. Of these lonely individuals, nearly half experienced high levels of depression.

Over the 12 years, researchers said people who were lonely experienced cognitive decline at a 20 percent faster rate than people who were not lonely.

This is the same rate of cognitive decline associated with high levels of depression itself. Even having a single symptom of depression was enough to cause an 8 percent increase in cognitive decline.

“We found that those effects were overlapping. In those analyses it looked like loneliness and depression were operating under the same mechanism, so we think that they’re related risk factors, but we also believe that loneliness or even one or two depressive symptoms in and of themselves represent a risk factor,” said Dr. Nancy Donovan, associate psychiatrist at Brigham and Women’s Hospital, in an interview with Healthline.

Related Reading: New Brain Scan Technique Captures Early Signs of Dementia »

Although Donovan’s study didn’t dig into the biological mechanisms of how loneliness and cognitive decline are linked, she is planning follow-up studies to examine this further.

Until then, she has some ideas on what might be going on. The link may not be causal, but instead two symptoms of the same root problem.

“Loneliness may be a symptom or marker of psychosocial stress,” she explained. “If psychosocial stress is something that someone’s experiencing, they may report it as loneliness, and that may then be associated with adverse health consequences.”

Psychosocial stress isn’t so different from other forms of stress, such as starvation, sleep deprivation, or physical abuse. All forms of stress, in the end, pack the same punch to the brain.

“There’s some evidence that psychosocial stress has effects on the brain and also may increase inflammation that can have kind of a global effect on peoples’ health, so that might explain why you see so many adverse health consequences from loneliness and depression,” Donovan said.

Donovan is hopeful that her findings can help improve interventions to prevent cognitive decline.

“Increasingly, there’s an interest in lifestyle factors and modifiable risk factors for cognitive decline, and identifying people who are socially isolated, or lonely, or depressed,” she said. “Helping to remediate those symptoms may actually reduce the incidents of cognitive decline and dementia.”

Keep Reading: How Stress Permanently Changes Our Genes »