Karen Schneider, a 69-year-old in East San Jose, Calif., went through an acrimonious split from her husband in the mid-1990s that left her estranged from her two daughters and without anywhere to live. Friends let her sleep on couches and a garage as she scraped by on jobs as a home health aide and Walmart greeter. Sometimes she slept in her car.

Over the years, that support network shriveled as people moved away or died, she says. When Ms. Schneider landed in the hospital with a heart attack six years ago, she had no one to call for help. “When you get older you don’t have as many friends,” she says. “Everything changes.”…

In a review of 148 independent studies on loneliness, covering more than 300,000 participants, Julianne Holt-Lunstad of Brigham Young University and colleagues found that greater social connection was associated with a 50% lower risk of early death.

Research suggests that those who are isolated are at an increased risk of depression, cognitive decline and dementia, and that social relationships influence their blood pressure and immune functioning, as well as whether people take their medications.