"This study confirms what has also been indicated in previous research regarding the serious health consequences of loneliness," said lead researcher Anne Vinggaard Christensen, of Copenhagen University Hospital in Denmark.
"Loneliness should be considered a serious risk factor in patients with cardiac disease and should be included in risk evaluation of patients," added Christensen, who is with the hospital's heart center.
The Danish researchers cautioned, however, that their findings can't prove that loneliness caused people to die, only that loneliness and the risk of death appear to be connected. But the differences in mortality between those who felt lonely and those who didn't can't be explained by their medical condition alone, they said.
Loneliness is a subjective experience, one that is distressing and unpleasant, the researchers noted.
The connection between loneliness and health is complex, Christensen said.
People who are lonely or socially isolated tend to have more unhealthy lifestyles. They smoke more, are less likely to be physically active and don't take their medication, she said.
"Having a social network helps motivate people to make healthier choices," Christensen said. "A social network can act as a buffer for stress."
Also, lonely people have been found to have higher levels of stress hormones and lower immune function, she added.
"These different pathways are interconnected and help us understand why people feeling lonely or socially isolated experience worse health," Christensen said.
James Maddux is a professor emeritus in the department of psychology at George Mason University in Fairfax, Va.
"This study adds to the growing research on the perils of social isolation among people in general and among older adults over 60 in particular," said Maddux, who wasn't involved with the study.
"These results help reaffirm what we've learned so far about how unhealthy social isolation can be," he added.
For the study, Christensen and her colleagues collected data on more than 13,400 heart patients after they left the hospital from 2013 to 2014. Their average age was in the mid-60s.
Participants completed questionnaires on their health, psychological well-being, quality of life and levels of anxiety and depression.
Compared to people who didn't feel lonely, those who said they were lonely were nearly three times more likely to be anxious and depressed and have a lower quality of life, the researchers found.
A year later, Christensen's team found that feeling lonely had a significant impact on participants' health.
Regardless of other factors, lonely women were nearly three times more likely to die than women who weren't lonely, and lonely men were more than twice as likely to die, the researchers found.
Living alone, however, is not necessarily equal to feeling lonely, and it was linked to a lower risk of depression and anxiety than living with others.
But among men, living alone was tied with a 39% greater risk for poor heart health. This may be because men tend not to have an extensive support network after divorce or death of a spouse, compared with women, the researchers noted.
"Loneliness can have many causes and can occur even if you have people around you," Christensen said.
For some, it would help to have a family member who remembers to ask how they are doing and is ready to listen, she said.
"For others, help with practical things might be what they need, and for some, the opportunity to talk to other patients who have gone through the same thing is helpful," Christensen said.
She also stressed that the effects of loneliness are not confined to heart disease. "It's also [detrimental] for individuals who do not suffer from an illness. Loneliness seems to be damaging to your health no matter what," Christensen said.
The report was published online Nov. 4 in the journal Heart.