We live in a time when loneliness is more present and health providers should take this into account when assessing risk.
Lonely people are twice as likely to die from heart problems, according to a research.
The study found that feeling lonely was a stronger predictor of poor outcomes than living alone, in both men and women.
"Loneliness is more common today than ever before, and more people live alone," said Anne Vinggaard Christensen of The Heart Centre, Copenhagen University Hospital, Denmark.
"Previous research has shown that loneliness and social isolation are linked with coronary heart disease and stroke, but this has not been investigated in patients with different types of cardiovascular disease."
The study investigated whether poor social network was associated with worse outcomes in 13,463 patients with ischaemic heart disease, arrhythmia (abnormal heart rhythm), heart failure, or heart valve disease.
Data from national registers was linked with the DenHeart survey, which asked all patients discharged from April 2013 to April 2014 from five heart centres in Denmark to answer a questionnaire about their physical and mental health, lifestyle factors such as smoking, and social support.
Social support was measured using registry data on living alone or not, and survey questions about feeling lonely - Do you have someone to talk to when you need it? Do you feel alone sometimes even though you want to be with someone?
"It was important to collect information on both, since people may live alone but not feel lonely while others cohabit but do feel lonely," explained Vinggaard Christensen.
Feeling lonely was associated with poor outcomes in all patients regardless of their type of heart disease, and even after adjusting for age, level of education, other diseases, body mass index, smoking, and alcohol intake. Loneliness was associated with a doubled mortality risk in women and nearly doubled risk in men.
Both men and women who felt lonely were three times more likely to report symptoms of anxiety and depression, and had a significantly lower quality of life than those who did not feel lonely.
"Loneliness is a strong predictor of premature death, worse mental health, and lower quality of life in patients with cardiovascular disease, and a much stronger predictor than living alone, in both men and women," said Vinggaard Christensen.
Vinggaard Christensen noted that people with poor social support may have worse health outcomes because they have unhealthier lifestyles, are less compliant with treatment, and are more affected by stressful events. But she said: "We adjusted for lifestyle behaviours and many other factors in our analysis, and still found that loneliness is bad for health."
She concluded: "We live in a time when loneliness is more present and health providers should take this into account when assessing risk. Our study shows that asking two questions about social support provides a lot of information about the likelihood of having poor health outcomes."
The study was presented at EuroHeartCare 2018, the European Society of Cardiology's annual nursing congress.