At a time when more Americans than ever are living alone, evidence is mounting that isolation can be bad for one's health for reasons that can range from the absence of a ride to the hospital to the lack of some chemical response to human contact.
The latest clues about the health hazards of living alone come in two studies published today that find people who have had heart attacks and who live alone and have no close friends are more likely than others to suffer additional heart attacks.
The reasons for the pattern are unclear, but previous studies have found higher rates of death in general among the unmarried. In addition, accidents, suicides, psychiatric disorders and even tuberculosis have been found by some to be more common among the socially isolated.
"The data is very strong and suggestive that we need better social contacts," said Dr. Robert B. Case, a New York cardiologist and author of one of the new studies. "It may be very important to maintain these relationships, particularly in today's society."
Nearly 12 percent of American adults, 22.6 million people, were living alone at the time of the 1990 census. According to census officials, that is an all-time high, up from 7 percent, 10.8 million adults, in 1970.
The rise of the one-person household coincides with advances in knowledge about the health risks of being alone. Some researchers have suggested that social disintegration in the United States is helping to undermine public health.
The new studies, reported in the Journal of the American Medical Association, focused on people with documented heart disease, for whom stress is known to have an especially insidious effect and on whom the benefits of social contacts might be especially apparent.
The first study, by Dr. Case and others at St. Luke's-Roosevelt Hospital and Columbia University in New York City, found that TC heart attack patient who lives alone is nearly twice as likely as other heart attack patients to suffer another heart attack -- and more likely to die of a heart attack -- within six months.
The researchers compared the impact of living alone to that of physical risk factors such as previous heart damage and heart rhythm disturbances. They found that living alone was "a major independent risk factor" comparable to physiological factors.
To Dr. Case's surprise, he did not find a similar increase in health risk associated with divorce, separation or the death of a spouse. Half of the people with disrupted marriages in his 1,234-patient study were living with someone else at the time.
"What's particularly significant is the magnitude of the effect," said Nan Case, a clinical psychologist, co-author and Dr. Case's wife.
"We know that emotions and [social] integration have an effect, but we never knew it could come close to the physiological factors in heart disease."
In the other study, Dr. Redford B. Williams, a professor of psychiatry and psychology at Duke University Medical Center in North Carolina, found that unmarried heart patients without close personal confidants were significantly less likely than others to survive for five years.
Dr. Williams and his colleagues found a similar pattern among the poor in their study of 1,368 men and women: Patients with incomes of less than $10,000 a year were much less likely than those with incomes of $40,000 or more to survive for five years.
The link between social relationships and health remains largely unexplained. Researchers speculate that the reasons may include some mundane, practical factors as well as more mysterious emotional factors with physical effects.
For example, research has shown that people with social support are more likely to follow medical advice to take medication, alter their diet or stop smoking. They are also more likely to have someone around to help them in a time of need.
But Dr. Williams and others also suggested that people who are alone may experience more stress, without the emotional buffer offered by a companion. Stress can trigger the release of hormones, such as adrenalin, that can harm the heart.