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 may 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 aloneness are two studies being published today that found people with heart disease who live alone and have no close friends are more likely than others to suffer another heart attack.
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, 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.
Nearly 12 percent of American adults, or 22.6 million people, were living alone at the time of the 1990 census. According to census officials, those figures are believed to represent an all-time high, up from 7 percent, or 10.8 million adults, in 1970.
The rise of the one-person household coincides with advances in knowledge of the health risks of being alone. Some have gone so far as to suggest that social disintegration in the United States is helping to undermine public health.
The new studies, reported in the Journal of the American Medical Assn., focused on people with documented heart disease -- people 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 heart attack patients living alone were nearly twice as likely as others to suffer another heart attack -- and more likely to die of an 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.
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.
In the other study, Dr. Redford B. Williams, a professor of psychiatry and psychology at Duke University Medical Center, found that unmarried heart patients without a close personal confidant 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 less than $10,000 a year were much less likely than those with incomes of $40,000 or more to survive for five years.
Like Dr. Case, Dr. Williams found that the impact of poor social contact and inadequate income was independent of the effect of underlying physical factors. That is, the poorer or more isolated patients were not simply sicker to start with and therefore more likely to die.
The link between social relationships and health remains largely unexplained.