In other words, the results suggested that people suffering from social isolation may be more prone to inflammation and less able to fight viral infections—which could be one reason why lonely elderly people are more susceptible to illnesses ranging from the common cold to dementia, and why they have higher mortality rates than their peers.

To bolster their findings, the researchers did a second round of experiments in rhesus macaques, who demonstrated the same connection between social isolation and genetic changes. The study authors then randomly assigned rhesus macaques to two groups: Some of them would be exposed to new social relationships, while others would not. The macaques given new social exposures had higher numbers of circulating blood monocytes, a key type of immune cells, compared to those who had no new social exposures.

A few decades ago, a link between social life and immune-cell genetics wasn’t even on researchers’ radars. We now know much more than we used to about the genetic and molecular sources of mental illness, but social isolation and loneliness are still often considered purely social or psychological issues, rather than valid medical concerns.

Yet the more we learn about the rich interplay between mental illness and physical disease, the clearer it becomes that no illness can be reduced to the simple expression of an abnormal gene, or to the failure of a mutated protein to bind to its receptor. Consider the fact that young women with depression have double the risk of heart attack, and that people with schizophrenia have greatly elevated susceptibility to heart disease and cancer. The psychological and the physical are inextricably linked; mental illness is physical illness.

Loneliness, it appears, is no exception, but the science linking loneliness to health is far from settled. The next step will be to assess whether initiatives to reduce loneliness, like Lyme’s Veggie Cares program, are actually correlated with better immune function and improved health outcomes in the elderly.

While access to good health care and nutrition are essential to physical health in older people, social connections may be just as important. Imagine if the most powerful health intervention for the at-risk elderly isn’t a high-tech surgery or a handful of expensive pills, but the simple exchange of stories over a steaming cup of tea.

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