Scientists have long known that spending time with loved ones is good for our long-term health and may reduce our risk of cognitive decline, whereas loneliness is linked to high blood pressure, inflammation and a weakened immune system.
But why exactly does loneliness have such bad effects on our health and well-being?
One reason, according to a new study, may have to do with the way loneliness triggers cellular changes in our bodies that can make us more susceptible to viral infections.
"Feeling lonely means you are not in a socially affine environment but rather in a relatively hostile environment," Dr. John Cacioppo, a professor of psychology and behavioral neuroscience at the University of Chicago and a co-author of the study, told The Huffington Post in an email.
"In socially affine environments, protection against viral infections is especially important, whereas in hostile environments, protection against bacteria is important," Cacioppo wrote. "The pattern of gene expression in the lonely [environment] decreases protection against viral infections and instead may increase protection against bacterial infections."
In other words, as Live Science notes, the cellular changes that result in a shift toward protection against bacteria may come at the cost of the ability to protect against viral infections.
For the study, researchers analyzed the regulation of the leukocyte gene -- which is involved in protecting the body against both bacteria and viruses -- in 141 older adult humans over a five-year period, and in a separate group of rhesus macaque monkeys that displayed behavior indicative of social isolation.
The researchers noticed increased activity in genes that produce inflammation in the body and less activity in genes that help to fight off illness in the adults who were lonely and in the monkeys, The Telegraph reported.
In the monkeys, researchers also found that loneliness causes the body to produce "fight-or-flight" stress signals, which can impair the body's antiviral responses.
For instance, when the researchers infected monkeys with simian immunodeficiency virus, the virus grew faster in the monkeys that were classified as socially isolated than in monkeys that were not "lonely," according to Live Science. This may be a result of the immune system releasing monocytes, a particular kind of immune cell linked to high levels of inflammatory proteins and low levels of antiviral proteins.
The researchers concluded that inflammation and impaired antiviral responses contribute to the bad health effects of loneliness. The findings appear to support previous research suggesting that for older adults, feelin
"This study specifically showed loneliness causes a physiological reaction in people," Dr. Matthew Lorber, acting director of child and adolescent psychiatry at Lenox Hill Hospital in New York City, who was not involved in the new research, told CBS News.
"This is the first study I have seen that has actually gone into the details of showing loneliness leading to a decreased production of leukocytes (disease-fighting cells) and an increased production of immature monocytes," Lorber said. "Leukocytes are what our body needs to fight infection. The fact that loneliness is leading to a decreased production of the leukocytes is really fascinating to me."
But to be clear, the research doesn't conclusively prove anything. The U.K.'s National Health Service pointed out in a blog post on Tuesday that "this study has not proved that socially isolated humans are more likely to become ill or die earlier. ... Feelings of loneliness and social isolation can be complex emotions that may be influenced by many personal, health and life circumstances."
Still, the NHS went on, "what is fairly apparent from this and previous research is that, whatever the biological mechanism(s) that may be behind it, loneliness and social isolation do seem to be associated in some way with disease and illness."
The researchers said that they plan to continue examining how loneliness leads to poor health outcomes, and how these effects can be prevented in older adults.
The study was published in the Proceedings of the National Academy of Sciences on Monday.
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