Even in this age of Facebook, Twitter and 24/7 social connectedness — or perhaps because of it — an awful lot of people feel lonely. Half of all Americans report they have no one outside their families they can talk to about their troubles or joys, according to the 2010 General Social Survey of the National Science Foundation, which drew its data from 1500 face-to-face interviews.

And their health is in jeopardy as a result.

Being alone — social isolation, scientists call it — has long been known to make people susceptible to poor health, even early death. In fact, studies done at Brigham Young University found having few emotional connections to other people is as great a risk factor for early death as obesity, smoking 15 cigarettes a day or being an alcoholic.

“If you’re alone and not lonely, you generally do okay,” explains Bruce Rabin, MD, PhD, medical director of the University of Pittsburgh Medical Center Healthy Lifestyle Program and an expert on the effects of stress on the immune system. “But if you’re with people and you are lonely, you are at increased risk of disease.” 

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Why? Stress plays a major role. Humans are social creatures and they feel threatened when they are alone or even just feel alone, says John T. Cacioppo, PhD, a University of Chicago professor who co-founded the field of social neuroscience and is a leading expert on the effects of loneliness. Loneliness, he noted in a 2011 study, is the equivalent of physical pain, hunger or thirst — a signal from the brain to seek life-saving relief (in this case, other people who can restore a feeling of safety and well-being). 

Think of loneliness as any other unrelenting stressor, like an abusive boss, a nightmare commute or a chronic illness. Left untreated, loneliness “can cause an increase in the concentration of stress hormones that can affect both your physical and mental health,” says Rabin.

For example, stress hormones can send your blood pressure soaring, increase how rapidly cholesterol accumulates in your blood vessels and promote unchecked inflammation, priming you for a heart attack. “They also change the way your immune system works so you’re more susceptible to infection and, if you have an autoimmune disease, more susceptible to relapse,” says Rabin.

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Mentally, he says, “there is an increased risk of depression because these hormones actually cause changes in the number of brain cells in area of brain that causes depression.”

Stress also can shorten your telomeres, says Rabin. These are the small caps at the end of each strand of DNA that protect the chromosomes. Theoretically, shortened telomeres equal a shorter life.

The solution isn’t as simple as “go out and make friends,” says Rabin. Cacioppo’s studies found that people who are lonely think differently and see things in a more negative way than when they’re not lonely. So therapy that attempts to reframe the thoughts behind a person’s behavior (which cognitive behavioral therapy aims to do) can be more effective than other kinds of therapy.

For people who may be depressed or shy, trying to be social is as painful as loneliness. They need an ice-breaker.

“What we tell people is to identify something they have an interest in, and become a volunteer,” says Rabin. “Do you like gardening? Volunteer in your local community garden and meet people with similar interest. If you like to read, volunteer at your local library to read to children or join a book club.”

You don’t need your calendar to overflow with social events though, says Rabin. “Just knowing you have one afternoon a week with people is enough.”

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Denise Foley is a veteran health writer and a former contributing executive editor at Prevention magazine.