One is the loneliest number and, sometimes, it could even be the most dangerous.

Loneliness is just as much of a public health hazard as obesity, if not more so, according to research presented at the American Psychological Association annual conference last week. Research from 148 studies, involving 300,000 participants, showed people who had greater social connections had a 50% reduced risk of dying early, and another set of research involving 70 studies representing 3.4 million people in North America, Europe, Asia and Australia found social isolation, loneliness or living alone each played a significant role in premature death.

Over 42 million Americans over the age of 45 suffer from chronic loneliness, according to the AARP. More than one-quarter of the population lives alone and more than half are unmarried, according to the U.S. Census Bureau. People that considered themselves lonely were less likely to engage in social activities, such as going to religious services, volunteering or finding a hobby, the AARP study concluded. (It’s based on a scale created by UCLA, where respondents were asked about various characteristics linked to loneliness. (The AARP promotes group activities for seniors.)

One theory: Loneliness and isolation can lead to anxiety, said Fran Walfish, a family and relationship psychotherapist in Beverly Hills, Calif. and author of “The Self-Aware Parent.” And without taking action, it’s only likely to get worse. “The antidote to loneliness is activity,” Walfish said. Going outside, doing chores in the house or meeting someone for dinner.” What’s more, friends and family also play an important role when people get older and sick, helping them through their recovery or, even, spotting symptoms that a person might let go untreated.

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Lonely people are more likely to have a higher use of medication and higher incidence of falling and, as such, an increasing their risk of requiring long-term care, according to the Campaign to End Loneliness, a U.K.-based nonprofit group to help reduce the number of isolated people, particularly the elderly. Obesity can also lead to early death, and shortens the life of a person more so than other preventable health problems, including smoking and high blood pressure. (On the upside, they re more likely to visit their general practitioner, the Campaign to End Loneliness found.)

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There’s a body of research to support this theory: A study from the University of Arizona published in January showed that lonely people were less likely to have close relationships, manage daily stresses, maintain their health or sleep well. It found loneliness is highly individual and determined by a person’s expectations and needs. Another study, published in the “Aging & Mental Health Journal” in 2003, suggested loneliness,was most significant in older men. Older people in particular may face a higher risk of loneliness, because of the loss of family and friends or a job.

And merely being single or living alone doesn’t necessarily mean a person is unhappy or, indeed, lonely, particularly for many millennials. Young single people feel more of a connection to others than their married counterparts, according to a 2007 paper called “Social Embeddedness and Late-Life Parenthood.” The number of people who never marry has been growing over the years, and a record percentage (25%) may never marry, according to Washington, D.C.-based nonprofit think tank Pew Research Center.