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Is loneliness lethal? According to two new studies published online Monday in the Archives of Internal Medicine, living alone or feeling lonely can increase your chances of disability and early death.

In one study, researchers at Harvard Medical School followed nearly 45,000 people who had heart disease or were at high risk of developing it. Over four years, the study authors tracked the participants’ health and found that those who lived alone were more likely to die from heart attack, stroke or other heart-related problems than those who lived with others.

The association was especially marked by age: for the youngest participants, aged 45 to 65, living alone increased the risk of early death by 24%; in people aged 66 to 80, solitary living was associated with a 12% increased risk of death; among those over 80, there was no link between living arrangements and risk of heart-related death.

Why the differences? It could be that for middle-aged people, for whom living alone is much less common than it is for the elderly, the single life may be a marker for other psychological or social problems that can affect health — a poor support system, depression, loneliness, job- or relationship-related stress. For the elderly, however, living alone may be a marker of strength; if you’re 80 and living solo, you might be healthier and more independent than your peers who can’t manage on their own.

Living alone affects well-being in other, more practical ways, too: people who don’t have a partner or family member to remind them to eat right or take their medicines or to recognize signs of health problems are less likely to maintain good health.

(PHOTOS: Senior Love Triangle: Photographs by Isadora Kosofsky)

Much previous research has linked social isolation with poor health outcomes, including depression, heart disease, sleep problems and other disorders, but it has never been clear what it is exactly about being alone that may be so harmful. In the second study published Monday, researchers led by Dr. Carla Perissinotto, an assistant professor of medicine at the University of California, San Francisco, report that it’s not just living alone, but having actual feelings of loneliness and isolation that matters.

In Perissinotto’s study, which involved 1,604 participants, average age 71, the researchers defined loneliness not by gauging the participants’ living situations, but by asking them to answerthree questions regarding feelings of aloneness: did they feel left out, isolated or that they lacked companionship? If the participants answered “often” or “some of the time” to any of these questions, they were considered lonely; if they responded “hardly ever” to all three, they were not.

The researchers checked in with the participants every two years for six years to evaluate certain day-to-day abilities like bathing, dressing, eating, walking and climbing stairs — all measures of overall health in the elderly. By the end of the study period, lonely people showed significantly more disability: they were 59% more likely to have lost the ability to perform tasks of daily living. For example, they were 18% more likely to suffer mobility problems and 31% more likely to have trouble climbing stairs than those who didn’t report feeling lonely.

Even more concerning, the lonely participants were 45% more likely to have died by the end of the study than those who weren’t lonely. “I was surprised by how strong the relationship actually was,” says Perissinotto. The association held even after she and her team adjusted for other factors that could influence the health outcomes, such as a previous diagnosis of depression or other medical conditions that could account for declining health.

(MORE: Feeling Alone Together: How Loneliness Spreads)

Loneliness can be detrimental in many ways, some of which are biological and some of which are more behavioral. Feeling isolated can trigger changes in brain chemicals and hormones that can increase inflammation in the body, for example, which can exacerbate conditions like heart disease and arthritis. Loneliness may also lead to other problems — poor sleep, depression, a disinterest in one’s own health care — which can in turn contribute to disability and early death.

Which is why the researchers were particularly concerned over another finding — many of the elderly who said they felt lonely were not actually living alone. Rather, they were married or living with family members. That suggests that the size of a person’s social network isn’t the only measure of loneliness, and that studies that look only at the number of people’s contacts may miss an entirely separate factor that can have a significant impact on health, says Perissinotto. “I think that from a public health and policy level, we are doing a disservice by not asking [people] about their subjective feelings of loneliness,” she says. “We focus on their diabetes control and treating their hypertension, but are we missing something that may be more distressing to patients and have more of an impact on their health?”

Many elderly patients experience declines in their daily activities, and doctors are often at a loss to explain why some slip more quickly into poor health, while others with similar circumstances tend to remain healthier longer. Feelings of loneliness, and not just social isolation, could in part explain this difference. Perissinotto encourages more doctors, particularly those who treat the elderly, to ask their patients about their feelings. “We are trained to ask about a patient’s physical environment to assess how they are functioning, but we aren’t traditionally taught to ask about things like their feelings of loneliness,” she says. “It’s challenging, but it deserves more delving into, and that should be the next step to addressing loneliness and its impact on health.”

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Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.