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Absence might make the heart grow fonder, but loneliness could do it serious damage, according to a new study.

Researchers at the University of Chicago looked at 229 people aged 50 to 68 and found that over a five-year period the loneliest individuals saw their systolic blood pressure rise by 14.4 mm more than those who were socially contented. That rise could push anyone with normal blood pressure, that below 120 mm, into the category called pre-hypertension.

"The lonelier you are, the bigger the effect," says Louise Hawkley, a psychologist and lead author of the study, published in the journal Psychology and Aging.

While loneliness has been linked to other factors that could cause an increase in blood pressure, such as stress and depression, those factors could not account for the rise observed in the study.

"It's certainly the case that loneliness is related to depressive symptoms and depression, to stress, to hostility, to how much social support you perceive you have. And we looked at each of those as possible explanations for the effect, and it wasn't there," Dr. Hawkley says. "It seems to be that there's something unique about loneliness."

For the purposes of the study, loneliness was roughly defined as "perceived social isolation," Dr. Hawkley says. "Not everybody feels isolated even if they're alone or if they don't have a lot of friends. They just don't have a need for a lot of connections. They're perfectly content. Other people can have a lot of connections, a lot of relationships, and yet feel lonely."

To determine the level to which they perceived themselves as lonely, study participants were asked to respond to a series of statements, including, "I have a lot in common with the people around me," "My social relationships are superficial" and "I can find companionship when I want it."

Over the course of the five-year study, during which blood pressure was measured four times, the degree to which individuals perceived themselves as lonely was directly linked to how much their blood pressure increased.

The loneliness that participants reported feeling at the beginning of the study predicted "how much their blood pressure is increasing over those intervals," Dr. Hawkley says.

High blood pressure affects health in several ways, Dr. Hawkley says. Not only does it increase the risk of heart attack and stroke, but any measurement over 115 mm increases the risk for cardiovascular disease. As well, hypertension, or a systolic blood-pressure measurement greater than 140 mm, is the primary or contributing cause of roughly 18 per cent of deaths in the United States.

"Every millimetre matters in terms of health outcomes," Dr. Hawkley says.

Unlike other causes of high blood pressure, such as smoking and alcohol consumption, loneliness is far more difficult to treat.

"Unfortunately, there aren't a lot of people yet who have done loneliness interventions to any great success. So even if we know that we should be doing something about it, to find somebody who's effective at helping may be a challenge," Dr. Hawkley says.

Researchers suspect that evolution may explain the link between loneliness and high blood pressure.

"Loneliness reflects a very fundamental need to feel connected," Dr. Hawkley says. "In evolutionary terms, you had to have the sense that you had your back covered. You travelled in groups, in tribes, in families. Anything to fend off threats from the environment … and that, I think, persists. We still need to feel like we belong, like we're connected."