No one is an island. For years, studies have been showing how important a strong social network is to good health. Social connectedness is a better predictor of a child's well-being as an adult than their grades are, and people with strong social connections live far longer than those without them.

What's been missing is information on exactly how social networks do this. A new study offers insight into the physiological reasons why having friends helps our health, and it suggests that social networks operate differently at different stages of life.

This is the first study to link social relationships with concrete measures of physical well-being such as blood pressure.

In youth and late in life, the size of the social network you have appears to matter most; but in middle adulthood, quality, the social support connections provide and how well they help ease the stresses of life, is what matters more than quantity.

Those people who had more social connectedness at all stages of life had lower values for four negative health markers: blood pressure, waist circumference, body mass index and level of C-reactive protein, a general marker of inflammation.

The study looked at data from four large surveys of U.S. health that collectively covered lifespan from adolescence to old age, including the National Longitudinal Study of Adolescent Health, which has been tracking middle and high school students since 1994.

In adolescence, social isolation increased risk of inflammation by the same amount as physical inactivity did, while social integration — the degree to which an individual feels connected to other people — protected against abdominal obesity. In old age, social isolation increased the odds of developing high blood pressure more than diabetes did.

It can be hard to reduce personal relationships to a string of numbers. This is the first study to definitively link social relationships with concrete measures of physical well-being such as blood pressure. Its take-home message is that friends and other social ties are an essential part of living a happy and healthy life, a practical lesson for both doctors and patients.

“Based on these findings, it should be as important to encourage adolescents and young adults to build broad social relationships and social skills for interacting with others as it is to eat healthy and be physically active,” study co-author Kathleen Mullan Harris, a professor in the Department of Sociology at the University of North Carolina at Chapel Hill, said in a statement.

The study appears in Proceedings of the National Academy of Science (PNAS) and is freely available.