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Having friends is good for your physical health, and the benefits appear to start early in life, according to a new study.

Researchers used data from more than 14,000 Americans in four large, nationally representative surveys of health from adolescence to old age. They measured social integration with an index that quantifies the number and nature of social connections — in romantic relationships, with family and friends, and by participation in religious and social organizations. The study appeared in Proceedings of the National Academy of Sciences

After controlling for education, smoking, depression, alcohol consumption, diabetes and other characteristics, they found a lower score on the social integration index was associated with higher levels of C-reactive protein,a measure of general inflammation, and with higher blood pressure, higher body mass index and larger waist circumference.

The associations were stronger at some ages than others. For example, social isolation in adolescence raised the risk of inflammation to about the same degree as physical inactivity. Being isolated in old age raised the risk of hypertension as much as having diabetes did.

The relationship between social isolation and poor health is “well known among aging people,” said the senior author, Kathleen Mullan Harris, a professor of sociology at the University of North Carolina. “But this is the first to study it starting in the very beginning of the life course, when people foster these relationships — early adolescence.”