In the quest for better health, many people turn to doctors, self-help books or herbal supplements. But they overlook a powerful weapon that could help them fight illness and depression, speed recovery, slow aging and prolong life: their friends.

Researchers are only now starting to pay attention to the importance of friendship and social networks in overall health. A 10-year Australian study found that older people with a large circle of friends were 22 percent less likely to die during the study period than those with fewer friends. A large 2007 study showed an increase of nearly 60 percent in the risk for obesity among people whose friends gained weight. And last year, Harvard researchers reported that strong social ties could promote brain health as we age.

“In general, the role of friendship in our lives isn’t terribly well appreciated,” said Rebecca G. Adams, a professor of sociology at the University of North Carolina, Greensboro. “There is just scads of stuff on families and marriage, but very little on friendship. It baffles me. Friendship has a bigger impact on our psychological well-being than family relationships.”

In a new book, “The Girls From Ames: A Story of Women and a 40-Year Friendship” (Gotham), Jeffrey Zaslow tells the story of 11 childhood friends who scattered from Iowa to eight different states. Despite the distance, their friendships endured through college and marriage, divorce and other crises, including the death of one of the women in her 20s.