Washington ? Exercising and maintaining a healthful weight when young can delay the onset of breast cancer in women at very high risk of the disease, according to a study of women with a genetic mutation that gives them an 82 percent lifetime risk of developing the disease.

Researchers also found that women with mutations in the BRCA1 or BRCA2 gene have a 23 percent to 54 percent risk of ovarian cancer, depending on which gene is affected.

The study, appearing today in the journal Science, showed that lifestyle during adolescence played a role in when these high-risk women developed breast cancer. The finding was consistent with earlier studies suggesting that among women in general, exercise and healthy weight early in life can reduce a woman’s risk of developing breast cancer after menopause.

In the new study, women who exercised actively when they were young — either dancing, or in team sports, or just walking a lot — and who maintained a healthful weight — that is, they were not obese — through the age of 21 were somewhat protected from breast cancer, said Mary-Claire King, first author of the study and a professor of genome sciences at the University of Washington, Seattle.

Earlier studies had suggested that women with the gene mutations have a risk ranging from 25 percent to 80 percent of developing breast cancer in their lives. The new study estimates the risk at 82 percent.