Dietary fat and sports activity as determinants for age at menarche.

A prospective study (1985-1991) was conducted to evaluate the role of nutrition, physical activity, and other life-style factors for the age at menarche, a known breast cancer risk factor. In 1988-1989, a total of 261 girls, aged 8-15 years, from a nationwide representative nutrition and activity survey (1985-1988) responded additionally to a mailed questionnaire on personal maturation and anthropometric data, physical activity, and information about nutritional habits, measured by a short food-frequency questionnaire. All girls without menarche at the time of questionnaire mailing were observed for 2 years or until the event of menarche. At the end of the follow-up period, 79% of the girls had experienced menarche. The survey and questionnaire data were analyzed for their relation to age at menarche using Cox's proportional hazards method. After excluding participants with largely deviating measurements between survey and questionnaire, the increased energy-adjusted fat intake was associated with accelerated menarche (relative risk = 2.1; 95% confidence interval 1.1-4.0; lowest vs. highest quartile), while increased sports activity was associated with a delay in menarche (relative risk = 0.3; 95% confidence interval 0.1-0.5; lowest vs. highest quartile). Parameters of physical development such as body weight or body fatness were found to be predictors of menarche. The study provides evidence that dietary effects on breast cancer risk might be indirect by influencing hormonal events like the age at menarche.