Women in Steady Exercise Research [Formerly Women, Oxidative Stress, Exercise and Estrogens (WOSEE)]

OBJECTIVES: Primary * Compare the changes in urine levels of a stable marker of oxidative stress (F\_2-isoprostanes) in healthy young women who undergo exercise training during 4 menstrual cycles vs no exercise. Secondary * Assess the changes in other metabolic factors that explain the association between physical activity and breast cancer, including: Estrogen metabolism (estrone, estradiol, estriol, 2-OHE\_1, 2-OHE\_2, 2-MeOHE\_1, 2-MeOHE\_2, 4-OHE\_1, 4-OHE\_2 \[as well as ratios of these metabolites\], insulin-like growth factor (IGF)-1 (as well as IGF-binding proteins-1, -2, and -3, insulin, glucose, body composition (including body mass index, lean mass, and body fat \[DEXA scan\]). OUTLINE: This is a randomized, controlled study. Participants are stratified according to baseline body fat percentage and age (18 to 24 years vs 25 to 30 years). Participants are randomized to 1 of 2 intervention arms. * Arm I: Participants exercise for 30 minutes 5 times a week. The intensity of exercise is increased every 4 weeks. The exercise regimen continues for up to 4 menstrual cycles. * Arm II: Participants are observed for 4 menstrual cycles. Participants in both arms undergo 24-hour collection of urine on days 7, 8, and 9 of menstrual cycles 1 and 6. F\_2-isoprostanes are measured via gas chromatography-mass spectrometry (GC/MS). Estrogens are also measured by GC/MS and include E\_1, E\_2, E\_3, 2-OHE\_1, 2-OHE\_2, 4-OHE\_1, 4-OHE\_2, 2-MeE\_1, 2-MeE\_2, 4-MeE\_1, 4-MeE\_2, and 16OHE\_1. Participants undergo blood collection, body mass measurement by dual-energy x-ray absorptiometry, and a fitness assessment twice during the 6-month study. PROJECTED ACCRUAL: A total of 400 participants will be accrued.