Heavier girls are at a decreased risk of breast cancer throughout life, but the underlying mechanism is not understood. Women with dense breasts are at an increased risk of breast cancer. Girls who are heavier have lower breast density as adults, which could contribute to lower breast cancer risk in women who were heavier as children and adolescents. The objective of this study is to identify dietary factors in youth that influence breast density. Identification of specific dietary factors in youth that are associated with adult breast density could lead to targeted recommendations that do not involve excess weight gain.