Nutrition and age at first birth in breast-cancer risk.

Urban/rural breast-cancer incidence ratios in the state of Iowa for 1950 and 1969-71 were contrasted with corresponding urban and rural distributions of age-at-first-birth and population nutrition, variable measured approximately 15 years before each morbidity survey and putatively related to breast-cancer incidence. Over the study interval, the decline in the urban/rural breast-cancer incidence ratio correlated better with changing nutritional patterns than with changing age-at-first-birth.