during infancy: one reported no difference in age at menarche,47 and the other reported that those given soy formula early in life (≤4 months through ≥6 months of age) had earlier menarche.48 Soy formula composition changed in the early 1960s from soy flour to a more highly digestible soy protein isolate,49 and so we evaluated whether the association between soy formula and age at menarche differed for women born before and after this time period. The association with late menarche did not change across birth decades, while the association with very early menarche was seen only in those born in 1960 or later (rRR = 1.50 [95% CI = 1.03-2.19]).
