O besity is an established risk factor for type 2 diabetes mellitus.1,2 Affected individuals show signs of insulin resistance and hyperinsulinemia, a process that may begin in childhood.3,4 Pre-adolescent obesity is also an important predictor of age of onset of breast development in young women, and of breast size after puberty. 5,6 Premature onset of puberty is preceded by childhood insulin resistance, hyperinsulinemia and hyperandrogenemia, 7 which may persist after puberty 8 and continue into early adulthood.
9Although an elevated body mass index (BMI)10,11 and central adiposity 12 are established risk factors for insulin resistance and the onset of type 2 diabetes mellitus, little is known about the contribution of extra-abdominal adipose tissue, including breast tissue, about 60% of which is fatty tissue, to this process.13,14 We hypothesized that a woman's breast size in late adolescence reflects her predisposition to insulin resistance and type 2 diabetes mellitus that is both additive to, and independent of, BMI. We explored this hypothesis in conjunction with the Nurses' Health Study II by relating bra cup size, a proxy for breast size, to the onset of type 2 diabetes mellitus.
Methods
ParticipantsThe Nurses' Health Study II is a prospective cohort study, which was initiated in 1989 and is ongoing. The study involves 116 609 women, more than 90% of whom are white, from 14 US states.15 At the time of enrolment, participants were between the ages of 25 and 42, and they completed a detailed baseline questionnaire about their health, lifestyles and anthrometric characteristics. They continue to complete follow-up questionnaires on a biennial basis. We included all women who had concomitant information about bra cup size at age 20 and BMI at age 18, for a total of 92 106 participants. We excluded women who had already received a diagnosis of diabetes mellitus or who had a history of gestational diabetes.