Age at onset of anorexia nervosa and breast cancer risk.

The objective of this study was to investigate breast cancer occurrence among women treated for anorexia nervosa (AN), with emphasis on age at the onset of this disorder. We conducted a register-based retrospective cohort with a total of 6009 women with at least one admission with an AN diagnosis during the period 1973-2003 in Sweden. During a mean follow-up of 13.4 years, information on 80 057 women-years was generated. The standardized incidence ratio (SIR)--the ratio of observed-to-expected number of cases--was used as the measure of relative risk. Overall, 16 women developed breast cancer versus 25.5 expected cases [SIR: 0.6, 95% confidence interval (CI): 0.4-0.9]. Among women who were first admitted for AN between the age of 10 and 24 years, four developed breast cancer versus 11.3 expected (SIR: 0.4, 95% CI: 0.1-0.9). In this group of women with early onset AN, only one parous woman developed breast cancer versus 6.3 expected (SIR: 0.2, 95% CI: 0-0.9). Among women first hospitalized for AN between the age of 25 and 40 years, 12 developed breast cancer, whereas the expected number was 14.2, a nonsignificant deficit. Our results suggest that early onset AN may play an important role in the development of breast cancer, possibly because of the extreme restriction of energy intake at a crucial period for mammary gland development. Late onset AN is likely to play a relatively less important role.