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.
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