Does breastfeeding affect the risk of gastric cancer?

Abstract

Dear Sir,Inoue et al. 1 recently studied relations between reproductive and menstrual factors and the risk of gastric cancer, considering case-control data from Japan. According to their working hypothesis, the lower risk of gastric cancer seen in women may in part be linked to factors involving fertility and pregnancies. Indeed the data of Inoue et al. 1 indicated that parous women with a short breastfeeding period per child experienced a lower risk than nulliparous women, although women with a long period of breastfeeding had a considerably higher risk. The positive association with the average duration of breastfeeding per child was especially pronounced for cancers in the upper third of the stomach.We have previously published summary estimates of potential associations between duration of breastfeeding and incidence of gastric cancer, considering follow-up data from 3 Norwegian counties. 2,3 To our knowledge, there are no other reports on this relation except the recent one by Inoue et al. 1 Prompted by the results in the Japanese study, we have now reanalysed our data in more detail on the basis of an extended follow-up.Our cohort included female residents of the counties of Nord-Trøndelag, Aust-Agder and Vestfold, born in 1886 -1928 and living mostly in rural areas, largely reflecting breastfeeding habits different from those seen later. Information on pregnancies and breastfeeding habits was collected at a screening for breast cancer during 1956 -1959. The cohort was followed in the period 1961-1998 with regard to cancer diagnoses reported to the Cancer Registry of Norway and with regard to deaths and emigrations. To avoid possible diagnostic problems in very old women, our analysis took into account the follow-up for each person only until the age of 80 years. Relations between the risk of gastric cancer and reproductive factors may be different in women aged less than 50 years 3 and in any case the study of Inoue et al. 1 included postmenopausal women only. Thus our present analyses were restricted to the age interval 50 -79 years.Our cohort included 48,225 parous women with data on breastfeeding contributing person-years to this age interval and similarly 11,415 nulliparous women. The mean duration of breastfeeding per child among parous women was 6.0 months, with a standard deviation of 4.0 months. During follow-up, a total of 605 cases of gastric cancer were diagnosed. The information recorded at the Cancer Registry of Norway allows a classification of gastric cancer into 4 categories with regard to subsite: 4 proximal stomach (cardia and fundus; 71 cases), middle stomach (body; 65 cases), distal stomach (antrum and pylorus; 190 cases) and subsite unknown (both curvatures, overlapping and unspecified subsites, and previous gastric resection; 279 cases).Relative incidence was assessed by Poisson regression analysis of log-rates, considering the number of person-years contributed by each woman during follow-up. Table I shows results in the categories of average duration of breastfeeding per child cons...