BACKGROUND: Sex hormones play a role in gastric cancer and colorectal cancer etiology, however, epidemiological evidence is inconsistent. This study examines the influence of menstrual and reproductive factors over the risk of both tumors. METHODS: In this case-control study 128 women with gastric cancer and 1293 controls, as well as 562 female and colorectal cancer cases and 1605 controls were recruited in 9 and 11 Spanish provinces, respectively. Population controls were frequency matched to cases by age and province. Demographic and reproductive data were directly surveyed by trained staff. The association with gastric, colon and rectal cancer was assessed using logistic and multinomial mixed regression models. RESULTS: Our results show an inverse association of age at first birth with gastric cancer risk (five-year trend: OR = 0.69; p-value = 0.006). Ever users of hormonal contraception presented a decreased risk of gastric (OR = 0.42; 95%CI = 0.26-0.69), colon (OR = 0.64; 95%CI = 0.48-0.86) and rectal cancer (OR = 0.61; 95%CI = 0.43-0.88). Postmenopausal women who used hormone replacement therapy showed a decreased risk of colon and rectal tumors. A significant interaction of educational level with parity and months of first child lactation was also observed. CONCLUSION: These findings suggest a protective role of exogenous hormones in gastric and colorectal cancer risk. The role of endogenous hormones remains unclear.

The study was supported by the Consortium for Biomedical Research in the “Acción Transversal del Cáncer," approved on/nthe Spanish Ministry Council on the 11th October 2007, by the Instituto de Salud Carlos III grants PI08/1770 (to M. Kogevinas), PI09/0773 (to J. LLorca), PI09/1286 (to V. Martín), PI09/1903 (to R. Peiró), PI09/1662 (to J.J. Jiménez-Moleón), PI11/01403 (to N. Aragonés), by the Instituto de Salud Carlos III, co-funded by FEDER funds – a way to build Europe – PI08-1359, PI14-0613 (to V. Moreno), by the European Commission grants FOOD-CT-2006-036224-HIWATE (to V. Moreno), by the Catalan Government DURSI grant 2014SGR647 (to V. Moreno), by the Fundación Marqués de Valdecilla grant API 10/09 (to J. Llorca), by the Junta de Castilla y León grant LE22A10-2 (to V. Martı´n), by the Consejería de Salud of the Junta de Andaluc´ıa grant 2009-S0143 (to J. Alguacil), and by the Conselleria de Sanitat of the Generalitat Valenciana grant AP061/10 (to R. Peiró). The funders had no role in the study design and data analysis