[Is breast-feeding a protective factor for infectious diseases? A case-control study].
AIM: To analyze if breast-feeding (BF) is a protective factor against infectious diseases. DESIGN: Case-control study. Retrospective study during 1998. SETTING: Primary care. Teaching health center. Population of 22,500 inhabitants. PATIENTS: We consider an alpha risk of .05 and a beta risk of .20, unilateral hypothesis. We estimated a sample of 48 children in the "case" group and 144 in the "control" group. Random sample. MEASURES: Cases were children with 3 or more acute respiratory or digestive infectious episodes during the first year of age. Controls were children with less than 3 episodes. We analyze exposure to breast feeding (BF) and we consider it if the child had at least 3 months of BF. In other case we define as a non-exposure child. We have calculated odds ratio (OR) and its 95% confidence intervals. RESULTS: 60.4% of the children were BF after birth. But only 28.5% lasted 3 months, and only 13.9% lasted 1 year. The median of BF was 3.3 months (SD 4.2). 79.2% of the children have a respiratory infectious disease in the first year and 14.6% diarrhea. In children less than 1 year old the OR was 0.81 (95% CI, 0.40-1.63). In the period 3-6 months old we have found an OR of 0.346 (95% CI 0.161-0.744). CONCLUSIONS: In children aged less than 1 year, we have not found protective effects of breast-feeding against acute respiratory infectious diseases. Only in the period 3-6 months old, we have found that BF protects against these diseases.