Duration of breast-feeding and adiposity in adult life.

Few studies have examined whether the duration of breast-feeding is associated with BMI in adult life. In the past, the heights and weights of infants and the duration of breast-feeding were routinely recorded at infant welfare clinics in Helsinki, Finland. Most infants in the city were taken to these free clinics. The Helsinki Birth Cohort comprises 13,345 people born in the city during 1934-1944; 84% were breast-fed. In 2001, a questionnaire was sent to members of the cohort asking about their weight and height. A random sample of 2003 men and women attended a clinic at which height, weight, and body composition were measured. We studied sibships that included 2 or more people from the cohort. There were 1823 subjects: 831 had completed the questionnaire; 129 had attended the clinic. We grouped the subjects according to duration of breast-feeding: 0-2 mo, 3-4 mo, 5-7 mo, and 8 mo or more. We compared siblings who were discordant for duration of breast-feeding. We found that a longer period of breast-feeding was associated with lower BMI at 1 y of age (P = 0.04 for a linear trend). This relation disappeared by the age of 7 y. People breast-fed for 5-7 mo had the lowest reported BMI at age 60 y, although this was not statistically significant; 8.8% more people breast-fed for 8 mo or more had reported BMIs that were overweight (25 to 30 kg/m(2)) compared with those breast-fed for shorter periods (P = 0.06). Breast-feeding for <2 mo or 8 mo or more was associated with an increased BMI and percentage body fat in later life, measured at the clinic (P = 0.08 and P = 0.03 for quadratic trends). We conclude that breast-feeding for <2 mo may be deleterious, possibly because of lack of exposure to protective factors in breast milk. Breast-feeding beyond 8 mo may be deleterious because mother's hormones in breast milk reset the infant's hypothalamic-pituitary-thyroid axis.