Could associations between breastfeeding and insulin-like growth factors underlie associations of breastfeeding with adult chronic disease? The Avon Longitudinal Study of Parents and Children.

OBJECTIVE: The influence of infant feeding method (breast/formula) on growth factor levels could underlie associations of breastfeeding with childhood growth and risk factors for cardiovascular disease. We investigated associations of having been breastfed with serum IGF-I and IGFBP-3 in childhood.
METHODS: Prospective birth cohort study (subsample of the Avon Longitudinal Study of Parents and Children, UK) based on 871 children born in 1991/1992 who underwent clinical follow-up and blood tests at age 7-8 years. A total of 488 (56%) children had complete data.
RESULTS: In children with complete data, the age- and sex-standardized IGF-I levels of those who were partially or exclusively breastfed were 6.1 and 13.8 ng/ml higher, respectively, than those who were never breastfed (increase in IGF-I levels per category of breastfeeding exclusivity: 7.1 ng/ml; 95% CI: 0.3-13.9; P = 0.04). In models also controlling for birthweight, gestational age, mother's age, and socioeconomic and dietary factors, the breastfeeding-IGF-I association was attenuated (regression coefficient: 3.3 ng/ml; -4.2-10.7; P = 0.4); further adjustment for IGFBP-3 made little difference (regression coefficient: 4.1 ng/ml; -2.8-10.9; P = 0.2). There was little evidence for an association between breastfeeding and IGFBP-3 or the molar ratio IGF-I/IGFBP-3.
CONCLUSIONS: The positive association between breastfeeding and IGF-I could be due to residual confounding or to chance. Nevertheless, the magnitude of the fully adjusted effect estimate and the novelty of the association suggest that larger studies should now be conducted to confirm or refute the hypothesis that variations in IGF-I by infant feeding mode explain associations of breastfeeding with health in later life.