The relationship between breast-feeding and later cardiovascular health has been investigated in randomised trials and observational studies. This review focuses on randomised control trials, regarded as the ‘gold standard’ in establishing causal relationships between interventions and outcomes. Since it is not ethical to randomise healthy term infants to be breast- or formula-fed, only two randomised control trials have examined effects of breast-feeding on later health. In one randomised control trial, preterm infants randomised to receive banked donor breast milk had significantly lower blood pressure (BP), more favourable plasma lipid profile and reduced leptin resistance at age of 13–15 years compared with those who were fed preterm formula; with a dose–response relationship between the proportion of human milk and later outcomes. In contrast, a cluster-randomised control trial of a breast-feeding promotion intervention in healthy term infants (Promotion of Breast-feeding Intervention Trial study) found no effect of the intervention on adiposity or BP at 6 years, despite increased incidence, duration and exclusivity of breast-feeding. Potential explanations for the discrepancy between the two studies include: (i) beneficial effects of breast milk on cardiovascular health might be confined to preterm infants; (ii) effects on cardiovascular outcomes may not manifest until adolescence, a concept supported by other studies; (iii) if the underlying mechanism for the effect of breast-feeding on later cardiovascular outcome is slower early growth; a concept supported by data from animal models, human observational studies and now experimental studies in human subjects; it is plausible that differences in early growth between groups in the Promotion of Breast-feeding Intervention Trial were insufficient to produce a detectable effect on these outcomes.
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