Prolonged breast-feeding (six months or more) and milk fat content at six months are associated with higher developmental scores at one year of age within a breast-fed population.

Since dietary fats may affect brain composition and function in early life, we evaluated developmental indices at 1 year of age in relation to the duration of breast-feeding and the milk fat composition in a breast-fed population. A blinded monitor administered the Bayley test (2nd edition) to 1-year-old subjects born at term and exclusively breast-fed for at least 3 months. Weaning foods were introduced from the 5th month onward. Mothers' milk lipid composition (fat [wt/dL], fatty acid [wt/dL], FA% [% of total fatty acids]) was determined at 0, 1, 3, 6, 9, and 12 months. Statistics used were Student's t-test, Pearson's r, and multiple regression. Forty-four infants out of 95 recruited at birth met inclusion criteria. There was a progressive reduction of the number of breast-fed babies to 29 (6 mo), 17 (9 mo), and 10 (12 mo). Breast-feeding for 6 months or longer gave a 6.6-point advantage (95% confidence interval, -0.6, 13.8; P = 0.07 for the Bayley psychomotor developmental index (PDI) and 2.0 for the Bayley mental developmental index (MDI) (95% confidence interval, -3.2, 7.3; NS) compared with the 15 subjects breast-fed for fewer than 6 months. Among the milk lipid factors, the fat weight (mg/dL) at 6 months showed the highest association with the MDI (r = 0.55, P = 0.002). Thus, prolonged breast-feeding during the weaning process may result in a better developmental performance at 12 months, possibly due to the supply of fats contributing energy and/or affecting brain composition.