Infant feeding, cortisol metabolism and weight gain in the first six months of life

Abstract

Breastfeeding has a protective effect on obesity. Little is known about the mechanisms involved. Breast-milk contains cortisol, which is involved in appetite regulation (Wolkowitz, Epel, & Rues, 2001)[[Au: The references “Wolkowitz, Epel, and Rues, (2001) and Epel et al. (2001)” are cited in the text but not listed. Please check.]] and energy balance (Epel et al., 2001). This study is the first to investigate the role of breastfeeding and cortisol metabolism on weight-gain throughout the first year. It was hypothesised that breastfed infants would be lighter than formula-fed infants and that there would be a difference in their cortisol metabolism. Sixty-one infants (33 males, 28 females) and their mothers (mean age 30.0 years [SD 6.0]) were tested at 1-week, 1-, 3- and 6-months postpartum. Maternal and infant weights and feeding method were measured. Single urine samples from mothers and 24-h nappy collections were taken for cortisol metabolism analysis. ANCOVAs examining differences in infant weight showed at 1-week, there were no significant differences in weight F (2, 70) = 1.32, p = 0.27. At 1-month F (2, 65) = 3.31, p < 0.05 and 6-months F (2, 48) = 4.10, p < 0.05, there were significant differences between groups. Breastfed infants weighed less than formula-fed infants at 1-month, t (65) = −1.3, p = 0.05 and 6-months, t (48) = −2.09, p < 0.05. A MANCOVA examining differences in cortisol metabolism at 6-months between these groups showed a significant difference F (4, 2) = 21.31, p < 0.05; Wilks’ λ = 0.001 in steroid ratio UFF/UFE (indicative of higher cortisol) F (2, 2) = 86.99, p = 0.01. Breastfed infants have higher levels of UFF/UFE than formula-fed and mixed-fed infants ( p < 0.05). This is the first study to indicate that cortisol metabolism may be one mechanism by which breastfeeding is protective against greater weight gain in infancy.