The effect of between-breast differences on human milk macronutrients content

Objective: Little is known about the effect of maternal handedness and preferential side of breastfeeding upon macronutrients concentration in human milk (HM). We aimed to compare macronutrients content of HM from both breasts, taking into account the self-reported preferential feeding ('dominant') breast, breast size and handedness (right versus left). We tested the null hypothesis that macronutrients content of HM is not affected by breast dominancy, breast size or maternal handedness. Study Design: Fifty-seven lactating mothers were recruited. HM macronutrients were measured after mid manual expression using infrared transmission spectroscopy. Results: Out of the 57 mothers recruited, 12 were excluded from the analyses because they brought in insufficient samples. Among the 22 who reported a size difference, 16 (73%) had a larger left breast (P<0.001). Approximately a third of women reported no breastfeeding side dominance, a third reported a right dominance and another third reported a left dominance. Breastfeeding side dominance was unaffected by either handedness or breasts size. When size asymmetry was reported (n=22) the dominant side was also the larger breast in 16 (73%) women, the smaller breast in 2 (9%) women, whereas 4 (18%) additional women with asymmetry had no preferential breastfeeding side. There were no statistically significant differences in macronutrients between the right and the left breasts. In multiple stepwise backward regression analysis, fat, carbohydrate, protein and energy contents were unaffected by maternal handedness, breast side dominance or breast size asymmetry. Conclusions: Macronutrients content of mid expression HM is unaffected by maternal handedness, breast size or breast side dominance.