Macronutrients in human milk at 2, 12, and 16 weeks postpartum

This study evaluated changes in human milk composition from 2 to 16 weeks postpartum. Milk from 12 mothers was analyzed for lipid (utilizing a modified Folch), lactose (enzymatic hydrolysis), and nitrogen (semi-micro Kjeldahl). Energy was calculated by fractional analysis and bomb calorimetry. All samples were from well-defined subjects, and uniform collection procedures were used. Milk lipid, total nitrogen, and energy content differed significantly from one woman to another. Lipid and energy content increased from 3.98 +/- 1.0 to 5.50 +/- 1.1 gm/100 ml and 68.5 +/- 9.8 to 83.0 +/- 11.1 kcal/100 ml, respectively, and nitrogen content decreased significantly from 0.24 +/- 0.05 to 0.16 +/- 0.02 gm/100 ml from 2 to 16 weeks postpartum. Lactose remained statistically stable, increasing from 6.3 +/- 0.7 to 7.0 +/- 0.7 gm/100 ml. Estimates of energy from fractional analysis of macronutrients produced lower caloric estimates, especially at 2 weeks postpartum. Statistical differences over time and between and among mothers were found at all periods studied; therefore, on the basis of these data, we caution against reliance on single sample values as representative of mature milk either for individuals or for groups.