Relationships among lactation performance, maternal diet, and body protein metabolism in humans

The relationships between lactation performance and maternal diet and body protein metabolism were determined at 1, 5, and 12 months postpartum in lactating women who consumed a controlled diet of measured protein and energy. Milk production was measured by the 24-h test weighing procedure. Maternal body protein metabolism was evaluated by nitrogen balance and a primed, constant infusion of [1-13C]leucine and [alpha-15N]lysine. Milk production was associated positively with lysine flux (P less than 0.05, r = 0.59), leucine incorporation into body protein (P less than 0.05, r = 0.58), nitrogen intakes (P less than 0.05, r = 0.56), and energy intakes (P less than 0.01, r = 0.69). When adjusted for postpartum time, significant associations between total nitrogen concentrations in milk and nitrogen balance also were present (P less than 0.05, r = 0.77). These observations document associations among lactation performance, maternal diet, and the metabolic responses of body protein stores in well-nourished women and suggest strategies for the improvement of milk production in settings where nutrient insufficiency and malnutrition prevail.