WHOLE-BODY PROTEIN-TURNOVER IN THE FED STATE IS REDUCED IN RESPONSE TO DIETARY-PROTEIN RESTRICTION IN LACTATING WOMEN

We examined the adaptive responses of body protein metabolism in the f ed state to dietary protein restriction in lactating women to determin e whether rates of body protein degradation and synthesis were lower t han those of nonlactating women. Thirteen healthy women (five lactatin g, four nonlactating postpartum, four nulliparous) aged 28-32 y were g iven protein intakes of 1.5, 0.4, and 1.0 g . kg(-1). d(-1) over three consecutive 3-d periods, respectively. At the end of each period, whi le in the fed state, subjects received orally a single bolus dose of [ 1-C-13]leucine. A 24-h urine collection was obtained simultaneously. W hole-body protein metabolism was characterized by using the end produc t model based on nitrogen excretion and leucine catabolism. Nitrogen f lux and rates of protein degradation and synthesis in the fed state we re significantly lower at a dietary protein intake of 1.0 g . kg(-1). d(-1) in lactating women than in their nonlactating postpartum counter parts. Net protein retention in the fed state was significantly higher at a dietary protein intake of 1.0 g . kg(-1). d(-1) in lactating tha n in nonlactating postpartum and nulliparous women because of the rela tively greater reduction in protein degradation than in protein synthe sis. These studies suggest that lactating women rapidly adapt to dieta ry protein restriction by down-regulating protein metabolism, and that C-13-labeled amino acid tracers in combination with urinary nitrogen excretion serve as useful metabolic markers for the adequacy of the di etary protein content of lactating women.