Lysine and protein metabolism in the young lactating woman

Five lactating and five postpartum non-lactating women of similar ages, times postpartum, body weight and height consumed a liquid formula diet that supplied 1.3 g protein and 32 kcal/kg per day (lactating subjects) and 1.1 g protein and 26 kcal/kg per day (non-lactating subjects). Their last meal supplied 25% of the daily intake and was consumed 4 h before they received L-[13C1]lysine (27 mumol/kg) by a single intravenous injection and L-[15N2]lysine (27 mumol/kg) orally. Frequent plasma and breath samples were collected for 6 h during which time they consumed no food. On a separate day, subjects received NaH13CO3 (10 mumol/kg) as a single intravenous dose and breath samples were collected for 6 h. Plasma tracer lysine levels were determined by gas chromatography-mass spectrometry isotope ratiometry, and breath 13CO2 levels were measured by gas-isotope-ratio mass spectrometry. Averaged tracer data for the two groups were fitted to a multicompartmental model of lysine and protein metabolism that partitioned lysine kinetics between a central and two tissue compartments. The tissue compartments had characteristically fast and slow rates of lysine turnover. The results were compared with those previously obtained in nulliparous women. The postpartum state was associated with a reduction in protein turnover in a compartment with a rapid rate of protein turnover and postpartum women catabolized significantly less lysine than nulliparous controls. Lactating women catabolized slightly more lysine than the non-lactating postpartum subjects, especially when lysine catabolism was expressed as a proportion of lysine flux. Lactation was associated with smaller splanchnic and extracellular pools of free lysine and with an increase in the rate constant for absorption of orally administered lysine. Lysine flux was significantly lower in the lactating subjects and this was associated with a decrease in the rate of lysine turnover in the slowly turning over lysine compartment. The results suggest that lactation is associated with a slower rate of protein turnover in a peripheral tissue compartment. We conclude that an intake of 1.3 g protein/kg per day may be inadequate to support the protein needs of lactation and body protein metabolism and may result in metabolic adaptations that maintain lactation at the potential expense of other aspects of maternal protein turnover.