INSULIN, CORTISOL AND THYROID-HORMONES MODULATE MATERNAL PROTEIN STATUS AND MILK-PRODUCTION AND COMPOSITION IN HUMANS

The partitioning of dietary and endogenous nutrients during lactation is not well understood. To examine associations between plasma hormone and substrate profiles and indices of either maternal body protein me tabolism or lactational performance, we measured plasma insulin, corti sol, prolactin, thyroxine, triiodothyronine, individual amino acid, bl ood urea nitrogen, and prealbumin concentrations in lactating and null iparous women in the postabsorptive state. We related these measuremen ts to the subjects' nitrogen balance urinary 3-methylhistidine excreti on, [1-C-13]leucine metabolism and milk production. Insulin concentrat ions showed significant positive relationships with nitrogen balance a nd prealbumin concentrations; cortisol levels showed a significant neg ative relationship with nitrogen balance and a significant positive re lationship with leucine incorporation into protein. Thyroid hormone co ncentrations showed significant positive relationships with urinary 3- methylhistidine excretion, leucine incorporation into protein, and mil k production. Proline concentrations were associated positively with n itrogen balance and negatively with leucine incorporation into protein , whereas glutamate-glutamine concentrations showed positive associati ons with leucine oxidation and milk nitrogen concentrations. We propos e that insulin and cortisol modulate the channeling of nutrients betwe en anabolic and anti-anabolic aspects of maternal body protein metabol ism, whereas thyroid hormones and cortisol modulate nutrient partition ing toward milk production and visceral protein synthesis. We suggest that some nonessential amino acids (proline, glutamate-glutamine) may become limiting during lactation because of their unique contributions to milk protein synthesis.