Kj. Motil et al., INSULIN, CORTISOL AND THYROID-HORMONES MODULATE MATERNAL PROTEIN STATUS AND MILK-PRODUCTION AND COMPOSITION IN HUMANS, The Journal of nutrition, 124(8), 1994, pp. 1248-1257
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.