Does protein intake alter the precursors for synthesis of lactose and non‐essential amino acids by the mammary glands of lactating mice?

Abstract

The aims were to: 1) develop a [U-13C]glucose tracer approach to establish the pathways of and substrates used for milk lactose and casein synthesis in the mouse mammary gland and 2) determine the influence of protein intake on this partition and use for milk synthesis. In Study 1, we determined the time-course to reach isotopic plateau in milk lactose and casein during the feeding of [U-13C]glucose. Four mid-lactation mice (C57/BL) were fed a standard diet with 10% of dextrose replaced by [U-13C]glucose for 5 d with milk collected after 1, 3 and 5 d of tracer feeding. 13C-Isotopic and isotopomer plateau was attained in milk lactose (glucose, galactose) and all non-essential amino acids (NEAA) in milk casein by 3 d. In Study 2, six paired (intake and pups equal) sets of lactating mice were fed either a normal (N, 20%) or low (L, 10%) protein diet from d 2 to 15 of lactation. The last 3 d, [U-13C]glucose replaced 10% of diet dextrose. On d 15, milk and mammary tissue were collected. In Study 1 and 2, 13C-mass isotopomer distribution (MID) in lactose-derived glucose and galactose did not differ, indicating common mammary metabolic pools. In Study 2, L mothers and pups weighed less (P<0.01). In addition, 13C-MID in blood vs. casein indicated significant mammary synthesis of glutamate (N:51%, L:50%), alanine (N:32%, L:29%), and serine (N:18%, L:37%, P<0.05). This NEAA synthesis suggests additional requirements for glucose and(or) EAA.