De novo synthesis of milk triglycerides in humans

Abstract

Mohammad MA, Sunehag AL, Haymond MW. De novo synthesis of milk triglycerides in humans. Am J Physiol Endocrinol Metab 306: E838 -E847, 2014. First published February 11, 2014 doi:10.1152/ajpendo.00605.2013 de novo lipogenesis contributes significantly to milk fat in animals but little is known in humans. Objective: To test the hypothesis that the incorporation of 13 C carbons from [U-13 C]glucose into fatty acids (FA) and glycerol in triglycerides (TG) will be greater: 1) in milk than plasma TG, 2) during a high-carbohydrate (H-CHO) diet than high-fat (H-FAT) diet, and 3) during feeding than fasting. Seven healthy, lactating women were studied on two isocaloric, isonitrogenous diets. On one occasion, subjects received diets containing H-FAT or H-CHO diet for 1 wk. Incorporation of 13 C from infused [U-13 C]glucose into FA and glycerol was measured using GC-MS and gene expression in RNA isolated from milk fat globule using microarrays. Incorporation of 13 C2 into milk FA increased with increased FA chain length from C2:0 to C12:0 but progressively declined in C14:0 and C16:0 and was not detected in FAϾC16. During feeding, regardless of diets, enrichment of 13 C2 in milk FA and 13 C3 in milk glycerol were ϳ3-and ϳ7-fold higher compared with plasma FA and glycerol, respectively. Following an overnight fast during H-CHO and H-FAT diets, 25 and 6%, respectively, of medium-chain FA (MCFA, C6 -C12) in milk were derived from glucose but increased to 75 and 25% with feeding. Expression of genes involved in FA or glycerol synthesis was unchanged regardless of diet or fast/fed conditions. The human MG is capable of de novo lipogenesis of primarily MCFA and glycerol, which is influenced by the macronutrient composition of the maternal diet. fatty acid; glycerol; GC-MS; gene expression; lipogenesis; stable isotopes TRIGLYCERIDES (TG) make up 98% of the lipid content and, in humans, contributes 40 -50% of the total energy content (28). Total milk lipid content ranges, depending on the species, from 0 to 50% of the total milk volume (20). The fatty acid (FA) composition of the TG is dependents on the dietary FA composition, the dietary carbohydrate/lipid ratio and the species. Milk TG contains FA that are derived from three sources: de novo mammary gland (MG) synthesis, dietary lipids, and endogenous fat stores (adipose or hepatic lipids) (7). Data from a study comparing the milk fats from seven different species (4) revealed that the milk fats of humans, dogs, and guinea pigs are largely (92, 93, and 97%, respectively) made up of longchain FA (LCFA). The milk fats of horses contain large amounts (ϳ33%) of medium-chain FA (MCFA); however, the milk fats of cows, sheep, and goats are enriched (13-15%) in short-chain FA (SCFA) (4). The MG is one of three primary lipid-synthesizing organs in the body; the other two are liver and adipose tissue (48). In fact, many of the initial pathways of FA biosynthesis were defined using mammary tissue from lactating ruminants and rodents (1). The unique feature of FA synthesis in the MG is th...

Precursors of hexoneogenesis within the human mammary gland

“…The analyses were performed using SP17-1 capillary columns, and conditions for the GC-MS were as follows: temperature program 70°C for 1 min, a temperature ramp at 25°C/min up to 320°C followed by a 6-min hold, monitoring in the EI mode for ions of m/z 242-245 allowing detection of the incorporation of 13 C into 1-3 carbons, while m/z 217-220 to detect the incorporation of 13 C into 4 -6 carbons. The triacetate derivative of free glycerol in plasma and milk was prepared using acetic anhydride as described previously (19). Additionally, breast milk triglycerides were hydrolyzed using alkaline saponification (17) and total glycerol enrichments, and concentrations were measured (26).…”

“…Additionally, breast milk triglycerides were hydrolyzed using alkaline saponification (17) and total glycerol enrichments, and concentrations were measured (26). The pentafluorobenzyl-acetyl derivative of lactate was analyzed for the isotopic enrichments of [U- 13 C]lactate in both plasma and milk by GC-MS using an HP-5 capillary column, monitoring for ions of m/z 131-134 in the NCI mode (19). The conditions for the GC-MS were as follows: temperature program 70°C for 1 min and a temperature ramp at 15°C/min up to 320°C followed by a 2-min hold.…”

“…For the determination of milk glutamine enrichments, defatted and deproteinized samples were run through an AG-50W resin column prior to esterification and derivatization. Enrichments of acetate in plasma and milk were performed using the PFB derivative as has been described (19). We encountered problems in detecting 13 C enrichments in milk acetate; accordingly, the M 2 enrichment of lauric acid (C12:0) was measured as described (19) and was used as a surrogate for acetate enrichment in milk (after being divided by 6, i.e., number of acetate units in C12:0 FA) (19).…”

“…Additionally, breast milk and plasma samples derived from previous published studies (19,20) were analyzed for isotopomer distributing analysis under similar study conditions except that these studies were preceded by a 7-day high-carbohydrate/low-fat diet (HC) or low-carbohydrate/high-fat diet (HF) and the subject received an infusion of only [U- 13 C]glucose during 12-to 14-h fasting (19,20).…”

“…From our pervious study of an infusion of [U- 13 C]glucose alone following an overnight fast on both high-carbohydrate and high-fat diets (19,20), the mass isotopomer distribution analyses (MIDA) utilizing the pentaacetate derivative of plasma and milk hexoses indicated lower (P Ͻ 0.01) M 6 but higher (P Ͻ 0.05) M 1 , M 2 , M 3 , and M 4 in galactose compared with glucose in either plasma or milk ( Table 2).…”