Submitted to: Journal of Clinical Endocrinology and Metabolism
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/15/2001
Publication Date: 1/1/2002
Citation: Sunehag,A.L., Louie,K., Bier,J.L., Tigas,S., Haymond,M.W. 2002. Hexoneogenesis in the human breast during lactation. Journal of Clinical Endocrinology and Metabolism. 87(1):297-301. Interpretive Summary: The present studies demonstrate that although the plasma glucose pool is the principal source of milk lactose in humans, a significant amount of lactose is generated within the breast, a process we have denoted "hexoneogenesis". Thus, during feeding of a mixed macronutrient drink or glucose alone, plasma glucose contributed nearly all of the glucose but only 65% of the galactose in milk lactose. In the fasted state, 25% of the glucose and 50% of the galactose was produced within the breast. In addition, if one can extrapolate from the ingestion of glucose labeled with 13C to that of a normal mixed meal, our data suggest that 75% of the milk lactose is derived directly from the meal carbohydrate.
Technical Abstract: Lactose is the major osmotic agent in milk. Therefore, lactose synthesis indirectly regulates milk volume. The aim of this study was to determine the source of glucose and galactose in lactose. Six healthy lactating women were studied twice, during a 24 h fast and during ingestion of a mixed macronutrient drink (Sustacal) using [U-13C]glucose and [2-13C]glycerol. Six additional lactating women were studied on one single occasion during ingestion of glucose labeled with [1-13C]glucose. Using the ratios of [13C6] enrichments of glucose in lactose and plasma glucose and that of galactose in lactose and plasma glucose, we determined that 98 ± 3% of glucose and 68 ± 7% of galactose in lactose were derived from plasma glucose in the fed state, and 72 ± 4 and 51 ± 3%, respectively, following a 24 h fast. Virtually identical results (97 ± 6 and 64 ± 4%, respectively) were obtained during the glucose feeding study. Based on the [13C1] enrichment of glucose and galactose in lactose (derived from [2-13C]glycerol), glycerol contributes to the production of galactose but not glucose within the breast. Thus, plasma glucose is an important source of lactose, but significant amounts of glucose and galactose in lactose are generated within the breast, a process denoted "hexoneogenesis". In this process glycerol is a precursor for milk galactose but not glucose.