Submitted to: Journal of Clinical Endocrinology and Metabolism
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/20/2002
Publication Date: 1/1/2003
Citation: J Clin Endocrinol Metab 88:225-229, 2003 Interpretive Summary: During ingestion of galactose at 22 mmol/kg min, 9.2 mmol/kg min appeared in the systemic circulation, 5.5 mmol/kg min appeared as plasma glucose and 4 mmol/kg min was oxidized i.e., we accounted for 85% of the oral galactose. Since plasma lactate concentrations doubled during ingestion of galactose, a portion of oral galactose was most likely converted to plasma lactate. Plasma galactose directly contributed ~10% of milk lactose with a larger proportion directed towards galactose in lactose as compared to glucose in lactose (12 vs. 7%). The direct contribution from plasma glucose to lactose was similar to that observed in the fasted state in our previous study. Thus, the carbon provided by plasma galactose replaced part of the carbons provided by "hexoneogenesis" during fasting. This study demonstrates that although plasma glucose is the primary source of milk lactose, plasma galactose is a potential substrate for milk lactose. In normal individuals, the plasma concentrations of galactose are very low, thus, the contribution of galactose to lactose is small.
Technical Abstract: We have previously demonstrated that plasma glucose contributed 80% in the fed and 60% in the fasted state to lactose synthesis in humans, while de novo synthesis in the breast contributing to both the glucose and galactose moieties accounted for the remaining 20 and 40%, respectively, of lactose. The present study was conducted to determine, in lactating women, whether oral galactose is directly incorporated from plasma galactose into glucose and galactose in milk lactose or via conversion of galactose to glucose in the liver. Six healthy exclusively breast feeding women (30 ± 2 y) (mean ± SE) ingested galactose at 22 mmol.kg-1.min-1 for nine hours following an overnight fast during infusion of [6,6-2H2]glucose and [1-13C]galactose. We observed that 69 ± 6% of glucose and 54 ± 4% of galactose in lactose were derived directly from plasma glucose, whereas 7 ± 2 and 12 ± 2% of glucose and galactose in lactose, respectively, were derived directly from plasma galactose. De novo synthesis of glucose and galactose via "hexoneogenesis" accounted for 25 ± 8 and 35 ± 6%, respectively. We conclude, that during ingestion of galactose the contribution from plasma glucose to glucose and galactose in lactose was similar to that of a short term fasting, while part of the de novo synthesis of glucose and galactose in the breast was replaced by direct uptake of galactose.