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Hexoneogenesis in the Human Breast during Lactation

Department of Pediatrics, Children’s Nutrition Research Center, U.S. Department of Agriculture/Agricultural Research Service, Baylor College of Medicine, Houston, Texas 77030

Address all correspondence and requests for reprints to: Morey W. Haymond, M.D., CNRC, 1100 Bates Street, Houston, Texas 77030. E-mail: mhaymond{at}bcm.tmc.edu

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, after a 24-h fast. Virtually identical results (97 ± 6 and 64 ± 4%, respectively) were obtained during the glucose feeding study. On the basis of 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.

This project was supported by NIH Grants RO1DK 55478 and HD 37857, U.S. Department of Agriculture (USDA) Cooperative Agreement 58-6250-6-001, and Baylor General Clinical Research Center Grant MO1-RR-00188-34.

This work is a publication of the USDA/Agricultural Research Service, CNRC, Department of Pediatrics, Baylor College of Medicine, Houston, TX. The contents of this publication do not necessarily reflect the views of policies of the USDA, nor does mention of trade names, commercial products, or organizations imply endorsement from the U.S. Government.

Abbreviations: CNRC, Children’s Nutrition Research Center; GCMS, gas chromatography mass spectrometry; USDA, U.S. Department of Agriculture.

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