Fructose Metabolism in Dairy Cows

Abstract

Metabolism of C14 fructose has been studied in dairy cows and the results compared to those obtained earlier with C14 glucose. C14 fructose disappeared rapidly from the plasma; simultaneously there occurred an immediate rise in plasma glucose C14. The pattern of isotope distribution in milk C and the peak expired CO2 specific activity was the same after fructose as after glucose injection. These results suggest that glucose is a key intermediate in the metabolism of fructose—the conversion of fructose to glucose presumably occurring in the liver. That fructose is also metabolized to a considerable extent by other pathways was indicated by the earlier peak—and more rapid decline—in expired CO2 specific activity. Further, considerably less fructose C14 was found in expired CO2 and in milk C 24 hours after injection. The extent to which fructose was metabolized by pathways which did not involve plasma glucose was estimated by comparing the integrated specific activity/time curves after fructose and glucose injection. It appears that up to 50% of the transfer of fructose C to expired CO2 and to milk C goes by pathways exclusive of blood glucose.