Abstract
Abstract When the 13C/12C ratios in lactose and milk fat of normally fed healthy cows are compared to 13C/12C in feed a metabolic depletion of 13C in the milk constituents becomes obvious. Due to a carbon isotope effect in the decarboxylation step of pyruvate to Acetyl-Co-A resulting in a 13C depleted acetate, depot fat and milk fat are especially reduced in 13C relative to 12C. Under pathophysiological conditions after longer fasting periods or in ketosis, long chain fatty acids from depot fat are metabolized and their 13C depleted carbon atoms enter the citric acid cycle via Acetyl-Co-A. Then the intermediary products of this cycle become 13C depleted too and with a drain on the oxalacetate pool by gluconeogenesis 13C depleted carbon atoms are channeled into lactose. This hypothesis is apt to explain a 13C/12C decrease in lactose of ketotic cows relative to healthy controls. It is supported by the observation that 13C-rich fatty acids from corn oil dosed orally to a ketotic cow increase the 13C/12C rati...
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