Butyrate as a precursor of milk constituents in the intact dairy cow.

Butyric acid is a major product of feed fermentation in the intestinal tract of various animals, especially the rumen of cattle and sheep. This has been known for 70 years (1). Elsden, Hitchcock, Marshall, and Phillipson (2) noted that the rumen and reticulum of an ox contained an average of 329 gm. of volatile acids (calculated as acetic acid), and that the mixture of volatile fatty acids in the intestines of cattle, sheep, horses, pigs, and rabbits had a similar composition, namely an average of 67 per cent acetic, 19 per cent propionic, and 14 per cent butyric acid. (Th ese figures presumably represent molar percentages.) The relative rate of absorption of fatty acids from the rumen depends on the acidity of the rumen content (3). Kiddle, Marshall, and Phillipson (4) suggest that a greater part of the butyrate than of the other fatty acids is catabolized in the rumen wall, and Pennington (5) demonstrated that tissue slices of rumen epithelium convert butyrate to acetoacetate. Butyrate is thus a most interesting substance in the nutrition of cattle and sheep, its importance in biochemistry, however, not being limited to ruminants. It may play a role as a link between fat and carbohydrate metabolism. Its function as such in rats may, at least in part, be independent of acetate (6), and a considerable part of its contribution to the synthesis of animal carbohydrate by-passes the carbonate pool (7). We injected C14-labeled butyrate as a single dose into the jugular vein of lactating cows. We then measured the Cl4 concentration in the expired COz and in lactose, casein, milk albumin, and butter fat as a function of time after injection of the tracer. These measurements permitted estimates of the relative rates at which butyrate provides carbon for respiratory COZ and for the synthesis of the organic milk constituents.