Availability and Metabolism of Various Substrates in Ruminants. V. Entry Rate into the Body and Incorporation into Milk Fat of d(−)β-Hydroxybutyrate

Abstract

Aeetate-l-l~C or D (-)fl-hydroxybutyrate-1,3-14C (BHBA) was infused into the left front quarters of the udders of cows fed either a normal (control) or a high-grain, low-fiber diet. Approximately twice as much 14C was recovered in the milk fat during 20 hr after acetate-14C infusion as after BttBA-14C infusion. Also, about twice as much 14C was recovered in the fat of control cows as in those fed a high-grain diet. Specific radioaetivities indicated that 50% of the four c~,rbon atoms at the methyl-terminal end of the C4-Cle milk fatty acids arose from BHBA and that only one molecule of BHBA was incorporated into any fatty acid molecule. It was calculated that BHBA contributes a maximum of 8% of the milk fatty acid carbon. Labeling patterns after acetate-14C infusion indicated an orderly condensation of C 2 units in the formation of milk fatty acids from C 4 to Cla; however, about 40% of the milk fat palmitate and all of the Cls milk fatty acids arose from a source other than mammary synthesis. No differences were observed between dietary treatmerits in the plasma concentrations of BHBA, the entry rate of BHBA, its oxidation to carbon dioxide, or its contribution to the total expired carbon dioxide. About one-half of the total entry of BHBA appeared in milk fatty acids and expired carbon dioxide; the fate of the remainder was unaccountable.