Methylmalonic acid in low-fat milk syndrome.

The possible association between low-fat milk syndrome and methylmalonic acid accumulation in blood was investigated. Blood was sampled from the internal iliac artery of nine lactating dairy cows fed ad libitum roughage plus grain or high grain, restricted roughage diets. Daily milk fat percent and milk fat production were decreased 44% and 47% on the high grain, restricted roughage diet. No differences in blood methylmalonate concentrations could be detected in the cows fed the two diets. The effect of methylmalonate and its metabolic precursor, propionate, on mammary rates of fatty acid synthesis was determined with bovine mammary tissue slices. Neither of these metabolites affected acetate incorporation into fatty acids. Methylmalonate and propionate were incorporated into fatty acids at extremely low rates. Low-fat milk syndrome is not caused by accumulation of methylmalonic acid.