Increasing Dietary Palmitic and Stearic Acid Decreases De Novo Synthesis of Milk Fatty Acids in Dairy Cows Based on Modeling Natural Carbon Isotope Enrichment.

Background: Mammary de novo lipogenesis is dynamic, but total lipogenesis is not easily quantified because palmitic acid is also available from dietary absorption.

Objectives: This study aimed to quantify the effect of dietary palmitic and stearic concentrations on mammary de novo lipogenesis using natural ¹³C enrichment differences in ingredients.

Methods: Twelve midlactation Holstein cows were used in a 4 × 4 Latin square design with 21-d periods. Treatments were a low-fat basal diet that contained ∼85% corn-derived ingredients (CON) and the basal diet with an additional 1.95% dietary fatty acid (FA) from a high palmitic acid (HP), high stearic acid (HS), or blend of palmitic and stearic acid (PS) supplement derived from palm oil. Carbon (C) isotope enrichments in feed and milk FAs were determined by isotope ratio mass spectrometry, and de novo lipogenesis calculated. Data analyzed as a mixed model and means separated using a protected least significant difference test.

Results: In the low-fat control, 83% ± 2.62% of the 16-C FA in milk fat were calculated to be from de novo lipogenesis. Increasing dietary palmitic acid in PS and HP decreased de novo 16-C FAs by 27 and 82 g/d respectively (P < 0.001) compared with CON but total de novo synthesized FAs decreased by 130 g/d in HP only (P < 0.001). The transfer efficiency of dietary 16-C FAs to milk fat was between 44% and 49%. The basal diet accounted for 98.3% ± 2.56% of the 18-C FAs in milk fat in the control.

Conclusions: Natural ¹³C isotope enrichment allowed calculation of the contribution of de novo lipogenesis to 16- and 18-C FAs in milk fat. The proportion of de novo synthesized 16-C FAs was higher than expected when feeding a lower fat diet and was differentially decreased by increasing dietary palmitic and stearic acid.