Mammary Lipogenic Enzyme Activity, trans Fatty Acids and Conjugated Linoleic Acids Are Altered in Lactating Dairy Cows Fed a Milk Fat–Depressing Diet

Abstract

The objectives of the present study were to examine the effect of a milk fat-depressing (MFD) diet on: 1) the activity of mammary acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS), 2) ACC mRNA relative abundance and 3) distributions of conjugated linoleic acids (CLA) and trans-18:1 fatty acids (tFA) in milk fat. Twelve lactating Holstein cows were used in a single reversal design. Two diets were fed: a control diet (60:40% forage/concentrate) and an MFD diet (25:70% forage/concentrate, supplemented with 5% soybean oil). The MFD diet decreased (P: < 0 0.001) milk fat by 43% and ACC and FAS activity by 61 and 44%, respectively. A reduced ACC mRNA relative abundance (P: < 0.001) corresponded with the lower ACC activity. The fatty acids synthesized de novo were decreased (P: < 0. 002), whereas tFA were increased from 1.9 to 15.6% due predominantly to a change in trans-10-18:1 isomer (P: < 0.001). With the MFD diet, the trans-7, cis-9 and trans-10, cis-12 CLA isomers were elevated (P: < 0.001), in contrast to the decrease in trans-11-18:1 (P: < 0. 001) and cis-9, trans-11-18:2. The data were consistent with a dietary effect on mammary de novo FA synthesis mediated through a reduction in ACC and FAS activity and in ACC mRNA abundance. The results were compatible with a role of trans-10, cis-12 CLA in milk fat depression, but alterations noted in tFA and other CLA isomers suggest that they also may be important during diet-induced milk fat depression.

Recent advances in the regulation of milk fat synthesis

“…Mammary lipogenic capacity may be regulated at multiple levels including transcription, translation, protein turnover and enzyme activity. Decreased mammary expression of genes for key enzymes and proteins involved in FA uptake, synthesis, transport and esterification occurred during both diet-and CLA-induced MFD (Piperova et al, 2000;Ahnadi et al, 2002;Baumgard et al, 2002b;Peterson et al, 2003;. Combined, these observations define MFD as a decreased mammary capacity for lipid synthesis due to a coordinated transcriptional down-regulation of enzymes and proteins involved in milk fat synthesis.…”

“…Piperova et al (2000) reported decreased mammary enzyme activity of acetyl-CoA carboxylase (ACC) and fatty acid synthase (FASN) during diet-induced MFD and Baumgard et al (2002b) observed decreased mammary lipogenic capacity during CLA-induced MFD based on 14 C acetate incorporation into FA by mammary tissue explants. This demonstrates an inhibition of milk fat synthesis in the mammary gland and is in agreement with the decrease in de novo-synthesized FA previously discussed.…”

Effect of silage type and concentrate level on conjugated linoleic acids, trans-C18:1 isomers and fat content in milk from dairy cows

“…A number of studies have demonstrated that increasing amounts of readily digestible carbohydrates in the diet are associated with increased proportions of trans10,cis12-CLA and trans10-C18:1 in milk fat [4][5][6]16]. The fact that the concentrate level only affected trans10,cis12-CLA and trans10-C18:1 but also the cis9,trans11-CLA and trans11-C18:1 concentration in milk when maize silage was fed, suggests that increased proportions of concentrate (starch) in the diet does not severely affect the ruminal biohydrogenation pattern unless combined with a forage naturally high in starch.…”

“…However, diets that provide large amounts of readily digestible carbohydrates such as starch, with [4,5] or without [6,7] a supplement of vegetable oil often lead to increased proportions of trans10-C18:1 and trans10,cis12-CLA in milk fat. This response is usually caused by a decline in rumen pH [4,6] and shifts in the bacterial populations [8] whereby the biohydrogenation pathway of C18:2n−6 is altered [9].…”

“…The effects of including oilseeds and plant oils with different PUFA profiles in high concentrate/low roughage diets on milk fat CLA concentration and milk fat depression have been thoroughly investigated [4,5,12]. Less well studied are the effects of commonly used forages differing in PUFA composition and starch content in combination with different levels of concentrate.…”