Effects And Mechanisms Of Dietary Unsaturated Fatty Acids On Milk Conjugated Linoleic Acid (CLA) Synthesis

Based on optimized milk fatty acid analysis, experiments were conducted to investigate the effects of 1) parity and days in milk, 2)dietary linoleic acid and linolenic acid content, 3)forage to concentrate ratio with or without dietary linoleic acid addition, 4)fish oil, soybean oil or their blend on trans 11-18:1 (TVA), and c9,tl 1 CLA content of milk from cows and goats fed the expected treatments.In the first study, a simple milk fat separation method was proposed. Milk fat was separated by double centrifugation or centrifugation with hexane:isopropanol and was transesterified by the suggested procedures of Kramer et al.(1997) and then the FAMEs were quantified with GC equipped with a highly polar capillary column (SP2560). The fatty acid proportions were statistically identical when compared with other two reported mehods.In the second experiment, 77 Chinese Holstein cows were fed with the same diet for 35days, milk was sampled on day 35th of feeding. Frequency distributions demonstrated that milk content of c9,tl 1 CLA and CLA-desaturase index varied among individuals. Parity and days in milk had little relationship to the individual variation for these two CLA variables. Milk fat of CLA content and CLA-desaturase index were independent of milk yield, and milk fat yield.In experiment 3, forty dairy cows were randomly assigned to 4 treatments. Cows in four treatments were fed diets containing low linoleic and linolenic acid (LLALEA), high linoleic(HLA), high linolenic acid(HLEA), or high linoleic and linolenic acid (HLALEA). The proportions of TVA and c9,t11 CLA were 1.48^c, 6.19^a, 3.04^b, and 4.53^ab (p=0.01), 0.64^c, 2.39^a, 1.60^b, and 1.81^b% of total FAMEs in LLALEA, HLA, HLEA, and HLALEA, respectively. Feeding free oil rich in linoleic acid was 100% and 50% more effective in enhancing TVA and c9,tl 1 CLA in milk fat, respectively, than feeding free oils rich in linolenic acid fed at the similar levels in the diet. The milk fat content of c9,t11 CLA across the whole treatment increased linearly with increased intake of dietary LA. Milk fat concentrations of TVA in HLEA, HLALEA, and HLA were 1.56, 3.05, and 4.7ig/100g higher than LLALEA respectively;whereas, c9,t11 CLA were 2.5, 2.8, and 3.7 times higher, respectively.In experiment 4, four Holstein cows were used in a 4 x 4 Latin square to determine duodenal flow of TVA, c9,tl 1 CLA, and milk fatty acid profiles of cows fed four different diets (30:70CW, 30:70CCA, 50:50CCA, and 65:35CCA). Duodenal flow of TVA were 7.44, 10.33, 11.37 and 12.66g/d (p=0.0926) in 30:70CW, 30:70CCA, 50:50CCA, and 65:35CCA, respectively, and the duodenal flow of TVA increased numerically (p=0.0926) in response to the increased dietary concentrate. However, milk fat content of c9,tll CLA and duodenal flow of c9,tll CLA were not different among treatments(p>0.05).In experiment 5,fourty Holstein cows were allocated to four treatments arranged in a 2 x 2 factorialdesign with forage to concentrate ratio (60:40 vs 40:60,namely HC vs LC) and addition of soybean oil(FAT or CK) as two factors. Milk fat concentrations of TVA were 5.80, 5.53, 1.08, and 1.09%(p<0.01) , and c9,tll CLA were 4.04%, 4.10%, 0.58%, and 0.76% (p<0.01) in HCFAT, LCFAT,LCFAT, and HCCK,respectively.In experiment 6, relative to the control, adding 2% fish oil (FO) to dairy cow's diet induced significant milk fat depression from 3.63% to 2.50% (p<0.05), while cows fed 2% fish oil plus 2% soybean oil (SBFO) did not experience MFD. Concentrations of DHA, TVA, C18:3, and c9,tll CLA in milk were increased in both FO and SBFO group (p<0.01) .In experiment 7, the concentration of most milk fatty acids synthesized de novo (8:0-14:0) decreased (P<0.001), while those derived from pre formed fatty acids (18:0), TVA, and c9,t11 CLA increased in blood plasma and milk. Feeding 2% soybean oil to goats or 2% fish oil plus 2% soybean oil to dairy cows decreased the mRNA expression in mammary tissue of SCD, FAS, and LPL. Compared to the 2% soybean oil supplement, 2% fish oil and 2% soybean oil decreased the mRNA expression of SCD, ACC, FAS, and LPL by 14%, 27%, 34%, and 35%.Significant implications from the above studies include: 1) Intake of dietary LA and LEA provide the available TVA, through increased ruminal production, for endogenous synthesis milk of c9,t11 CLA via SCD in the mammary gland. LA is more effective than LEA in enhancing milk c9,tll CLA concentration;2) EPA and DHA, through fish oil supplementation, provide substrate for milk c9,t11 CLA endogenous synthesis by regulating LA metabolism in the rumen;3) total dietary unsaturated fatty acid content is necessary for enhancing milk c9,t11 CLA by manipulating dietary forage level. When dietary LA and LEA content is limited, milk fat c9,tll CLA content will not differ significantly regardless of forage to concentrate ratio. However, high forage diets supplemented with high LA content will produce and maintain a high level of c9,tl 1 CLA in milk fat.