Effects of dietary n-3 polyunsaturated fatty acids and ... - FINS

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1 st WorkshopXIII International Feed Technology Symposiummodules (36, 40), and reduce adipose tissue (4, 6). Milk and dairy products are the bestnatural sources of CLA and mainly contain a cis9, trans11-isomer originating in therumen of ruminants during biological biohydrogenation of linoleic and linolenic acidoriginating from vegetable feeds (5).Carbohydrates and fats ingested by animals via food are very important for forming fattyacids in milk (37, 40). In food, carbohydrates are found in the form of structuralcarbohydrates (cellulose, hemicellulose, pectin), sugars, and starch. Fodders such as hay,straw and silage contain relatively high quantities of structural carbohydrates, and theirdigestion in the digestive tract depends to a high extent on the fodder itself (hay isdigested to a higher extent than straw), and this is important for the presence of milk fatin milk (37, 38, 29). Sugars and starch (nonstructural carbohydrates), which dominate inseeds, can also be converted to milk fat by disintegration to butyric acid (39). Fattysubstances that are not degraded in the rumen (protected fats), and are absorbed in thesmall intestine of goats can also be used for this purpose.DIETARY FACTORS AND THEIR INFLUENCE ON CLACONTENT IN MILK FATMilk fat is the richest natural source of conjugated linoleic acid with a content rangingfrom 2.4 to 28.1 mg/g, however with a very pronounced seasonal variation, and with asummer concentration which is 2 to 3 times higher than in winter months (25).The typical composition of milk fat, depending on the source of fat in the ration, as wellas the content of fatty acids and CLA (conjugated linoleic acid) in various fat sources arepresented in Table 1.Table 1. Influence of fat in the diet on the composition of milk fat in cumulative cows'milk (27)Fatty acid4:0b 5.686:03.248:01.7410:03.5012:03.7314:010.1315:01.0716:030.4318:011.1918:1 trans1.7618:1 cis19.2218:22.7118:30.35CLA C 0.45acommercial protected fats, tallow and cottonseedbas percentage relevant to total fatty acid contentcconjugated linoleic acidDietNo added fat Whole soybeans Fat a5.103.161.763.633.8010.460.9227.9813.141.7419.144.520.750.504.992.861.543.213.4710.450.9830.3012.121.9520.402.840.300.65245
1 st WorkshopXIII International Feed Technology SymposiumGriinari and Bauman (31) have proposed that dietary factors which affect milk CLAcontent could be grouped into of two categories. The first would be factors that providelipid substrates for formation of CLA or trans C18:1 oleinic acid in the rumen. Thesecond would be factors that change the microbial activity associated with ruminalbiohydrogenation (4). Literature data are presented in Table 2. according to the ranges ofobserved milk CLA content.Table 2. Effects of dietary factors on milk fat CLA content in dairy cows (literaturereview) alow values(0.2–0.8%)- corn silage- grass silage/hay/pasture- animal or vegetable saturatedfats- raw soybeans- micronized soybeans- soybeans treated by heatprocessing- extruded soybeans orcottonseedsmedium values(0.8–1.6%)- fresh pasture/younggrass- low fiber diets- restricted feeding- extruded soybeans- peanut oil- rapeseed oil b- soybean oil b- linseed oil b- calcium salts ofrapeseed oilhigh values(> 1.6%)- rapeseed oil c- soybean oil c- sunflower oil c- linseed oil c- calcium salts ofsoybean and linseedoils- fish oilsa (1, 3, 5, 12, 17, 18, 21, 22, 33, 34, 35, 41, 43, 45).b At low dosesc At medium dosesHowever, no definitive conclusions can be drawn from these data, because a largeproportion of the data arose from indirect comparisons of experiments in differentlaboratories or experimental conditions, and also because a lot of the potentialinteractions in practical farm conditions have not yet been studied. It neverthelessappears that plant oils high in linoleic acid (e.g. sunflower, soya and rapeseed) are veryefficient at increasing milk CLA content. Besides directly increasing the yield of CLAand trans C18:1, it is likely that linoleic acid inhibits the final reduction of trans C18:1,thus increasing its accumulation in the rumen (31). Calcium salts of rapeseed alsoincrease milk CLA content (17), in agreement with the concept that calcium salts are notresistant to ruminal biohydrogenation. Limited research shows that the frequency offeeding cows fats and the physical form of oils (free or in raw seeds of oleaceous plants),as well as thermal processing of seeds of oleaceous plants, exert a relatively lowinfluence on altering milk fat composition (7, 27). Furthermore, vegetable oils are moreefficient than extruded seeds (which are themselves more efficient than raw seeds) atincreasing milk CLA content (32) (Tab. 2). This potency could be inversely related tothe protection of PUFA against biohydrogenation. On the other hand, supplementationwith animal fats is not very efficient at increasing CLA content because of their lowPUFA content. When soya oil was offered 24-times daily, instead of twice, the milk fatcontent increased, and the percentage of trans C18:1 decreased whereas that of C18:0246
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Effects of dietary n-3 polyunsaturated fatty acids and ... - FINS

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