Conjugated linoleic acid content of milk and milk products I. Fetors affecting the quantity of conjugated linoleic acid of milk (A review)

Abstract

According to our present knowledge, among the food-products, the meat and milk, and those products of the ruminants contain the most conjugated linoleic acid (0.2-2 g CLA/100 gfat). Only one tenth of the CLA content of ruminants and those products was found in the meat of monogastric animals, according to fishes and other sea animals this CLA content was even lower. The CLA in ruminants-derivedproducts comes into existence partly from the linoleic acid and partly from trans-C 18:1 fatty acids by the 9-desaturase reaction in the mammary gland. The increase of the CLA content of tissues was also experienced in the case of monogastric animals, so it is possible that in their digestive system, though in a smaller extent than in the rumen of ruminants, biological hydrogenation occurs. CLA can also be produced by the means of 9-desaturase reaction in the liver of rats, and besides the endogenic CLA production the CLA content of the products of monogastric animals can be substantially improved by taking into animal- origin feedingstuffs with substantial CLA content. In the case of ruminants the CLA content of the milk can be substantially increased by interfering the biological hydrogenation processes through feeds. One opportunity is the to use feeds containing a lot of polyunsaturatedfatty acids. When the fat is present at a free form in the feed, or in the case of a stabilised form, the structure of the oil career is fragile and/or the animals receive the daily amount of feed in small portions, than a great amount of linoleic acid arrives at the rumen that is easily reached by bacteria in a short time. Due to the impact of the high substrate concentration, the first two products of the fast reactions of the biological hydrogenation c9,t11-C18:2 and t11-C18:1 accumulate in the rumen then later proceeding on the alimentary after their absorption they spread to all directions in the organism of the animal. The other solution is to use high starch and low fibre content diet which entails the slowing down of the speed of the final hydrogenation step. It is obvious though that the increase of the fat and the decrease of the fibre content of the diet can only be done in a lesser degree owing to eliminate the undesirable physiological effects. For the time being the mechanism of the CLA decreasing effect of fish oil is not known and it is also unanswered to what measure does the biological hydrogenation and the 9-desaturase reaction influence the CLA level of the milk. One of the substrate of this reaction, the t11-C18:1, is the same with one of the interim products of the biological hydrogenation so theoretically the speed of the 9-desaturase reaction can be influenced with the amount of the absorbed t11-C18:1 fatty acids deriving from the biological hydrogenation. The increase of the CLA-content of the milk fat can be carried out by appropriate feeding but it usually entails a considerable change in the composition of milk. The fat and protein content of the milk might decrease, within the fatty acid composition the level of long-chain fatty acids increase, while the ratio of the medium-chain fatty acids decrease parallel with the increase of the quantity of trans-fatty acids. The increase of the CLA content of the milk is only a temporary phenomenon since the CLA content of the milk decreases after the introduction of the new feed in some weeks.