Discipline: dairy & health; Keywords: conjugated linoleic acid, feed conversion, milk fat, lean meat, metabolic functions.
Should CLA be increased through the diet of the cow, or supplemented in the dairy product? In this presentation I will attempt to address this question after studying the literature on the subject.
Conjugated linoleic acid, more commonly known as CLA, refers to a group of naturally occurring isomers of linoleic acid present in ruminant fats and dairy products. Dairy products are the richest source providing about 70% of dietary CLA, followed by beef (25%) and smaller amounts from other ruminant meats. CLA has been shown to offer several health benefits with respect to cardiovascular disease, cancer, obesity, osteoporosis and the immune system. Being the case, it is certainly of interest to establish whether the CLA content of milk can be increased by dietary or other means.
CLA isomers are produced by the bio-hydrogenation of linoleic acid by rumen bacteria, and they are also produced in animal tissues; in fact, by far the majority is produced in the tissues. The variation of CLA in milk is high, ranging between about 3 to 6 mg CLA per gram of milk fatty acids and the reasons for the high variation are not well understood. The CLA contents of milk are usually higher on pasture grazing systems than on TMR systems or preserved forages but even on pastures the contents vary considerably, apparently in relation to season, time of day (morning vs evening milk) and quality of the pasture. The contents can be increased by supplementing cows with oils such as canola, particularly if the oil is protected from rumen bacterial action and then absorbed from the small intestine. Presumably this technique provides more precursors for synthesis of CLA in the tissues.
A number of feeding trials have shown that when supplementing with CLA, milk production and feed conversion ratio benefit and milk fat content declines. Similarly, when pigs are supplemented with CLA in the feed, growth and the feed conversion ratio benefit because more protein (lean meat) and less fat is produced. The net effect is not more CLA in milk or carcass fat, but the farmer benefits because of more efficient production. These are surprising results, but illustrate that CLA produced in the tissues are used for a vast number of metabolic and physiological functions in the animal body which may have preference to milk fat synthesis. Nevertheless, some oil seed supplements have shown increases in milk fat CLA contents, but the predictability of the response appear low. Being the case, and given the fact that fat of milk in the market is reduced to 2% or to fat-free (which reduces the CLA content accordingly), one wonders whether the easier option is not to fortify the end product, rather than try to do it through the cow. More studies in this context are required.
Bernal-Santos, G. et al., 2003. Production responses of dairy cows to dietary supplementation with conjugated linoleic acid (CLA) during the transition period and early lactation. J. Dairy Sci. 86, 3218-3228.
De Veth, M.J. et al., 2006. Response to conjugated linoleic acid in dairy cows differing in energy and protein status. J. Dairy Sci. 89, 4620-4631.
Dhiman, T.R. et al., 2000 Conjugated linoleic acid (CLA) content of milk from cows offered diets rich in linoleic and linolenic acid. J. Dairy Sci. 83, 1016-1027.
Dugan, M.E.R. et al., 2001. Effects of feeding different levels of conjugated linolenic acid and total oil to pigs on live animal performance and carcass composition. Can. J. Anim. Sci. 68, 761-767.
Griinari, J.M. et al., 2000. Conjugated linoleic acid is synthesised endogenously in lactating dairy cows by Δ9 – desaturase. J. Nutr. 130, 2285-2291.
Lawson, R.E. et al., 2001. The role of dairy products in supplying conjugated linoleic acid to man’s diet: A review. Nutr. Res. Reviews 14, 153-172.
Lin, H. et al., 1995. Survey of the conjugated linoleic acid contents of dairy products. J. Dairy Sci. 78, 2358-2365.
Palmquist, D.L., 2001. Ruminal and endogenous synthesis of CLA in cows. Austr. J. Dairy Technol. 56, 134-136.
Tricon, S. et al., 2006. Effects of dairy products naturally enriched with cis-9, trans-11 conjugated linoleic acid on the blood lipid profile in healthy middle-aged men. Am. J. Clin. Nutr. 83, 744-753