Nutritional regulation of milk fatty acid composition

There is increasing evidence that nutrition plays an important role in the development of chronic diseases in the human population including cancer, cardiovascular disease, insulin resistance and obesity. Developing foods that enhance human health is central to dietary approaches for preventing and reducing the economic and social impact of chronic disease. Numerous studies in human subjects have implicated a high consumption of medium-chain (12:0-16:0) saturated fatty acids (SFA) and trans fatty acids (TFA) as risk factors for cardiovascular disease risk, with emerging evidence of a possible role in the development of insulin resistance and inflammation. Milk and dairy products are a major source of 12:0, 14:0, 16:0 and TFA in the human diet. However, developing public health policies promoting a decrease in milk, cheese and butter consumption ignores the value of these foods as a versatile source of nutrients and bioactive lipids, including 4:0, branch-chain fatty acids, cis-9, trans-11 conjugated linoleic acid (CLA), vitamins A and D, β-carotene and sphingomyelin. Therefore, altering milk fatty acid composition through sustainable, environmental and welfare acceptable means is an integral component of an overall strategy for preventing human chronic disease. Nutrition is the major environmental factor regulating milk fat composition. Strategies for enhancing the nutritional value of milk fat have been directed towards reducing the proportion of 12:0, 14:0 and 16:0, increasing cis monounsaturated fatty acids (MUFA) and polyunsaturated fatty acid (PUFA) content and/or enhancing the concentration of bioactive lipids. Formulation of diets to alter milk fat composition to meet these targets typically involves 1) inclusion of plant oils and oilseeds, 2) supplements of marine lipids, 3) increasing the proportion of dietary energy derived from fresh grass, 4) replacing ensiled grass, maize or whole-crop cereal with forage legumes or 5) inclusion of rumen-protected lipids in the diet. Nutritional strategies for reducing 12:0, 14:0 and 16:0 and enhancing cis-MUFA in milk fat are dependent on increasing the supply of C18 or longer-chain fatty acids to inhibit mammary de novo fatty acid synthesis. Due to incomplete metabolism of dietary unsaturated fatty acids in the rumen, inclusion of oils or oilseeds in the diet also increases milk fat TFA content. Both the concentration and distribution of TFA isomers in milk is dependent on the amount and type of lipid supplement fed, composition of the basal diet and interactions between these factors. Altering the diet of lactating cows can be used as a means to significantly enrich milk fat cis-9, trans-11 CLA content. Increases in the concentration of CLA in milk are reliant, in the most part, on enhancing ruminal trans-11 18:1 outflow for endogenous cis-9, trans-11 CLA synthesis in the mammary gland. The potential to increase n-3 fatty acids in milk is relatively limited. Inclusion of fresh or ensiled red clover in the ruminant diet significantly increases 18:3 n-3 content, while increases in milk 20:5 n-3 and 22:6 n-3 concentrations to marine lipid supplements is relatively limited due to extensive ruminal metabolism of n-3 PUFA. Greater enrichment of 20:5 n-3, 22:5 n-3 and 22:6 n-3 in milk can be achieved using rumen-protected fish oil supplements. Overall, recent research has highlighted the important role of nutrition in attempts to modify milk fat composition for improved long-term human health.