Environmental impacts on the n-3 content of foods from ruminant animals

Dietary intake of n-3 polyunsaturated fatty acids (PUFA) reduces the risk of cardiovascular disease, cancer and type-2 diabetes, and is critical for proper brain and visual development in the foetus and maintenance of neural and visual tissues throughout life. Most attention has focused on the long chain n-3 PUFA, eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA; 22:6n-3). Plants are the primary source of n-3 PUFA, both in the terrestrial and marine ecosystems. Plants have the unique ability to synthesise de novo α-linolenic acid (18:3n-3) which is the building block of the n-3 series of essential fatty acids and elongation and desaturation of this fatty acid results in the synthesis of EPA and DHA. The formation of these long chain n-3 PUFA by marine algae and their transfer through the food chain to fish, accounts for the high amounts of these important fatty acids in fish oils. Hence, fish oils are the predominant source of long chain n-3 PUFAs for man but since consumption of fish is low in western diets much attention is given to enriching these fatty acids in other foods. It is acknowledged that the natural reserves of marine fish stocks are declining creating an unsustainable situation (Naylor et al., 2000) and this has provided further impetus for more sustainable and environmental benign sources of these important fatty acids for man (Napier, 2006). Despite the low concentration of n-3 PUFA in ruminant foods considerable effort has been given to increasing these fatty acids in meat and milk. A number of reviews examining the impact of nutrition, including the significant role the rumen plays in the extensive metabolism of dietary lipids, on milk and meat fatty acid composition have recently been published (Chilliard and Ferlay, 2004; Bauman and Lock, 2005; Shingfield and Grinarii, 2007; Scollan et al., 2006a; Sinclair, 2007; Wood et al. 2008). This chapter is directed towards the role of forages in the diet on lipid metabolism in the rumen and opportunities to beneficially enhance the composition of ruminant lipids, particularly in relation to n-3 PUFA. Forages rich in 18:3n-3 such as grass are an important feed for ruminants in both conventional and organic farming systems and hence may offer opportunities for increasing the delivery of n-3 PUFA to man.