13 Fat Metabolism and Turnover D.W Pethick,1 G.S Harper2 and F.R Dunshea1,3 School of Veterinary and Biomedical Sciences, Murdoch University, Murdoch, WA 6150, Australia; 2CSIRO, Division of Livestock Industries, St Lucia, QLD 4067, Australia; 3Department of Primary Industries, Werribee, VIC 3030, Australia Introduction This chapter will emphasize the metabolism of non-esterified fatty acids (NEFA) although some discussion will relate to triglyceride (TAG) and ketone bodies Plasma NEFA are a highly labile form of lipid that are transported between tissues in the circulation bound to albumin Although plasma NEFA are only a small proportion (5%) of total plasma lipid they represent an important source of oxidizable energy, particularly during periods of negative energy balance or when there is an acute need for energy such as during exercise This chapter will deal with the composition and sources of plasma NEFA, the fate of NEFA during different physiological states as well as the acute and chronic regulation of NEFA metabolism In addition, this chapter will cover methodology and principles of NEFA metabolism and also describe adaptations to different physiological states Although these areas have been covered in several reviews (Lindsay, 1975; Emery, 1979; Annison, 1984; Wiseman, 1984; Chilliard et al., 2000), this chapter will emphasize the quantitative aspects of NEFA metabolism Composition and Sources of Plasma NEFA and TAG The metabolically most active pool of long-chain fatty acids is transported and metabolized as either NEFA (bound to albumin) or TAG In the fed animal NEFA represent less than 5% by weight of total plasma lipid (2 g/l) with the remainder incorporated into various lipoprotein fractions (Kris-Etherton and Etherton, 1982) TAG (