Fatty acids are the major source of energy for most tissues during periods of negative energy balance; however, fatty acids can, in some circumstances, have pathological effects. Fatty acids are stored as triacylglycerols (TAG), mostly in the various adipose tissue depots of the body. However, if blood unesterified fatty acid (NEFA) levels are elevated for prolonged periods, as may occur during lactation or obesity, TAG can accumulate in other tissues including liver and muscle cells (myocytes), and this can have pathological consequences such as the development of ketosis (Grummer, 1993; Drackley et al. 2001) or type 2 diabetes (Boden & Shulman, 2002; McGarry, 2002).