Coronary artery disease (CAD) is the leading cause of death in the United States and is a major complication of type 2 diabetes (T2DM). Dyslipidemia, characterized by elevated low density lipoprotein cholesterol (LDL-C) and triglyceride levels and decreased high density lipoprotein cholesterol (HDL-C) levels, is associated with increased risk of CAD. The high TG/ low HDL-C phenotype is particularly prevalent in individuals with insulin resistance or T2DM. Apolipoprotein C-III (apoC-III), an 8.8 kDa protein component of both triglyceride-rich lipoproteins (TRLs) and HDL particles, inhibits lipoprotein lipase activity and hepatic uptake of TRLs. ApoC-III may also enhance very low density lipoprotein (VLDL) assembly and secretion, but the findings on this based on animal models and human studies are conflicting. In addition, apoC-III activates inflammatory signaling pathways, a cardiovascular risk factor of emerging importance. We recently provided evidence that apoC-III deficiency is cardioprotective by identifying a null mutation found in ~5% of the Lancaster Old Order Amish that reduces apoC-III levels by ~50% and confers favorable lipid profiles and less coronary calcification. The mutation appears rare or absent in the general population but has reached a relatively high prevalence in the Amish by a founder effect. While these recent findings strongly implicate direct apoC- III lowering as a promising therapeutic intervention, the enhanced lipolytic activity conferred by apoC-III reduction could theoretically promote disease risk by increasing visceral and muscle fat stores, which would be expected to promote insulin resistance. Alternatively, the enhanced flux of fatty acids into peripheral tissues may promote sufficient compensatory increases in fatty acid oxidation to make direct lowering of apoC-III levels a suitable target in the prevention of cardiovascular disease in insulin resistant individuals. To enhance our understanding of the metabolic effects of decreased availability of apoC-III and the role apoC-III plays in VLDL secretion in humans, we are proposing to compare individuals with heterozygous apoC-III deficiency to their non-deficient relatives to (1) evaluate the effect of apoC-III defiency on glucose and insulin metabolism and fat distribution; (2) evaluate the effect of apoC-III defiency on lipid and lipoprotein turnover; and (3) evaluate the effect of apoC-III deficiency on fat cells and their function.