been shown to decrease expression of muscle LPL. Muscle and myocardial LPL is instead activated by glucagon and adrenaline. This helps to explain why during fasting, LPL activity increases in muscle tissue and decreases in adipose tissue, whereas after a meal, the opposite occurs. Consistent with this, dietary macronutrients differentially affect adipose and muscle LPL activity. After 16 days on a high-carbohydrate or a high-fat diet, LPL activity increased significantly in both tissues 6 hours after a meal of either composition, but there was a significantly greater rise in adipose tissue LPL in response to the high-carbohydrate diet compared to the high-fat diet. There was no difference between the two diets' effects on insulin sensitivity or fasting LPL activity in either tissue. The concentration of LPL displayed on endothelial cell surface cannot be regulated by endothelial cells, as they neither synthesize nor degrade LPL. Instead, this regulation occurs by managing the flux of LPL arriving at the lipolytic site and by regulating the activity of LPL present on the endothelium. A key protein involved in controlling the activity of LPL is ANGPTL4, which serves as a local inhibitor of LPL. Induction of ANGPTL4 accounts for the inhibition of LPL activity in white adipose tissue during fasting. Growing evidence implicates ANGPTL4 in the physiological regulation of LPL activity in a variety of tissues. An ANGPTL3-4-8 model was proposed to explain the variations of LPL activity during the fed-fast cycle. Specifically, feeding induces ANGPTL8, activating the ANGPTL8–ANGPTL3 pathway, which inhibits LPL in cardiac and skeletal muscles, thereby making circulating triglycerides available for uptake by white adipose tissue, in which LPL activity is elevated owing to diminished ANGPTL4; the reverse is true during fasting, which suppresses ANGPTL8 but induces ANGPTL4, thereby directing triglycerides to muscles. The model suggests a general framework for how triglyceride trafficking is regulated. Clinical significance Lipoprotein lipase deficiency leads to hypertriglyceridemia (elevated levels of triglycerides in the bloodstream). In mice, overexpression of LPL has been shown to cause insulin resistance, and to promote obesity. A high adipose tissue LPL response to a high-carbohydrate diet may predispose toward fat gain. One study reported that subjects gained more body fat over the next four years if, after following a high-carbohydrate diet and partaking of a high-carbohydrate meal, they responded with an increase in adipose tissue LPL activity per adipocyte, or a decrease in skeletal muscle LPL activity per gram of tissue.