Storage of liver fat can only occur when daily calorie intake exceeds expenditure. Sucrose overfeeding for 3 weeks has been shown to cause a 30% increase in liver fat content (37). The associated metabolic stress on hepatocytes was reflected by a simultaneous 30% rise in serum alanine aminotransferase (ALT) levels, and both liver fat and serum ALT returned to normal levels during a subsequent hypocaloric diet. Superimposed upon a positive calorie balance, the extent of portal vein hyperinsulinemia determines how rapidly conversion of excess sugars to fatty acid occurs in the liver. In groups of both obese and nonobese subjects, it was found that those with higher plasma insulin levels have markedly increased rates of hepatic de novo lipogenesis (2,38,39). Conversely, in type 1 diabetes the relatively low insulin concentration in the portal vein (as a consequence of insulin injection into subcutaneous tissue) is associated with subnormal liver fat content (40). Initiation of subcutaneous insulin therapy in type 2 diabetes brings about a decrease in portal insulin delivery by suppression of pancreatic insulin secretion and, hence, a decrease in liver fat (41). Hypocaloric diet (42), physical activity (43), or thiazolidinedione use (23,44) each reduces insulin secretion and decreases liver fat content. Newly synthesized triacylglycerol in the liver will be either oxidized, exported, or stored as hepatic triacylglycerol. Because transport of fatty acid into mitochondria for oxidation is inhibited by the malonyl-CoA produced during de novo lipogenesis, newly synthesized triacylglycerol is preferentially directed toward storage or export. Hence, hepatic fat content and plasma VLDL triacylglycerol levels are increased.
Robert Ferry Jr., MD, is a U.S. board-certified Pediatric Endocrinologist. After taking his baccalaureate degree from Yale College, receiving his doctoral degree and residency training in pediatrics at University of Texas Health Science Center at San Antonio (UTHSCSA), he completed fellowship training in pediatric endocrinology at The Children's Hospital of Philadelphia.