Function of Fat Cells
Under the normal physiological condition, fat cells are primarily sensitive to insulin. During postprandial or fed state, there is a temporary increase in plasma insulin which acts to block the action of the enzyme lipase from breaking or hydrolyzing the stored triglycerides. It also facilitates the uptake and conversion of dietary and endogenous free fatty acids to triglycerides for storage into fat cells. In other words, during eating, fat storage (lipogenesis) predominates due to an increase in plasma insulin. However, during fasting or physical activity, increased release of free fatty acids (lipolysis) to meet energy demands of distant organs is favored as plasma insulin is low.
In the fed state, fat cells are also able to pick up about 10% of blood glucose, part of which is used for oxidation while the rest is converted to triglycerides. Fat tissue, therefore, contributes to glucose clearance during feeding to prevent glucotoxicity. The switch from fat oxidation to glucose oxidation involves the translocation of a special protein called glucose transporter 4 from inside the cell to the cell surface which then allows the passage of glucose to the fat cell.
As endocrine cells, fat cells produce and secrete hormones that affect actions of distant organs. Leptin and adiponectin are the most discussed hormones exclusively produced by the fat cells. Leptin is released in response to food intake and acts centrally on the hypothalamus to simultaneously regulate appetite and stimulate satiety. It also facilitates lipid oxidation and upgrades mitochondrial synthesis in peripheral organs such as skeletal muscle and therefore contributes to overall energy expenditure and weight loss. Adiponectin promotes insulin sensitivity in skeletal muscle and suppresses hepatic glucose production during feeding. It is also a potent anti-inflammatory hormone as it limits the pro-inflammatory action of macrophages resident in fatty tissue and promotes vascular health by blocking the production of a pro-inflammatory protein called plasminogen activator inhibitor-1 in the blood vessels.