Summary
Highlights
Lipid metabolism focuses on the breakdown and synthesis of fats, which are triglycerides composed of glycerol and three fatty acids. Fats can originate from diet, adipose tissue stores, or be synthesized from carbohydrates in the liver.
Dietary fats are primarily digested in the small intestine. Bile salts emulsify fats into smaller micelles, making them accessible to pancreatic lipase. Lipase then breaks triglycerides into monoglycerides, free fatty acids, and glycerol. These components are absorbed by enterocytes, where they re-form into triglycerides.
Triglycerides, along with cholesterol, are packaged into chylomicrons, which are lipoprotein particles. Chylomicrons transport these water-insoluble fats through the lymphatic system and into the bloodstream, delivering them to tissues. Liver-synthesized fats are transported by VLDL (Very Low-Density Lipoproteins).
In capillary walls, lipoprotein lipase hydrolyzes triglycerides in chylomicrons and VLDL into fatty acids and glycerol. These enter tissues for energy oxidation or re-esterification for storage. Stored fats in adipose tissue can be mobilized for energy by hormone-sensitive lipase in response to hormones like epinephrine.
Lipid metabolism is closely linked to carbohydrate metabolism. Glycerol enters glycolysis, while fatty acids undergo beta-oxidation to produce acetyl-CoA. Each round of beta-oxidation shortens fatty acids by two carbons, releasing acetyl-CoA for the citric acid cycle and producing high-energy molecules for the electron transport system.
Excess acetyl-CoA is converted into ketone bodies, which serve as an important fuel source, especially for the brain during glucose starvation. However, excessive production of acidic ketone bodies can lead to metabolic acidosis (ketoacidosis), a serious condition in diabetics or those on extreme low-carb diets, potentially causing coma and death.
Diets high in carbohydrates lead to excess acetyl-CoA, which can be converted into fatty acids, stimulated by citrate and inhibited by excess fatty acids. These fatty acids combine with glycerol (from glycolysis) to form triglycerides for storage or the synthesis of other lipids.