The general category of lipids in the body is composed of triglycerides, phospholipids, and cholesterol. The basic building blocks of triglycerides and phospholipids are long-chain hydrocarbon organic acids known as fatty acids. Cholesterol does not contain fatty acids, but its sterol ring is synthesized from fatty acids, and thus it shares many similarities with the other lipids. Lipids serve various bodily functions, but primarily act as an energy source. Phospholipids and cholesterol are key components in cellular membranes.
Fats in the diet are mainly absorbed from the intestines into the intestinal lymph. In the intestines, the majority of triglycerides are split into monoglycerides and fatty acids via pancreatic lipase. After passing through intestinal epithelial cells, they are re-synthesized into new triglycerides, and then enter the lymphatic system as small droplets called chylomicrons. The chylomicrons travel through the thoracic duct and are emptied into the bloodstream at the juncture of the jugular and subclavian veins. The chylomicrons are removed from the circulation in the capillaries of tissues containing lipoprotein lipase, namely adipose, skeletal muscle, liver, and heart tissue. Lipoprotein lipase hydrolyzes the chylomicron triglycerides, releasing fatty acids that diffuse into the cells, where they can act as a fuel source or regenerated into intracellular triglycerides.
A small amount of ingested fat, in the form of short-chain fatty acids, is directly absorbed through intestinal mucosal villi, and transported via the portal vein with the aid of lipid carrier proteins to the liver. The liver has multiple roles in fat metabolism, including fatty acid breakdown for energy, triglyceride synthesis from carbohydrates as well as proteins, and synthesis of functional lipids, that is cholesterol and phospholipids.
Fats are the primary storage source for energy in the human body, and yield 9 kcal/g, compared to 4 kcal/g for carbohydrates. Lipolysis is the first step in utilizing a triglyceride for energy. Lipases hydrolyze triglycerides into glycerol and free fatty acids. Free fatty acids diffuse into the bloodstream and attach to plasma albumin for transport to their destination tissue. Once in the cytosol, fatty acids are transported into the mitochondria with the assistance of the carrier protein carnitine. Inside the mitochondria, fatty acids are processed into the two-carbon moiety acetyl-CoA, which enters the citric acid cycle to produce NADH and FADH2. NADH and FADH2 are then used in the electron transport chain to create ATP.
Excess accumulation of acetyl-CoA can result in the formation of acetoacetic acid, which can then be converted to β-hydroxybutyric acid and acetone. These three compounds, known as ketone bodies, are acids and can cause an extreme metabolic acidosis. Most commonly occurring in insulin-depleted diabetics, the body’s cells do not absorb glucose from the bloodstream. The body shifts into starvation mode and catabolizes fatty acids, resulting in ketone ...