Lipids

Lipids are a diverse group of hydrophobic biological molecules that are soluble in nonpolar solvents but largely insoluble in water. They include fats and oils (triglycerides), phospholipids, sterols such as cholesterol, and waxes. Lipids serve as a concentrated energy reserve, provide insulation and protection, and form structural components of cell membranes. They also function as signaling molecules and vitamins.

The major lipid types are triglycerides, which consist of glycerol esterified to three fatty acids; phospholipids, with two fatty acids, a glycerol backbone, and a phosphate-containing head group; and sterols, notably cholesterol, which is a precursor to steroid hormones and bile acids. Fatty acids may be saturated (no double bonds) or unsaturated (one or more double bonds); cis-unsaturations prevail in nature, while trans fats can alter membrane properties.

In metabolism, lipids are digested in the small intestine with bile salts emulsifying fats and pancreatic lipase hydrolyzing triglycerides into free fatty acids and monoglycerides. These are absorbed by enterocytes, re-esterified into triglycerides, and packaged into chylomicrons for transport through the lymphatic system. In tissues, fatty acids undergo beta-oxidation to acetyl-CoA, feeding the citric acid cycle; glycerol can be used for gluconeogenesis or glycolysis. The liver also synthesizes fatty acids in lipogenesis when energy is plentiful.

Dietary lipids come from oils, fats, butter, dairy, meat, and fish. Lipids supply essential fatty acids and enable absorption of fat-soluble vitamins A, D, E, and K. Excess intake of saturated and trans fats is linked to health risks, while unsaturated fats are generally favorable when consumed in place of saturated fats.