Lipid

Lipids are a broad group of naturally occurring molecules that are largely hydrophobic or amphipathic. They are insoluble in water but soluble in nonpolar solvents. Lipids include fatty acids, triglycerides, phospholipids, sphingolipids, sterols, and related compounds. They are not polymers and are typically built from hydrocarbon chains with various functional groups.

Key structural categories include simple lipids (fats and oils, waxes) composed mainly of fatty acids and glycerol; compound lipids such as phospholipids and glycolipids that contain additional chemical groups; and derived lipids such as steroids and fat-soluble vitamins derived from isoprenoid or sterol skeletons. Fatty acids are carboxylic acids with long hydrocarbon chains; saturated fatty acids have no double bonds, while unsaturated fatty acids contain one or more double bonds. Essential fatty acids must be obtained from the diet.

Triglycerides, with three fatty acids esterified to glycerol, are the body's major energy reserve stored in adipose tissue and provide about 9 kcal per gram. Phospholipids possess a glycerol backbone, two fatty acids, and a phosphate-containing head group, giving them amphipathic properties that enable the formation of lipid bilayers in cell membranes. Sterols, such as cholesterol, modulate membrane fluidity and serve as precursors to steroid hormones and bile acids.

Digestion and transport involve emulsification by bile, enzymatic breakdown by pancreatic lipase to monoglycerides and fatty acids, absorption, and reassembly into triglycerides for transport by lipoproteins (chylomicrons, VLDL, LDL, HDL). Lipids play essential roles in energy storage, membrane structure, signaling, and the transport of fat-soluble vitamins, while imbalances can impact health, particularly cardiovascular risk with certain fats.