By Kevin Beck Updated Mar 24, 2022
Lipids are organic molecules composed of carbon, hydrogen, and oxygen that are insoluble in water but dissolve readily in fatty solvents. In the human body—and in the foods we consume—three principal classes dominate: triglycerides, phospholipids, and sterols. In nutrition discussions, the terms “lipid,” “fat,” and “oil” are often used interchangeably; solid lipids are commonly referred to as fats, while their liquid counterparts are called oils.
Just as nucleotides build DNA, fatty acids serve as the building blocks of triglycerides and phospholipids. Sterols, by contrast, are defined by a distinctive four‑ring carbon skeleton.
Triglycerides consist of a glycerol backbone bonded to three fatty acids via ester linkages. Glycerol is a three‑carbon alcohol (C(H2)OH‑C(H)OH‑C(H2)OH). When a hydroxyl group loses a hydrogen, a fatty acid attaches to the oxygen, forming a C–O–C ester bond. Fatty acids range from 4 to 24 carbons; a single double bond classifies them as unsaturated, otherwise they are saturated.
Triglycerides represent the majority of lipids in nature, comprising 99 % of body lipids and 95 % of dietary lipids. Their primary role is energy storage, providing 9 kcal per gram. Elevated triglyceride levels are a well‑established risk factor for cardiovascular disease, while certain essential fatty acids—such as omega‑3 linolenic acid—must be obtained through diet.
Phospholipids share a glycerol backbone but attach to two fatty acids and a phosphate group rather than three fatty acids. They contain a nitrogen‑bearing base, often a choline moiety. This amphipathic nature makes phospholipids indispensable components of cellular membranes, which regulate transport and signal transduction.
Beyond their biological role, phospholipids like lecithin function as food emulsifiers, keeping oil and water phases together in products such as salad dressings. Sources include wheat germ, peanuts, egg yolks, soybeans, and organ meats such as liver.
Sterols are defined by a fused four‑ring core of carbon atoms. Cholesterol is the most familiar example, essential for membrane fluidity and a precursor for steroid hormones, bile acids, and vitamin D. Although dietary cholesterol is only found in animal products, the body synthesizes sufficient amounts, so intake is not required for most individuals.
Plant sterols (phytosterols) are structurally similar but can interfere with the absorption of dietary cholesterol, offering a modest protective effect against hypercholesterolemia.
Saturated fatty acids are typically solid at room temperature, whereas unsaturated fatty acids remain liquid. Monounsaturated fatty acids contain one double bond; polyunsaturated fatty acids contain two or more. Fatty acids supply immediate energy, can be stored for later use, provide insulation and protection, influence satiety, and transport fat‑soluble vitamins.
