Fat

Fat, in biology and nutrition, refers to a broad class of lipids that store energy and perform other essential roles. In animals, fat commonly denotes triglycerides stored in adipose tissue. This tissue serves as an energy reserve, provides insulation and cushioning, and secretes signaling molecules that influence metabolism and appetite. Fat depots vary: subcutaneous fat lies under the skin, while visceral fat surrounds internal organs; visceral fat is more strongly associated with metabolic risk.

Chemically, dietary fats are mainly triglycerides—glycerol bound to three fatty acids. The fatty acids vary in chain length and level of saturation. Essential fatty acids, such as linoleic acid (an omega-6) and alpha-linolenic acid (an omega-3), must be obtained from the diet. Fats also include cholesterol and fat-soluble vitamins, which depend on fat for digestion and transport.

Digestion and metabolism: Dietary fats are emulsified by bile and broken down by pancreatic lipase into free fatty acids and monoglycerides, which are absorbed by intestinal cells and reassembled into triglycerides for transport as chylomicrons. In tissues, triglycerides are hydrolyzed again to release fatty acids for energy. Fatty acids undergo beta-oxidation in mitochondria to acetyl-CoA, entering the citric acid cycle; glycerol can feed into glucose metabolism.

Health and diet: Fats provide 9 kcal per gram and are necessary for absorbing fat-soluble vitamins A, D, E, and K. They also contribute to cell membrane structure and hormone production. Dietary guidance typically favors reducing saturated and trans fats, while prioritizing unsaturated fats (monounsaturated and polyunsaturated), including omega-3 and omega-6 fatty acids, and moderating total energy intake to maintain healthy body weight.