Carboxylic acids are very important biologically. The drug aspirin is a carboxylic acid, and some people are sensitive to its acidity. The non-aspirin pain reliever ibuprofen is also a carboxylic acid. Carboxylic acids that have very long chains of carbon atoms attached to them are called fatty acids. As their name suggests, they are important in the formation of fat in the body. Many carboxylic acids are present in the foods and drinks we ingest, like malic acid (found in apples), tartaric acid (grape juice), oxalic acid (spinach and some parts of the rhubarb plant), and lactic acid (sour milk). Two other simple carboxylic acids are propionic acid and butyric acid. Propionic acid is partly responsible for the flavor and odor of Swiss cheese. Butyric acid is responsible not only for the smell of rancid butter, but also contributes to the odor of sweat. Lactic acid is generated in muscles of the body as the individual cells metabolize sugar and do work. A buildup of lactic acid, caused by overexertion, is responsible for the fatigue one feels in the muscles by such short-term use. When one rests, the lactic acid is gradually converted to water and carbon dioxide, and the feeling of fatigue passes. A form of vitamin C is called ascorbic acid and is a carboxylic acid.
A special form of carboxylic acids are the amino acids, which are carboxylic acids that also have a nitrogen-containing group called an amine group in the molecule. Aminoacids are very important because combinations of amino acids make up the proteins. Proteins are one of the three major components of the diet, the other two being fats and carbohydrates. Much of the human body, like skin, hair, and muscle, is composed of protein.