A metabolic disorder is a medical disorder which affects the production of energy within individual animal cells. Most metabolic disorders are genetic, though a few are "acquired" as a result of diet, toxins, infections, etc. Genetic metabolic disorders are also known as inborn errors of metabolism. Because of the enormous number of these diseases and wide range of systems affected, nearly every "presenting complaint" to a doctor may have a congenital metabolic disease as a possible cause, especially in childhood.

Digestion is the process whereby the foods we eat pass through our bodies and are directed toward the purposes of either providing the body with energy or building new cellular material, such as fat or muscle. The parts of food that the body cannot use, along with other wastes from the body, are eliminated in the form of excrement. Aspects of digestion, particularly the production of waste and intestinal gas, are not exactly topics for polite conversation, yet without these and other digestive processes, life for humans and other organisms would be impossible.

The term metabolism refers to all of the chemical reactions by which complex molecules taken into an organism are broken down to produce energy and by which energy is used to build up complex molecules. All metabolic reactions fall into one of two general categories: catabolic and anabolic reactions, or the processes of breaking down and building up, respectively.