Foods may be classified in various ways: a. According to Source, as animal, vegetable, and mineral (including oxygen for combustion).

b. According to Chemical Composition, as organic and inorganic, the organic foods being further subdivided into nitrogenous and non-nitrogenous substances.

c. According to Function, as tissue-formers, or body-builders; energy (or work and heat) producers; and regulators of body processes.