Food

The U. S. legal definition of food is any substance, whether natural or processed, intended to be ingested by humans.   It includes drink, chewing gum and additives incorporated into the food during its manufacture.   Food does not include feed, live animals, plants prior to harvesting, medicinal products, cosmetics, tobacco and tobacco products, narcotics and psychotropic substances. ^{EurLex n.p.}   Food is obtained by farming, ranching, fishing, hunting and foraging.

Scientifically, food is fuel for living organisms.   Since fuel is any substance that provides energy, food is any substance that provides energy to a living organism, including humans.   Unlike the legal definition, the scientific definition excludes (pure) water, which has no energy, and includes beverages that contain sugar or alcohol.

Many foods contain stored (potential) chemical energy to produce and repair organ parts and functions.   Food energy, when released through reduction of food molecules (respiration) in the body, performs work and heat to keep body cells, tissues and organs functioning normally.   The chemical energy in food usually is measured in Calories   The chemical compounds in food can be categorized into functional groups, called nutrients:

Energy density of food components in Cal / g: ^{Wiki n.p.}

(Legal) Food Components:

Carbohydrates
Proteins
Fats and Oils
Water and Beverages
Vitamins
Minerals
Organic Acids
Phytochemicals
Food Additives
Salt (= Sodium Chloride = NaCl. See sodium & chlorine listed under Minerals and Chemical Elements.)

Note: In food chemistry, the word "essential" means that a nutrient is required for normal body functioning, but cannot be synthesized sufficiently by the body itself, and therefore must be included in the diet.   Food additives are not essential and phytochemicals presently are not known to be essential.   Common table salt (NaCl) is essential because it contains 2 essential elements, sodium and chlorine, included as Minerals in the above list.

The usual food of a person is called a diet.   A healthy diet combines foods to achieve an appropriate balance of the above nutrients.   A diet may be prescribed to achieve desired body appearance or function.   General recommendations for healthy diets often are illustrated by food pyramids, which advise kinds of food to be eaten.   For adults, those of the U. S. Department of Agriculture (Ag Pyramid) and Harvard Univ. School of Public Health (H.U. Pyramid) are common.   However, food pyramids represent general averages, so it is more accurate for people to use diets based on personal physiology and activities as prescribed by health professionals.   Recommended children's diets will contain foods that are different from those of adults, of course.   The diets of young adults will be different from those of seniors.   People with health problems will require special diets.   Vegetarian, vegan, kosher, halal, and other diets contain foods that promote health and other objectives.   Note that vitamin B₁₂ can be obtained naturally only from animals and animal products.