Animals use food for the replacement of worn-out parts of their bodies, turning over tissues like blood at a high rate, other tissues more slowly.
A sufficient diet for any animal must provide enough energy, must include enough protein of the right sort and enough of each vitamin that an animal needs.
Animals need food as a source of energy, and as a source of raw material for building tissues and cells and the repair of damaged tissues and cells.
The anatomy of an animal reflects what it eats, especially in the teeth and claws, and in those parts of the body used to reach or catch up with food.
The vitamin needs of different species of animal vary, depending on which of the needed vitamins they can make for themselves from their normal diet.
The teeth of an animal give a good clue as to what it eats: carnivores have large canines and shearing teeth to cut raw meat and no grinders.
The teeth of an animal give a good clue as to what it eats: herbivores have large grinding teeth to make a pulp of tough vegetation and no shearing teeth.
The teeth of an animal may be misleading about what it eats: male gorillas have large canine teeth, which they use in threat displays to other males.
Human starvation is a major killer of our fellow humans, both directly through starvation, and by weakening people so they cannot fight off disease.
Deficiencies in the diet can cause diseases such as pellagra, scurvy, rickets, goitre and beriberi, all of which may be cured by a better diet.
In 1657, James Cook (not the navigator) published the case notes on scurvy of John Hall, William Shakespeare's son-in-law, leading to some later confusion.
In 1753 James Lind published his Treatise on Scurvy, establishing that lemon juice cured scurvy, though it would be a long while before it was fully accepted.
In 1906, Frederick Hopkins suggested the existence of what we now call vitamins and that a lack of these essential compounds caused scurvy and rickets.
In 1921, Sir Edward Mellanby discovered vitamin D and showed that its absence causes rickets in dogs kept indoors, while cod-liver oil cured it.
In 1914, Goldberger had noted that only orphanage children between 6 and 12 years old got the disease, but no orphanage staff, and no younger or older children.
It turned out later that those under six were given plenty of milk, and those over twelve were given more meat. Everything pointed to some dietary deficiency.
In 1915, Joseph Goldberger showed pellagra was a vitamin deficiency disease, after he noted an age distribution that was later related to diet in an orphanage.
Some diseases are caused by environmental factors, including toxins in foods that may be produced by microbes, and carcinogens in food, air and water.
There are various methods of food preservation, which make it impossible for microbes to live in or on the food, or place a barrier between microbes and food.
Food preservation can be done by boiling to sterilize, followed by sealing to keep germs out, by adding strong solutions of salt or sugar, or by freezing.
Some animals rely mainly on metabolic water for their water, this being water that is formed during the respiration of lipids and carbohydrates.