Feeding the body food it cannot use can starve it. Preparing food so that it does not require activity of the entire digestive tract may incapacitate the body. Human food needs to be in wholesome condition, properly chosen and prepared, and the body itself be so cared for that it is well and works well in its living-processes. It is thus that the body is nourished.

Predigested foods are usually prepared with a ferment that does part of the work of the digestive juices. Such a food uses the alimentary tract only partially, whereas it needs to be fully active to be well itself. In illness, predigested foods are sometimes needed. Peptonized foods serve to nourish a body that cannot otherwise nourish itself. Fermented foods, as koumiss, may save the body similarly when this is its need.

Prepared foods may by factory preparation of food materials lessen home work. Cereals are commercially so prepared. They keep less well when partially cooked and are more expensive. They take up water as cooked, and do not then resist further changes that they would in the dry state. The moisture absorbed increases the weight. Such preparation saves work in the home, and home fuel for prolonged cooking. There is, however, in all such commercial food-preparation the danger that the home completion of the process may be insufficient. This often happens with cereals and results in such prepared foods being used underprepared. Other prepared foods may lose water; all condensed- and powdered-milk preparations do. The high heating of milk changes its composition.

All canned, preserved, dried, steamed, or otherwise cooked foods are prepared for keeping or for digestion. Any aid that makes a food digestible is desirable, but any effort to digest it externally may prove a deprivation by making unnecessary the body-activity that is essential to healthful digestive functioning.