Butter, like cream, from which it is separated by churning, is the most digestible animal fat. Fat gives over twice the heat-energy of the same amount of starch or sugar and gives it more rapidly than starch. But only one fourth or less of the energy food of the body can come from fat. Butter is the staple diet-fat, except where it cannot be afforded.

Some substitutes for butter are wholesome, and if sold for what they are and are worth are not fraudulent foods. Neither the digestion nor palatability of other fats fully equals that of butter, nor do they promote growth as it does.

All fats have some fixed fatty acids and some volatile; one of the latter is peculiar to butter. When other fats than milk-fat are used (as beef-fat), they are usually flavored with some butter, also colored to resemble it. The color of butter is not significant. Much butter that is yellow is not rich, only artificially colored. Colorless unsalted butter is the most delicate and expensive. It requires the freshest production, as salt is a preservative. The flavor of butter is due to the effect of bacteria upon cream; as the bacteria differ, so the flavor. Flavor is increasingly regulated by artificially "ripening" cream with bacteria selected to produce the flavor desired.

Oleomargarine or butterine is clarified beef-fat, often with cottonseed-oil too, churned in milk. It lacks casein or volatile fatty acids, so such characteristics of butter; also is without its aroma. Oleomargarine serves some purposes wholesomely and many claim palatably. In cooking some think it indistinguishable, except in cake and candy. It makes cake heavy when used alone; it fails to remain mixed in candy.

Renovated butter is rancid (or stale) butter remade by melting and pouring the fat off the casein that settles, then rechurning the fat. Such butter is improved, but is not the equivalent of fresh butter. Butter becomes rancid through changes in the casein or by the fats decomposing. Heating fat makes it less digestible.