_Fats_ are found in both animal and vegetable foods. Of animal fats, butter and suet are common examples. In vegetable form, fat is abundant in nuts, peas, beans, in various of the grains, and in a few fruits, as the olive. As furnished by nature in nuts, legumes, grains, fruits, and milk, this element is always found in a state of fine subdivision, which condition is the one best adapted to its digestion. As most commonly used, in the form of free fats, as butter, lard, etc., it is not only difficult of digestion itself, but often interferes with the digestion of the other food elements which are mixed with it. It was doubtless never intended that fats should be so modified from their natural condition and separated from other food elements as to be used as a separate article of food. The same may be said of the other carbonaceous elements, sugar and starch, neither of which, when used alone, is capable of sustaining life, although when combined in a proper and natural manner with other food elements, they perform a most important part in the nutrition of the body. Most foods contain a percentage of the _mineral_ elements. Grains and milk furnish these elements in abundance. The cellulose, or woody tissue, of vegetables, and the bran of wheat, are examples of _indigestible_ elements, which although they cannot be converted into blood in tissue, serve an important purpose by giving bulk to the food. With the exception of gluten, none of the food elements, when used alone, are capable of supporting life. A true food substance contains some of all the food elements, the amount of each varying in different foods. USES OF THE FOOD ELEMENTS.--Concerning the purpose which these different elements serve, it has been demonstrated by the experiments of eminent physiologists that the carbonaceous elements, which in general comprise the greater bulk of the food, serve three purposes in the body; 1. They furnish material for the production of heat; 2. They are a source of force when taken in connection with other food elements; 3. They replenish the fatty tissues of the body. Of the carbonaceous elements,--starch, sugar, and fats,--fats produce the greatest amount of heat in proportion to quantity; that is, more heat is developed from a pound of fat than from an equal weight of sugar or starch; but this apparent advantage is more than counterbalanced by the fact that fats are much more difficult of digestion than are the other carbonaceous elements, and if relied upon to furnish adequate material for bodily heat, would be productive of much mischief in overtaxing and producing disease of the digestive organs. The fact that nature has made a much more ample provision of starch and sugars than of fats in man's natural diet, would seem to indicate that they were intended to be the chief source of carbonaceous food; nevertheless, fats, when taken in such proportion as nature supplies them, are necessary and important food elements.