The principal forms of fat for food are those in meats of all kinds, butter, cheese, a few fruits (as olives) and several kinds of nuts.
The principal sources of carbohydrates are cereals of all kinds (wheat and rye flour in the form of bread and otherwise, corn, rice, oatmeal, tapioca, etc.), dried beans and peas, a few kinds of nuts, several varieties of dried fruits, potatoes, etc., and in a lesser degree some fresh fruits, as apples and bananas.
Sugar is 100% carbohydrate—but the system can only take it in moderate quanti ties, not being fitted for the exclusive use of full carbohydrates any more than for the use of large quantities of fat—it prefers both mixed with other components as in sweet, starchy or fatty foods, or to produce them itself by conversion.
The excess of fat and carbohydrates after supplying the immediate necessities of heat and energy, is stored in the body in the form of fat, which, except when excessive in amount, stands as a very real reserve store of energy when at any time the body requires more fuel than it can draw from the immediate supply of food. A person who for any reason does an unusually excessive amount of physical labor may lose several pounds in weight in a few hours—the energy and heat developed and utilized are necessarily drawn from the reserve force of the body as the food supply for the same period could not furnish so large an amount under such conditions of activity.
A well-regulated diet for an average person of normal digestion must, therefore, contain a variety of foods sufficient to supply the system with an abundance of water (as a beverage or in food), a sufficient amount of protein to repair the waste of tissue and "energy foods" (fats and carbohydrates) in accordance with his manner of life —the. more the labor required from the body, the greater should be the supply. Still further, the greater the proportion of hard physical labor in outdoor occupations, the greater, generally, the proportion of the "condensed fuel"—fats and sugars—that may be advantageously used.
A diet principally of lean meat would supply too great a proportion of protein —one of fruits and vegetables, except dried peas and beans, peanuts, etc., would
supply too little. Bread and good rich milk would make quite a satisfactory all-round diet--giving water, protein, fat and carbohydrates in very fair proportions and in easily digestible form, but, fortunately, we need not confine ourselves to such a very monotonous bill of fare! We can reach just as satisfactory a percentage by a judi cious mingling of a variety of other foods with all kinds of delightful properties.
It will be understood that all statements concerning human diet necessarily deal in averages—every individual must regulate the details of his diet to agree with the results of his own experience. Furthermore, all percentages of food components essential or desirable are subject to variation in different climates and countries and none is absolutely binding in the operation of the human machine—which is wonder fully adaptable in meeting exigencies. Lean meat, for example, contains very little fat and no carbohydrates, but the human body, when necessary, will obtain all its fats and carbohydrates by chemical transformation of the protein and a man might live for a long time on lean meat alone. But it would not be a safe or desirable diet! The tables following give the average percentages of a number of general food items, after discarding general waste, as skin, bone, etc. The body receives a large proportion of the values recorded in the cases of persons of good digestive organs, if the foods are properly prepared—which in most cases means properly cooked.
The importance of good cooking cannot be over-estimated—incompetent prepara tion often means the loss of much of the value of the foods eaten. The valuable carbo hydrates, for example, are chiefly very small starch grains enclosed in tiny cells with thick walls on which the digestive juices have little effect unless the walls have been broken by cooking—and this is only one of a great many examples that might be cited.