Another generality, to be thoroughly understood, is that while the figures given for proteid express quite clearly and with reasonable degree of accuracy the relative amounts of proteid matter present in the foodstuffs in question, there may be important differences in availability of which the percentage figures give no suggestion. In other words, the analytical data deal solely with the total content of proteid, while there is needed in addition information as to the relative digestibility, or availability by the body, of the different kinds of proteid food. For example, roast mutton, cream cheese, and dried peas contain approximately the same amount of proteid. Are we then to infer that these three foods have the same nutritive value so far as proteid is concerned ? Surely not, since no account is taken of the relative digestibility of the three foods. It is one of the axioms of physiology that the true nutritive value of any proteid food is dependent not alone upon the amount of proteid contained therein but upon the quantity of proteid that can be digested and absorbed; or, in other words, made available for the needs of the body. The same rule holds good for both fats and carbohydrates, but as proteid is the more important foodstuff, and is as a rule taken more sparingly, the question of availability has greater import with the proteid foods.

The availability or digestibility of foods can be determined only by physiological experiment. By making a comparison for a definite period of time of the amount of a given food ingredient consumed and the amount that passes unchanged through the intestine, an estimate of its digestibility can be made. The result, to be sure, is not wholly free from error, since we cannot always distinguish between the undigested food and so-called metabolic products coming from the digestive juices and from the walls of the intestine; but the errors are not large, and results so obtained are full of meaning. In a general way it may be stated that with animal foods, such as meats, eggs, and milk, about 97 per cent of the contained proteid is digested and thereby rendered available for the body. With ordinary vegetable foods, on the other hand, as they are usually prepared for consumption, only about 85 per cent of the proteid is made available. This is par-tially due to the presence in the vegetable tissue of cellulose, which in some measure prevents that thorough attack of the proteid by the digestive juices which occurs with animal foods. With a mixed diet i. e., with a variable admixture of animal and vegetable foods, it is usually considered that about 92 per cent of the proteid contained therein will undergo digestion.

Regarding differences in the availability of fats, it may be stated that, as a rule, the fatty matter contained in vegetable foods is less readily, or less thoroughly, digested than that present in foods of animal origin. In the latter, about 95 per cent of the fat is digested and absorbed. This figure, however, is generally taken as representing approximately the digestibility or availability of the fat contained in man's daily dietary, since by far the larger proportion of the fat consumed is of animal origin. Carbohydrates, on the other hand, are much more easily utilized by the body. Naturally, sugars, owing to their great solubility and ready diffusibility, offer little difficulty in the way of easy digestion; but starches likewise, though not so readily assimilable, are digested, as a rule, to the extent of 98 per cent or more of the amount consumed. It is thus evident that in any estimate of the food value of a given diet, chemical composition is to be checked by the digestibility or availability of the food ingredients.

As has been stated several times, the proteid foodstuffs are the more important, since proteid matter is essential to animal life. Man must have a certain amount of proteid food to maintain the body in a condition of strength and vigor. The other essential is that the daily food furnish sufficient energy to meet the needs of the body for heat and power. This means that in addition to proteid, which primarily serves a particular purpose, there must be enough non-nitrogenous food (either carbohydrate or fat or both) to provide the requisite fuel for oxidation or combustion to meet the demands of the body for heat and for work; both of which are subject to great variation owing to differences in the temperature of the surrounding air, and especially because of variations in the degree of bodily activity. The energy which a given foodstuff will yield can be ascertained by laboratory experiment, in which a definite weight of the substance is burned or oxidized in a calorimetric bomb under conditions where the exact amount of heat liberated can be accurately measured. The fuel, or energy, value so obtained is expressed in calories or heat units. A calorie may be defined as the amount of heat required to raise 1 gram of water 1' C, or, to be more exact, the amount of heat required to raise 1 gram of water from 15° to 16° C. This unit is usually spoken of as the small calorie, to distinguish it from the large calorie, which represents the amount of heat required to raise 1 kilogram of water 1° C. Hence, the large calorie is equal to one thousand small calories. When burned in a calorimeter, 1 gram of carbohydrate yields on an average 4100 gram-degree units of heat, or small calories; 1 gram of fat yields 9300 small calories. Both of these non-nitrogenous foods burn or oxidize to the same products - viz., carbon dioxide and water - when utilized in the body as when burned in the calorimeter; hence, the figures given represent the physiological heat of combustion, per gram, of the two classes of foodstuffs. Obviously, the fuel values of different foods belonging to the same group or class will show slight variation, but the above figures represent average values.

Unlike fats and carbohydrates, proteids are not burned completely in the body; hence, the physiological fuel value of a proteid is less than the value obtained by oxidation in a bomb calorimeter. In the body, proteids yield certain decomposition products which are removed through the excreta, and which represent a certain quantity of potential energy thus lost to the economy. The average fuel value of proteids burned outside of the body is placed at 5T11 calories per gram,1 or 5.7 large calories. Deducting the heat value of the proteid decomposition products contained in the excreta, the physiological fuel value of proteids is reduced on an average to about 4.1 large calories per gram.2 Rubner considers that the physiological fuel value of vegetable proteids is somewhat less than that of animal proteids; con-glutin, for example, yielding 3.96 calories, as contrasted with 4.3 calories furnished by egg-albumin, or 4.40 calories from casein. On a mixed diet, where 60 per cent of the ingested proteid food is of animal origin and 40 per cent vegetable, the fuel value available to the body would be about 4.1 calories per gram of proteid, on the assumption that the physiological heat value of vegetable proteids averages 3.96 calories per gram and that of animal proteids 4.23 calories per gram (Rubner).