For illustration, the energy value of a pound of edible material from a few foodstuffs is given as follows:

Available Energy

These figures mean that when a pound of each of these foods is wholly burned, the heat produced is as stated. It has been demonstrated, too, by severe and elaborate investigations that the law of the correlation and conservation of energy holds in the animal organism as it does with machinery, or, in other words, the energy given off as animal heat has been measured and found to be exactly equivalent to the food energy minus that in the various excreta.

168. Available Energy

We must distinguish, however, between the heat produced when any food substance is wholly oxidized in a calorimeter, and the heat or energy which is available when the same material is applied to physiological uses. It never happens that the combustible portion of a ration is entirely burned in the animal.

In the first place, food is practically never all digested, and, as only the digested portion furnishes energy, the available fuel value of a ration must be based primarily, not upon the total quantity of dry matter it represents, but upon the amount which is dissolved and passes into the blood. If all foods were digested in the same proportion and with the same ease, their total fuel values might show their relative energy worth, but as digestion coefficients for the various food materials vary greatly, it is evident that the fuel waste in the feces is not uniform.

In the second place, the digested proteins are never fully burned. A portion of these compounds always passes off in the urine unoxidized, the fuel value of which is lost to the animal. For this reason, the available energy of the digested protein is about one-fourth less than the total.

In the third place, there is an escape from the alimentary canal of unconsumed gases, due to the fermentations which take place during the latter states of digestion. These gases, mostly methane (marsh gas), have their source mostly in the carbohydrates. With farm animals the loss of energy in gases has been found to vary from 10 to 20 per cent of the digested dry substance of the food. With the human species, the loss is much less and is perhaps almost negligible.

We are to understand, then, that the available energy of food is represented by the fuel value of the dry matter which is digested from it, minus the dry matter of the urine and that lost in gases.

If, however, we wish to know the actual energy gain from a particular diet, we must go farther than a determination of its available energy.