169. Net Energy

Within a comparatively short time we have begun to speak of the net energy of foods, and as this is a practical consideration which is likely to be the subject of much future discussion, it is well to notice it in an explanatory way. As we have learned, food is not applied to use until it reaches the blood. Between the time when it is taken into the mouth and when it passes into the circulation, it rtiust have work expended on it in the way of mastication, solution, and moving it along the digestive tract, and it appears highly probable that the amount of this work per pound of food must vary greatly in different cases. In fact, we know this is so from the result of some masterly investigations conducted by Zuntz in Germany. By means of various devices and methods, a description of which would be out of place here, he measured the oxygen consumption necessary to sustain the mechanical energy of mastication and digestion with a horse, and he calculates from his determinations that the following heat units represented the energy used in chewing certain feeding stuffs: -

The differences revealed by these figures are interesting and important. Chewing green food cost in labor only about 62 per cent of the effort required to masticate its equivalent of dry hay, the proportions of labor for hay, oats, and corn being in the ratio of 100, 27, and 83.

This author goes farther and calculates that the work of mastication and digestion combined is 48 per cent of the energy value of the digested material from hay and 19.7 per cent of that from oats. To be sure, these results were obtained with a horse and do not apply to man, but they serve to illustrate the fact that the mastication and digestion of food is work and requires an expenditure of energy. The fact is also evident that the expenditure of energy varies with the mechanical condition of the foods, though not to the same extent perhaps. All these deductions are based upon the excess of oxygen used when the work of chewing and digestion is going on over that used in the absence of such effort.

If we wish to ascertain the comparative energy worth of two unlike foods, it would obviously be incorrect to multiply the total quantities of protein, carbohydrates, and fats in each, by the unit heat values, in order to ascertain the relative energy gain to the animal body.

To recapitulate, we may define available energy as total energy minus that which is lost in the excreta and in gases whicli escape, and net energy as available energy minus the cost of digestion and of preparing the food for use. Net energy is the balance of profit to the body.