If the daily income is reinforced by the addition of carbohydrate or fat, there is observed a decided influence on the outgo of nitrogen; the rate or extent of proteid metabolism is at once modified, fat and carbohydrate both having a direct saving effect on proteid. Neither fat nor carbohydrate can prevent the katabolism of proteid, but they can and do decrease it, and thus serve as proteid-sparers. In the fasting body, or where there is only an intake of proteid, the latter material, except for the fat contained in the tissues, must serve the double purpose of meeting the specific nitrogen requirements of the body and furnishing the requisite energy. The energy requirements, however, can be met more advantageously by either of the non-nitrogenous foodstuffs, and just so far as they are oxidized, so far will there be a saving of proteid. Herein lies the philosophy of a mixed diet, with its natural intermingling of proteid, fat, and carbohydrate. For the same reason, the body of a man rich in fat will in fasting lose far less proteid per day than the lean man; or, if fed with a given amount of proteid food, the fat man may attain nitrogen equilibrium, or even store up a little proteid, while on the same diet the lean man will lose proteid. Further, if a man is in nitrogen balance with a given amount of proteid food, the addition of fat or carbohydrate to the diet will permit of a reduction in the amount of proteid necessary to maintain nitrogenous equilibrium. Fat, however, when added to food, does not always protect proteid to the extent possibly suggested by the preceding statements. The following data from oft-quoted experiments by Voit[28] on dogs will serve to illustrate: [28] C. Voit: Hermann’s Handbuch der Physiologie des Gesammtstoffwechsels, Band 6, p. 130. +-----------------------++-----------------------------+ | Food. || Flesh. | +-----------+-----------++--------------+--------------+ | Meat. | Fat. || Metabolized. | On the Body. | +-----------+-----------++--------------+--------------+ | grams | grams || grams | grams | | 1500 | 0 || 1512 | -12 | | 1500 | 150 || 1474 | +26 | | | || | | | 500 | 0 || 556 | -56 | | 500 | 100 || 520 | -20 | +-----------+-----------++--------------+--------------+