The property of protein to build living tissue and replace tissue waste probably depends upon several factors; but certainly one of them is the presence of nitrogen. So intimately associated are the consumption of the tissue substance and the elimination of nitrogen that we have no better way of judging the amount of tissue substance used in the body than by determining the quantity of nitrogen that appears in its various waste products. From such investigations it has been found that the quantity of protein required to repair the breaking down of the tissues is not great. The average man consumes approximately a quarter of a pound (100 to 120 grams) of protein daily; but this quantity is in excess of his real needs. Indeed, Chittenden has shown that for various classes of individuals, namely, students, athletes and soldiers, half as much is sufficient. Other physiologists, though admitting that this is true, contend that it is inadvisable to regulate one's diet on such a slender basis. Very good reasons are assigned for the view that more protein is needed than just enough to counterbalance the tissue waste. Thus, in the case of animals, it has been found that a diet low in protein finally causes digestive disturbances and other ailments. Although it does not seem advisable to practise rigid economy in arranging the protein content of the diet, it is equally important that we should not go to the other extreme. The consumption of over- large quantities of protein, as would be the case if we lived exclusively upon meat, increases putrefaction in the intestines and throws unnecessary work upon the kidneys, which are the organs chiefly concerned in getting rid of the waste products of protein. _Carbohydrate_ is the name given the group of foodstuffs to which the sugars belong. The food value of cane sugar, the most familiar member of the group, was recognized even in prehistoric days by the natives of India. By boiling the plant we call sugar-cane they obtained a substance to which they gave the name Sakkara, and from this our word sugar evidently originated. The roots of this plant were carried into Europe and cultivated during the Middle Ages. Obviously, its value was and is appreciated, since the cultivation of the sugar-cane and the sugar-beet has become the foundation of a great modern industry.