If we desire to know how much sugar a certain food will add to the metabolism, that is, the sugar value of the food, we must know both the weight of the food and how much of this weight is carbohydrate, how much is protein, and how much is fat. The sugar value will be the sum obtained by adding the weight of the carbohydrate, 58 per cent of the weight of the protein, and 10 per cent of the weight of the fat. For instance, a slice of bread weighing 2 ounces or 60 gm., spread with one-third of an ounce or 10 gm. of butter would be 32 gm. of carbohydrate, 6 gm. of protein and 11 gm. of fat. The sugar value of this mixture would be 32 plus 3.5 plus 1.1 or 36.6 gm. A glass of whole milk, 6 ounces or 180 gm. of milk, would contain 9 gm. of carbohydrate, 5 gm. of protein and 7 gm. of fat, and its sugar value would be 9 plus 3 plus 0.7, or nearly 13 gm. After passing the liver, the blood enters the heart which then pumps it throughout the system of tubes called arteries into all the crevices of the body. Every tissue is thus bathed constantly with circulating fluids which contain sugar and the other elements of nutrition in forms suitable for utilization. _The liberation of energy in the tissues._--In the blood itself, no important chemical transformation occurs. The food fragments are picked out of the blood by the tissues and in the living protoplasms of each tissue, those important changes occur which constitute the most essential processes of life. These are, first, the building up of new protoplasmic matter, namely, growth and tissue repair, and, second, the liberation of energy by further decomposition and final burning or oxidation. Again each step of transformation requires the aid of some energizer or catalyst. The presence of many of these catalysts is only surmised. Some of them, however, have been captured and their mysteries are exposed. Among these is thyroxin, which Kendall showed to be the chief product of the thyroid gland. Thyroxin sets the pace of life. The rate of oxygen utilization is governed largely by it, so that when the thyroid gland is destroyed or not functioning, as in some forms of goiter, energy transformation proceeds at a slower tempo. _Insulin a catalyst._--We come now to diabetes, a condition which results from the lack or deficiency of a catalyst called insulin. The function of insulin, it seems, is to prepare sugar for utilization. _Diabetes a lack of insulin._--In diabetes, sugar escapes utilization in whole or in part. As it circulates normally in the blood, it is locked up, so to speak, and is unavailable either for storage as glycogen, or for oxidation or other transformation. Its energy is sealed, and before it becomes available a lock must be turned. The key to this lock is insulin, and in the absence of this key the normal means by which sugar is removed from the blood are unavailing, and in consequence, the level of sugar in the blood rises, and sugar appears in the urine.