In this connection it is to be remembered that the food of mankind may be classified under three heads, viz., _proteid or albuminous_, such as meat, eggs, casein of milk, gluten of bread, and various vegetable proteids; _carbohydrates_, as sugar and the starches of our cereals, and _fats_, including those of both animal and vegetable origin. The proteids are characterised by containing nitrogen (about 16 per cent), while the fats and carbohydrates contain only carbon, hydrogen, and oxygen. The two latter classes of food-stuffs are burned up in the body, when completely utilised, to carbonic acid (a gas) and water, while the proteid foods, beside yielding carbonic acid and water, give off practically all of their nitrogen in the form of crystalline nitrogenous products in the excreta of the body. Proteid foods have a particular function to perform, viz., to supply the waste of proteid matter from the active tissues of the body, and this function can be performed only by the proteid foods; hence the latter are essential food-stuffs without which the body cannot long survive. Fats and carbohydrates, on the other hand, are mainly of value for the energy they yield on oxidation, and in this connection it is to be remembered that the fuel value of fats per gram is much larger than that of carbohydrates, viz., 9.3: 4.1, or more than twice as great. Further, it is to be noted that the various food-stuffs cannot be utilised directly by the body, but they must first be digested, then absorbed and assimilated, after which they gradually, in their changed form, undergo decomposition with liberation of their contained energy, which may manifest itself in the form of heat or of mechanical work. The thoroughness with which foods are digested and utilised in the body must therefore count for a great deal in determining their dietetic or nutritive value. Moreover, it is easy to see how an excess of proteid food will give rise to a large proportion of nitrogenous waste matter, which, floating through the system prior to excretion, may, by acting on the nervous system and other parts of the body, produce disagreeable results. A mere excess of food, even of the non-nitrogenous variety, must entail a large amount of unnecessary work, thereby using up a proportional amount of energy for its own disposal, since once introduced into the body it must be digested and absorbed, otherwise it undergoes fermentation and putrefaction in the stomach and intestines, causing countless troubles. When absorbed in quantities beyond the real needs of the body, it may be temporarily deposited as fat; but why load up the system with unnecessary material, thereby interfering with the free running of the machinery? In other words, it is very evident that the taking in of food in quantities beyond the physiological requirements is undesirable, and may prove exceedingly injurious. It is truly uneconomical, and defeats the very ends we aim to attain.