students while writing a very difficult examination, and it was found that no more energy was set free from the body than when they were simply writing nonsense. However, a very small part of the potential energy of the food always takes the form of electrical energy and elec- trical energy is manifested whenever nervous tissue (brain, nerve, etc.) is active. It is possible then that the reason no extra heat energy was given off from the body when the brain was working hard is because this electrical energy simply took the place of other forms of energy (like the secretory processes of glands, etc.), in which case it would not be correct to say that brain work is not done at the expense of potential energy. The subject requires much more study than has yet been given to it.
It will be evident from the discussion thus far that it is the energy content of the food which a person is concerned primarily to know in order to judge the economy of its use. Ordinarily if one takes care merely to supply himself enough heat energy and takes care also to eat a variety of food the other requirements will be automatically regulated by the appetite. Failure properly to adjust the energy supply to the actual requirements result in depletion which may subject one to disease, or, on the other hand, may lead to obesity.
But it would be a serious mistake to ignore the other chief purpose of foods — namely, their value as tissue builders and restoratives. 'While not more than one fifth of the dry weight of our foods find a permanent lodgment in the body, it is obvious that the functional activity of the tissues could not be kept up indefinitely without these constituents of which tissues are formed. Still less could growth of the body in early life be maintained.
Among these constituents the most important is that class of sub- stances known to physiologists as proteins, exemplified by the white of egg, the casein of milk, the gluten of bread and par excellence the flesh of animals. The actual requirement of the body for these substances is much less than is ordinarily supposed. In starvation, when the body is living at the expense of its own substance, about 13 per cent, of its energy is derived from the breakdown of proteins or the nitrogenous substances. The remaining 87 per cent, is derived from the body fat and glycogen, which is the form in which the body stores up starches and sugars. Theoretically the body would be kept in perfect equilibrium then if the food contained 87 per cent, of the total energy in the form of carbohydrates and fats and 13 per cent, in the form of protein. In fact, many persons have found that they keep in better physical condi- tion if they take somewhat less than 13 per cent, in the form of protein, for with a generous suply of carbohydrate in the food the waste of proteins from the body is considerably less than in starvation. The dietaries suggested above for various sorts of workers, however, have
