Fat and water constantly replace each other in the tissues. The rule for nitrogenous matter is that from 20 to 22 parts are found in association with 78 to 80 parts of water, but most organs contain more solid and less water than this, the explanation being that other solids with less affinity for water are in intimate union or stored up in them. Fat, e.g., penetrates the interstices of the protoplasm as a dry, water-free mass, neither attracting nor repelling water, but as it increases the weight, the percentage of watery content is diminished. Lawes and Gilbert found that 80.8 per cent. of a fat sheep was made up of fat and water, the former amounting to 45.8 per cent. and the latter 35 per cent., whilst a lean sheep contained only 18.7 per cent. of fat and 53.7 per cent. of water. A man weighing 70 kilograms with 20 per cent. of fat might put on or lose 5 kilograms of fat without altering his composition otherwise; in the former case the percentage content of water would be lowered and in the latter raised. During inanition it is quite certain that the bone marrow replaces its loss of fat by withdrawing albuminous fluid from the tissues, and it is probable that when the fatty globules of the cells disappear and the fat cell collapses, small quantities of albuminous fluid pass into them, and make the tissues more watery.

With an excess of carbohydrates in the diet, the tendency of the body is to retain water in the tissues, whereas when fats largely predominate the tendency of the body is to lose water. Voit has demonstrated that dogs fed for a long time upon bread accumulate large quantities of water in their tissues, and in all probability the tissues of vegetarians are likely to become richer in water than when a mixed diet is used. It is in any case significant that when carbohydrate is lost from the body, it is always in association with a greater quantity of water than normal. It is known that glycogen can increase to 19 per cent. in the liver and to 36 per cent. in the muscles, and if it be sodden with water in the same manner as nitrogenous matter, this would explain Voit's results.

In unpublished experiments upon other low-protein advocates, Benedict came to the conclusion that they took food of 40 per cent. more caloric value than they believed.

The 2,000 calories per day which Chittenden suggests was his own requirement for energy is a little higher than the estimate of the energy obtained from his diet and might be sufficient for an inactive man of little weight. But Benedict can hardly conceive that such a vigorous man as Professor Mendel could subsist on less than 2,800 calories per day, or nearly one-fifth more than Chittenden's estimate. In the same way the muscular activity of the soldiers with their drills and gymnasium work could not be satisfied with the scheduled 2,800 calories, and the explanation suggested for the athletes' low estimate is that on the days on which the diet was weighed and analysed less food was eaten than usual.

There is a general agreement that the energy requirement is that necessary for the voluntary and involuntary muscular activity, and the amount of food required is a function not so much "of the actual body-weight as of the mass of the active protoplasmic tissue muscle." Hence in most cases where the body-weight is excessive, less food would, during a short experiment, be required by drawing upon the fatty deposits of the body. But the energy of the food can only be lowered by lowering the energy output a corresponding amount, and this must necessarily mean diminished capacity for muscular activity to obviate a consistent withdrawal of the bodily tissues themselves.

He concludes, therefore, that although these experiments of Chittenden and others throw most valuable light upon the metabolism of proteins in the body, the evidence at present is not sufficient to warrant a permanent and material diminution of the amounts of protein now in vogue. He even asserts that this might be attended with serious disadvantages and probably danger unless at the same time accompanied by a diminution of muscular activity.

In a paper published in the British Medical Journal (December, 1909) I expressed a doubt as to whether the average business man consumed anything like the amount of food or the allowance of protein suggested by the Voit standard, and I am pleased to be able to record an investigation made by Chittenden upon the daily protein consumption of 108 vigorous, healthy young men, with an average body-weight of 66.2 kilograms. The average daily amount of nitrogen eliminated in the urine was 12.87 grams or .194 grams nitrogen per kilo of body-weight, as compared with .228 gram of nitrogen per kilo of body-weight of the Voit standard. These men, selected at random, were found to be living by choice on a daily diet containing approximately 68 grams of protein, and yet there was nothing about their appearance, their physical or mental activity, or their state of health suggestive of lowered vitality or diminished efficiency. As Chittenden truly remarks, if natural instinct or primitive experience is of any value in the selection of a diet, "Why not grant as a possibility that these 108 persons were being guided by an instinct worthy of just as much credence as the instinct or appetite that prompts another group of men to consume daily twice the amount of protein food? Will not this kind of argument apply with equal force in either direction, and perhaps serve as an illustration of the questionable value of human instinct as a guide in meeting the physiological needs of the body?"