Physical exercise strongly influences the quantity of blood in the skin and its secretory functions, which are increased. Perspiration, as already said, may reach a very high amount. After physical exercise, and especially when resting after it, there is considerably more danger of taking cold than at other times, and the usual well-known measures should be taken for avoiding this.

In a "good mixed diet' protein, fat, and carbohydrate may, to a certain extent, take the place of one another according to the above value in calories. But it must be said that all diet for human beings, in order to be good, must be mixed and within certain limits must contain all the three organic food substances. The exclusion of fat is borne best, that of protein worst.

If protein is omitted the excretion of nitrogen by the urine continues, though diminished, till death, of which there is much more danger than when both fat and carbohydrate are excluded. Carnivorous animals are able to live ad infinitum on protein, salts, and water; in man and other omnivorous animals a similar diet produces loss of appetite, severe disturbances of digestion, etc.

The older men of science now living consider in general that protein is able to build up protein, fat, and carbohydrate in the human organism; that carbohydrate can build up fat and carbohydrate; and that fat is unable to make anything but fat. It seems to me not improbable that in the future these opinions may become better grounded and more generally accepted than they are now.

However, in these days the various proofs hitherto offered of the formation of fat from protein (by means of the separation from it of a molecule containing nitrogen) are not considered complete.* I consider the fact that protein can form glycogen, i.e., carbohydrate, has been proved beyond all possibility of doubt - by the important and minute investigations of E. Kulz particularly.

It is certain that carbohydrate is able to form carbohydrate and fat in the organism; we need not concern ourselves with the doubts as to the formation of fat which crop up here and there.

Skilful investigators find, what speaks for itself, that all which eventually is consumed for the production of energy within the organism must be carbohydrate (glucose). As no one doubts that fat maintains energy, the said investigators assume that fat within the organism can form carbohydrate. I consider that important well-known facts arc opposed to this view. If an animal is starved until its liver glycogen has sunk to a small but definite amount, and if the animal is then fed on fat alone, the liver glycogen continues to sink (see Seegen's figures, but not his opinions). In diabetics, with minimal power of assimilation of carbohydrates and maximal production of sugar in the urine, the sugar in the urine is not increased by eating butter. It is difficult to understand how either of these facts could be if fat under any conditions could form carbohydrate within the organism. If all vital force in the organism were produced by glucose, a certain calorie value of this material (as Zuntz remarks) should support more muscle work or give more warmth than the same calorie value of protein or fat, to change which into carbohydrate would mean a considerable loss of power. This is not the case in Zuntz's careful and exhaustive experiments. For the present, therefore, we have far more arguments against than for the opinion that fat can form carbohydrate, or that all that is finally oxidised within the organism is carbohydrate. We have long hesitated to accept the theory that muscle is able to break up the fat molecules, which to our minds appear so firm, and it is still a mystery where this breaking-up first takes place. But since we now know that fat, in being broken up to carbonic acid and water, certainly without a moments delay passes through the molecular formation of β oxybutyric acid, diacetic acid, and acetone, and as we believe that muscle or any other tissue has the power to oxidise β oxybutyric acid to diacetic acid, this to acetone, and the acetone to carbonic acid and water, we are more inclined than formerly to believe in the power of muscle to break up fat molecules.

* All scientists ought to be, and the majority arc, very sceptical, especially the elder ones who have seen so many rooted ideas unmercifully uprooted. But to make older doctors believe, e.g., that fat in a fatty liver after phosphorus poisoning does not come from fatty degeneration of cell protoplasm but from fatty infiltration is difficult enough. A priest said to me recently, quite seriously and solemnly, "We cannot but believe in the devil." I found the man a little obstinate, but with regard to the question of fatty degeneration I recall his remark, and for my own part say, "I cannot but believe in fatty degeneration."

The younger Cohnheim considers that he has shown that albumose and peptone in the mucous membrane of the alimentary canal arc broken down to form amino-acids by means of a ferment (erepsin), from which afterwards proteins peculiar to the body may be built up in the tissues. In addition, carbohydrates and, to a less extent, fats may also assist in the synthesis of proteins in the body by supplying the necessary carbon, hydrogen, and oxygen.

The question whether protein is able to maintain muscle work has been handled for a long time and by means of extensive experiments. There is no doubt that both fat and carbohydrates are able to do this. Neither is there any doubt that protein in the organism is able to form carbohydrate, so that it cannot be questioned that the former substance can at least indirectly maintain muscle work. But it is now generally accepted that as long as muscle work is not excessive and the supply of carbohydrate and oxygen sufficient, there is practically no change in the nitrogenous output. The final decision seems to be that the expenditure for muscle work is primarily borne by the glycogen of the liver, which is quickly changed to sugar in the blood and afterwards is fetched from the various storage places. Muscle glycogen is more stable and the sugar in the blood is inconsiderably diminished by muscle-work alone, and only late during starvation.