The experiments upon man show that men can live in health for several months and do routine work on a diet containing about 50 grammes of protein per day, or half the usual quantity. It is not shown that this diet is advantageous, or that it would do no harm taken permanently.
We have now to consider whether as much work can be done upon a low protein diet as upon a more plentiful supply. We have seen that muscular activity involves the using up of energy, and that if the work of any given individual be increased, he will require more food. Should this extra food contain protein? Or will the addition of carbo-hydrate and fat meet all the needs of a man subjected to severe exertion? Physiological experiment has shown that when energy is used in the contraction of muscle there is not a corresponding breakdown of protein material, for the nitrogen in the urine is only increased to a slight extent. For example, one of Atwater's subjects did in a day external work equivalent to 543 calories. This would necessitate the using up of probably at least 2,000 calories. The excretion of nitrogen, however, was practically unaffected. The energy for this work was therefore furnished by the oxidation of non-nitrogenous material, presumably carbo-hydrate or fat. It would therefore appear that when severe work has to be done the addition of carbo-hydrate or fat is all that is necessary.
It is not safe, however, in the present condition of our knowledge, to take this as proven, for we are faced with the fact that those doing really heavy and continuous work are usually found to take a diet which not only is high in caloric value, but is also rich in protein. We are not now referring to feats of strength or athletic tests which may be done at the temporary expense of the tissues, which afterwards recuperate, or to such work as that of Chittenden's soldiers or athletes, whose every exertion would be followed by an appropriate rest, and whose muscles would arrive at a state of proficiency in which they may be supposed to work with a minimum of waste and damage to their cells; but to work such as that done by labourers, who are toiling daily to the point of fatigue and exercising various groups of muscles in all kinds of movements. The table already-given on p. 135 shows the proportion of protein taken by such workers. These and other results are summarised by Benedict as follows :
Protein in Food.
People doing severe muscular work, lumbermen, etc.....
People doing ordinary muscular work, carpenters, labourers, etc........
People doing light muscular work, business men, etc.............
An interesting remark on this point is made by Darwin in The Voyage of a Naturalist round the World. He visited the Chilian copper mines, where at that time the ore was carried up a 450 foot shaft in loads of 200 lbs. at a time, by men who climbed up notched tree trunks placed zigzag in the sides of the shaft. The men were pale, and some of them with but little muscular development. They were fed on beans and bread, and themselves preferred bread which was in that country a luxury, but the masters, says Darwin, "finding that they cannot work so hard upon this, treat them like horses and make them eat the beans." In beans about 20 per cent of the energy is in the form of protein, in bread 9 to 13 per cent. Livingstone found in Central Africa that the grain-eating tribes could not endure fatigue so well as beef-eaters, who scorned " the idea of ever being tired." Carnivorous animals are capable of greater efforts and have more courage than the herbivorous. Irving Fisher, however, claims that a diet on the Chittenden scale is favourable to endurance, as measured by such actions as holding the arms horizontally as long as possible, or raising and lowering a dumb-bell. Observations were made upon nine students three times in the course of five months. During this time the protein was gradually reduced, but not so low as in Chittenden's experiments. The endurance was found to be much greater at the end than at the beginning of this period. The strength was, however, less, the grip of the right hand, for instance, being weaker at the end of the period in seven out of the nine men. No control experiments on other students are given. The loss of strength and loss of weight which these students suffered was attributed to overstudy, but the increase of endurance to the food. These considerations do not inspire confidence in the diet. Feats of endurance such as long walks have been done upon a vegetarian diet, but this is not necessarily deficient in protein. The great and undoubted increase in endurance as well as in mettle of a horse fed on beans or oats, as compared to one fed on grass or on hay seems to point to an opposite conclusion, namely that the increase of protein is beneficial to prolonged muscular exertion. Such horses can not only go more quickly, but can go much further than those on the low protein dietary, and horses fed on hay have more staying power if the hay have plenty of seed in it, which means more protein. Professor Dunstan, of the Wye Agricultural College, has kindly informed the writer that beans contain 23 per cent of digestible protein, oats 9 per cent, good hay (that is with plentiful seed in it) 7.4, and poor hay 34 per cent.
We may sum up the relation of protein to activity by saying that the bulk of the calorie value required for muscular work should be supplied by carbo-hydrate or fat, but that it is an advantage to also increase the protein in the food, and that a diet containing a fair amount of protein is likely to be favourable to an energetic existence. Since the actual energy for the contraction of muscle is supplied by non-nitrogenous material, we may suppose that a muscle can work more efficiently when the tissue lymph surrounding it is rich in protein molecules, or, again, the hypothesis may be put forward that it is an advantage to the body to have some of its non-nitrogenous oxidizable material derived from protein, the nitrogen of which may have been previously excreted.
We have considered the problem of the proportion of protein in the daily diet at some length on account of its great importance. The question must now be asked, Does a careful consideration of the experimental evidence lead to the conclusion that a reduction in this constituent of the food is desirable? Are the observations detailed above sufficiently convincing to justify us in regarding the consumption of protein by the average man as excessive or still more as harmful? It must be remembered that the dietetic habits of mankind are the result of a process which has been going on for ages; when, therefore, we find that both progressive nations and successful individuals take a fair proportion of protein, we must treat this fruit of accumulated experience with respect. The old idea that protein was the food par excellence, and that the more of it that was taken the better, has been succeeded by a more moderate and scientific view. In the opinion of the writer we shall be acting wrongly if we now go to the other extreme and adopt dietary standards in which the amount of protein is so low as to lie close to the physiological limit. Facts have been adduced above to show that such a course is not without danger to the individual and the nation.
A great excess of protein is not readily tolerated. A man cannot, as has been mentioned in a former chapter, exist in a natural state without a supply of carbo-hydrate in his food. The quantity of protein necessary to supply his caloric needs, if no other food be taken, is intolerable to the digestive system, and therefore a protein diet such as the Salisbury diet, is not exclusive of other foods for any length of time. Diaceturia and aceto-nuria occur, probably from the call on the body fat being so great that its oxidation is incomplete. The addition of fat to protein, without carbo-hydrate, will lessen the amount of protein required, but aggravates the acetonuria, whilst carbo-hydrate abolishes it. The amount of carbo-hydrate required is variable, but a small quantity, such as 70 grammes (2 1/2 oz.), is insufficient if much fat is being taken (Rosenfeld). In a diet containing plenty of carbo-hydrate as well as fat, a large quantity of protein can be dealt with, especially if severe work is being done; as much, for instance, as 250 grammes a day has been found to be eaten by miners in Russia. Or in treatment by "forced feeding," in which small quantities of food are administered at frequent intervals, the amount may be as great. A patient observed by Hale White and the writer, took an average of 243 grammes for thirty-eight days, with a maximum of 292 grammes in a day. In diabetes still greater quantities are consumed in the effort to meet the caloric needs of the body, which is partially or completely unable to utilize carbo-hydrate. To the rule that protein and fat alone cannot maintain man in health without carbo-hydrate, there appears to be an exception in the case of dwellers in arctic regions who have became accustomed to such a diet. These are all extraordinary conditions. In ordinary life excess in the consumption of protein is harmful and leads to digestive disorders, as we should expect from the experimental evidence : it is also regarded as leading to arteriosclerosis and chronic nephritis, though it is not often easy to dissociate this factor from others such as alcohol and bacillary toxins. In diseases of the kidney and liver, organs especially associated with the metabolism of nitrogen, it is obvious that particular care should be taken to avoid excess of protein. We may apply the results of the above-mentioned observations upon low protein to such cases, and prescribe a diet which shall include about 50 grammes of protein (contained, for instance, in 2 1/2 pints of milk); care must be taken, however, that the caloric value of the food is made up with carbo-hydrate and fat. In acute nephritis a diet containing no protein may be given for three or four days.
The form in which protein is taken is of importance. Experience shows that it is not advantageous to take more than about half the protein ration in the form of meat, and this applies with special force to those living sedentary lives.