The amount of protein regarded by Voit as a standard was 118 grammes, or just over 4 oz., for a man of 11 stone taking 2,800 calories a day. This is 1.7 grammes of protein per kilogramme. This figure, designated as Voit's standard, was arrived at by striking the average of a large number of observations, and may be taken to represent the diet chosen by a European of 11 stone doing a moderate amount of muscular work. Many people living sedentary lives take less both of protein and of caloric value, and of course those of smaller weight require a correspondingly smaller ration, other circumstances being equal. This may be illustrated by the following diet which was taken by a man just under 9 stone, and contains 90 grammes of protein, 67 of fat, and 385 of carbo-hydrate, total 2,300 calories : -
Bread, 19 oz.; meat, 4 oz.; potatoes, 8 oz.; milk, a pint; butter, 1 oz.; milk pudding, 4 oz.; tea, 1 pint.
This is as liberal in protein as Voit's diet for an 11 stone man being 16 grammes to each kilogramme. The pint of milk could be replaced by 3 oz. of cheese, which would slightly raise the proportions of fat and protein. Those living more active lives and those of greater weight commonly take a larger proportion of protein as well as of caloric value. A diet of 3,500 calories and containing about 130 grammes of protein is given by : -
Bread, 16 oz.; beef, 10 oz.; potatoes, 16 oz.; milk, a pint; butter, 1 oz.; oatmeal, 4 oz.; sugar, 3 oz.
In all dietary studies which include any large number of people we find that the quantity of protein amounts to 100 grammes or more. It is seldom below 90 grammes per day for an adult.
The following table of diets, arranged in order of protein value, will illustrate this point.
Compiled by Benedict and Atwater from various sources.
Protein in Food.
Caloric Value of Food..
Chinese laundrymen, United States ....
Printer, United States....
Machinists, United States....
Business men, United States....
Poor families in New York....
Sewing girl, London, earning 3s. 9d. per week .
Factory girl, Leipsic, earning 5s. per week .
It appears from these figures that in Europe, Asia, and America those who are free to choose their diet choose such food as shall include about 100 grammes of protein. It is uncommon to find less than 90 grammes taken except in conditions of poverty. Foods containing protein such as meat, eggs, milk and cheese are dearer than foods which contain a greater proportion of carbohydrate such as bread, potatoes, rice, and macaroni, and accordingly in the lowest social strata we find that there is, perforce, a scarcity of protein. It has, however, been urged with considerable force and by some whose opinions are worthy of great attention that it is not necessary or desirable for man to take so large a quantity of protein as 100 grammes a day. This view is founded upon experimental considerations which we may now examine.
If a man receives a diet the caloric value of which is sufficient but which contains little or no protein, for instance, a diet of starch matter with cream, the excretion of nitrogen in his urine sinks to a low figure, even to 3 grammes a day. This nitrogen is in the form of urea, creatinin, uric acid and ammonia, with small quantities of other bodies, and may be regarded as representing the lowest possible level of nitrogenous metabolism resulting from wear and tear of tissue. The patient is not in nitrogenous equilibrium because little or no protein is being given. If from 30 to 40 grammes of protein (1 to 1 1/2 oz.) be added to the diet, the caloric value being still made up by fat and carbo-hydrate, the individual can be kept in nitrogenous equilibrium excreting 4 to 5 grammes of nitrogen in the urine. If a further quantity of protein be given the nitrogen of this is excreted in the urine in a very short time. This does not necessarily mean that the extra protein has been used at once, but implies that the nitrogenous part of the protein molecule was not wanted and was therefore excreted rapidly. This nitrogen is turned out as urea; the residue of the extra protein is retained in the body for oxidation, and is referred to as the non-nitrogenous moiety. It is suggested as a deduction from these facts that 40 or 50 grammes of protein is all that is needed, and that when more is taken the rapid excretion of its nitrogen shows that it was in excess, and that its non-nitrogenous moiety is of no more advantage to the organism than fat or carbo-hydrate furnishing an equivalent amount of energy. These experiments were carried out by Folin. Many observations show that it is possible to live upon a diet containing only 40-50 grammes of protein. The following table from Jagerroos (Skand. Arch. f. Physiol. 13.375, 1902) summarizes a few of them.
Voit's standard .
For short periods still greater reductions can be sustained, but since we know that a man can live for a few days on a diet containing no protein at all, this is of no importance to our present inquiry.
The most thorough series of experiments upon a low protein diet has been made by Chittenden, who showed that a number of men of varying occupations and social position were able to live efficiently and carry out their ordinary work on a diet containing about 50 grammes of protein per day, which is • 7 grammes per kilo for a man of 11 stone and .9 for a man of 9 stone. These observations were made upon scientific men, soldiers working in a gymnasium, and students practising athletics. The latter were for five months in nitrogenous equilibrium upon a diet containing from 44 to 56 grammes of protein. Chittenden himself took remarkably small quantities of food and protein. One diet, for instance, consisted of : -
Milk, 2 1/2 oz.; cream, 5 oz.; sugar, 1 oz.; biscuit, 3 oz.; bread, 2 3/4 oz.; butter, 1/3 oz.; meat pie, 7 1/4 oz.
The conditions of the experiment must receive attention. The subjects were living quiet lives with regular work, sleep and food. The work was that to which they were accustomed and was performed at set times and rarely, if ever, to fatigue, and they were led to take a personal interest in the experiment. All these conditions, which apply also to most of the earlier experiments on this subject quoted above, are favourable to a minimal expenditure. Similar values have been found in a vegetarian married couple whose metabolism was investigated by Caspari and Glaess-ner. The protein in the food varied from 33 to 49 grammes per day. Hamill and Schryver found that a number of scientific men in London excreted about 9.6 grammes of nitrogen a day, equivalent to an intake of about 70 grammes of protein, a figure higher than those we have been dealing with, but lower than the Voit standard. Such results as these call for serious consideration. Voit set up a standard based upon experience gathered from those free to choose their own diet, of 1.4 to 1.7 grammes of protein to a kilogramme for an adult. The experiments detailed above give a standard of .7 to .9 grammes, roughly about half.