It is not necessary to trouble you here with the nature or names of the many amino acids which make up the molecule of a protein. Let me mention at random just two among them which, like several more, have been shown to be absolutely essential for the growth of the body and in smaller amount for its maintenance. I will choose cystine, which is an amino‐acid containing sulphur, and tryptophane, which is an indol derivative. Suppose at a particular period of its history the human body in order to grow and function normally demands half a gramme a day of cystine. Now of a protein containing 1 per cent. of that amino acid 50 grammes a day satisfies that particular demand, but of another protein containing less cystine a proportionately greater amount will be required, and it is always possible for a deficiency in cystine to become the factor which limits the flesh‐forming value of a protein. But, again, suppose the body at the same time requires 1 gramme of tryptophane a day. Now the protein of which 50 grammes gave an adequate supply of cystine might contain say 1 per cent. only of tryptophane. The latter amino acid would now become a limiting factor for the value of the protein, and 100 grammes instead of 50 will after all be required. This, however, would supply twice as much cystine as is necessary and probably excess of other amino acids. This excess cannot be used for the growth or maintenance of the tissues, but can only share in the less specific functions of fats and carbohydrates by supplying energy on oxidation. These considerations will perhaps make it clear that the food proteins which can be used with the greatest economy in the body are those which contain all the essential amino acids in such relative proportions as will correspond most nearly with the proportions required by the living tissues of the consumer. These are the proteins of so‐called high biological value; they are the “first‐class proteins” which nowadays, as I have said, receive mention whenever diets are evaluated. That different proteins have different values in this sense has been abundantly proved by controlled experiments on animals and to a less extent by experiments on humans. It will be easily understood that it is animal proteins which in general have the highest value. It was long accepted that a man doing average work required a daily ration of 100 grammes of protein. More recently we have come to believe that this figure is too high. I can testify as a result of experiments in practical classes involving estimations of the daily excretions of nitrogen, that the average consumption of Cambridge undergraduates (those in training doubtless excepted) is not above some 80 grammes. But in this the proportion of first‐class protein is probably higher than the average.
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