Protein like starch, sugar, and fat contains the elements carbon, hydrogen, and oxygen, but it differs from them in containing also a large proportion of the element nitrogen, which may be regarded as its characteristic constituent. When digested in the stomach and intestine it is split into a large number of simpler substances known by chemists under the name of amino-acids. Every animal requires these amino-acids in certain proportions. From the mixture resulting from the digestion of the proteins in its diet the amino-acids are absorbed and utilised by the body in the proportions required. If the proteins of the diet do not supply the amino-acids in these proportions, it is obvious that an excessive amount of protein must be provided in order that the diet may supply enough of that particular amino-acid which is present in deficient amount, and much of those amino-acids which are abundantly present must go to waste. This is undesirable for two reasons. Waste amino-acids are excreted through the kidneys, and excessive waste throws excessive work on these organs, which may lead to defective excretion, and thus cause one or other of the numerous forms of ill health which are associated with this condition. Again, excessive consumption of protein greatly adds to the cost of the diet, for protein costs nearly as many shillings per pound as starch or sugar costs pence. These considerations show clearly the wisdom of limiting the amount of protein in the diet to the smallest amount which will provide for wear and tear of the working parts. The obvious way to do this is to take a mixed diet so arranged that the various articles of which the diet consists contain proteins which are so to speak complementary. The meaning of this is perhaps best illustrated by a concrete example. The protein of wheat, gluten, is a peculiar one. On digestion it splits like other proteins into amino-acids, but these are not present from the dietetic point of view in well balanced proportions. One particular amino-acid, called glutaminic acid, preponderates, and unfortunately this particular acid does not happen to be one which the animal organism requires in considerable quantity. Other amino-acids which the animal organism does require in large amounts are deficient in the mixture of amino-acids yielded by the digestion of the protein of wheat. It follows, therefore, that to obtain enough of these latter acids a man feeding only on wheat products would have to eat a quantity of bread which would supply a great excess of the more abundant glutaminic acid, which would go to waste with the evil results already outlined. From this point of view it appears that bread should not form more than a certain proportion of the diet, and that the rest of the diet should consist of foods which contain proteins yielding on digestion little glutaminic acid and much of the other amino-acids in which the protein of wheat is deficient. Unfortunately information