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Page:Popular Science Monthly Volume 72.djvu/401

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SOME NEW VIEW POINTS IN NUTRITION

per cent., is of unknown composition. It is not unaltered protein, of that we can be sure, because all protein is destroyed in the hydrolysis. It is presumably composed of small fragments of some kind, not yet recognized by chemists. The next most noticeable feature is that no two of these proteins are alike in their chemical make-up. Proteins from the same grain are distinctly unlike; gliadin of wheat contains no lysine, while leucosin from the same kernel contains 2.75 per cent. of this basic substance; gliadin likewise contains 37 per cent. of glutaminic acid, while leucosin has less than 7 per cent.; gliadin shows 5.6 per cent. of leucine, and leucosin twice that amount; gliadin contains no glycocoll, while leucosin has nearly 1 per cent. of this amino-acid. Such marked differences in chemical composition speak plainly regarding the individuality of proteins, even of those which are associated in the same seed. Comparison of casein from cow's milk, as a typical animal product, with any of the vegetable proteins, brings to light equally strong points of difference, while in gelatin we see many of the familiar amino-acids reduced to a minimum or entirely lacking. While it is undoubtedly true that all proteins possess certain features in common, it is becoming strikingly manifest that they are more or less divergent in chemical constitution. It has been the custom of physiologists in the past to lay stress upon the general rule that proteins are substances capable of meeting the physiological necessities of the body and that their nitrogen exists in a form suited to the needs of the organism. We have been accustomed to point to gelatin as the one exception to the rule, and have classed it as a protein-like substance, with as much nitrogen or even more than most proteins, but not truly a protein, since it can not support life. I fancy, however, that many true proteins may prove, when taken alone, unable to support life. As a matter of fact, few isolated proteins have been tested in this respect. Most of our feeding experiments have been made with mixtures of proteins, and consequently a considerable variety of protein cleavage products have been available for nutritive purposes. Take, as an illustration, the zein of corn meal, which contains no tryptophane, glycocoll nor lysine whatever, and only 1.5 per cent. of arginine and histidine combined, but with 18.6 per cent. of leucine, to say nothing of other peculiarities of chemical structure. Is it not reasonable to suppose that such a protein, with so many of the ordinary chemical groups missing or in greatly diminished quantity, will prove inadequate to meet the demands of protein synthesis? Experiment with animals has, indeed, shown this to be the case.

Data along these lines are bound to bring us more definite information than we at present possess regarding the real merits of vegetarianism as contrasted with the use of animal foods. At present, so-called vegetarianism rests mainly upon sentiment, reinforced by the