Physiology: The Science of the Body

by Ernest G. Martin

While on the topic of diet, a word should be said about cooking. In a previous chapter the advantage of good cookery as an aid to digestion was emphasized. We would add here merely the comment that in defending the ancestral diet we do not intend to imply that their cookery was always what it should have been. Over most of America there has prevailed from pioneer days a habit of frying food in preference to other means of cooking it. Our hardy pioneer ancestors throve on fried meats; an outdoor life of muscular toil makes almost any kind of cookery both acceptable and digestible. As labor-saving machines tend more and more to diminish the amount of muscular labor that most of us do, we find it harder and harder to maintain good digestion on fried foods. The objection to frying is simple; fats are the hardest of all foods to digest and fried foods are smeared all over with fat. It is only logical to expect fried foods to be harder to digest than other kinds; it is undeniable that the flavor of many fried foods is so agreeable that we would be unwilling to omit them from the diet altogether. What is realty objectionable is the practice of smearing all the food with fat in the process of cooking it.

We have finished what we have to say about the use of food for the repair of bodily wastage. While we are on the topic, a few words about the use of food in growth will come in well, since the growth process is closely related to the process of tissue repair. The chief difference between them is that the process of growth comes to an end in all but a few of our body tissues as soon as we become adults. The tissues which go on growing are the layers of the skin just under the surface, the reproductive tissues, and the blood-corpuscle-forming tissues. Connective tissue has the power to grow at any time during life, although it does not actually keep on growing as does the skin. Whenever an injury is suffered which actually destroys muscular tissue or the deep layers of the skin, there is no growth of new tissue to take the place of either. Repair is made by a growth of connective tissue to fill up the space. The result is the formation of a scar. If the edges of the injury can be brought together skillfully enough, the outer layers of the skin which do have the power of growth may bridge across the space so that no scar will result.

A second thing to be mentioned about growth is a discovery which has attracted a great deal of attention of late years; we will realize of course that the chief thing in the making of new protoplasm is the building together of protein out of amino acids. It is evident that for the manufacture of new protein all the eighteen amino acids must be present in sufficient proportion to give enough of each. There are some plant proteins which lack one or two of the amino acids that are present in human proteins and when a growing animal goes on a diet in which these are the only proteins present it at once stops growing. Most of the experiments proving this have been done on white rats, and it has been found possible to keep a rat for more than a year at the size it was when only a few weeks old simply by feeding it proteins in which one or two amino acids were lacking. The fact that the animal lived during this time proves that his protoplasmic wastage was made good and therefore that there are proteins which can replace the body wastage but cannot manufacture new protein. There is only one conclusion to be drawn from this, namely that the daily wastage which the protein suffers does not include all of it; some amino acids when once built in are there for good; others, on the other hand, are constantly being lost in the process of wastage, and these are the ones which must be replaced. There is a common protein, gelatin, which is used a great deal for food, but which by itself will not serve either for repair or growth because it not only lacks some of the amino acids that are in living protoplasm, but also some that are lost in the process of wastage. Gelatin is useful as a food, therefore, only in combination with other proteins that contain the necessary amino acids; or after the nitrogen has been taken out, it becomes a good fuel.

One interesting question that was settled by the experiments described above was whether the ability to grow is exclusively a matter of youth; as we know, under ordinary conditions only young animals grow. When they reach a certain age, they become mature and thereafter no increase in size takes place. In the case of these white rats which were kept for more than a year at the size of partially grown rats it was found that as soon as their diet was changed to one in which all the necessary amino acids were present they would begin at once to grow and grow to full size, when they would stop growing just as they would have when young. This proves that growth is not a matter of age, but is a matter of achieving a certain size, and is controlled by factors which we do not at present understand. An animal that has not been able to attain