From a strictly chemical point of view, it makes little difference from which class of foods the different materials may be taken. They exist in varying proportions in all foods. A cow can grow fat on grass, but it usually has to eat for eighteen hours a day to do so. Physiological chemists have determined the exact quanta necessary to keep a man alive under different conditions (regarding work, etc.) ; and a dietary can be constructed to meet these demands. Comparative tables have been constructed to show the equivalents in food values, and a well-balanced dietary will include sufficient nourishment at a reasonable figure. Cheese, for instance, is one of the most valuable of foods. in proportion to its price. Another very important point to be considered is the subject of digestibility. For instance, on the surface it might be considered, and is very frequently so taught, that mushrooms are as useful as meat, because they contain an equally high percentage of proteids. It must be borne in mind, however, that the proteids of the mushrooms are not fully utilised by the digestive organs ; hence the very highly prized chemical factor is a misleading one. In fact, the chemical food experts have constructed very useful tables ; but they are all defective, and are merely suggestive, because the problems regarding digestion have not been taken into sufficient account. Personal idiosyncrasy of digestion is also a disturbing factor. Thus, enormous differences exist with reference to the ability of different individuals to utilise fats. For this reason the card philosophies of the " nut-food " faddists often come to grief because of the indigestible fats that many nuts contain.
The general question of digestibility is one to which the layman is accustomed to attribute a fixed, and, therefore, erroneous, meaning. For it may be said that there is scarcely a subject of the human economy which is exposed to greater variations ; and in no other respect does personal predisposition play a greater part than in that of the digestibility of food. A dish which may be readily digestible to one person may be very troublesome to the digestive organs of another. The digestibility of a foodstuff depends not only upon its natural qualities, but also upon numerous other factors, such as appetite, palatability, and proper mastication.
Numerous tables have been constructed purporting to show the com parative digestibility of various foodstuffs. These, however, should be interpreted only in the most general way. In view of the many factors which, as has already been pointed out, enter into the question, one might almost say that they are worthless. Many of the individual idiosyncrasies are due to chemical reasons ; others to mental reasons. There is very little
use arguing with the man who says that milk disagrees with him. It is bound to do so as long as he thinks it will ; for there is no part of the human body which is more profoundly subject to the impressions of the mind than the stomach. To reinforce the truth of this assertion, it is only necessary to refer to the relationship which the emotions bear to the appetite. An indi vidual can make himself, as well as others, very miserable by constantly wondering whether or not a certain kind of food will agree with him.
There are still other factors that determine the digestibility of the various foods. Whether a greater or less amount of bile is present is of importance with regard to fatty emulsification ; variations in the pancreatic secretion affect all three types of food-principles, the fats particularly ; and variation in the strength of the muscular coats of the intestines is also a determining factor in the question of digestibility. Then, again, all foodstuffs are composites. Their contained sugars may diffuse rapidly ; their proteids may soon be digested ; and their fats last. Hence, any table pretending to tell when a substance is fully digested is a delusion and a snare ; it may be helpful in a most general way, but it should not be interpreted too literally. The following table is one widely quoted, but it shares the defects of all the others in that it contains too many general statements. Thus, to illustrate, it states that celery is digested in hours, when, as a matter of fact, certain parts of the celery are never digested, but thrown out in the freces. It is inserted here as an instance of one man's guess about the digestibility of certain foods. It might be surmised that it represents his personal experiences. They are worth so much, and little more.
One hour: Boiled rice. One and one-half hours : Beaten eggs ; barley soup ; roast venison ; stewed fruits ; boiled trout ; spinach ; asparagus ; celery ; brains. Two hours : Boiled milk ; turkey ; boiled pickled pork ; baked potatoes ; peas ; beans ; roast beef. Three hours : Smoked ham ; stewed mutton ; boiled carrots ; cabbage. Three and one-half hours: Boiled beef ; roast pork ; corned beef ; boiled potatoes ; fresh wheat-bread. Four hours : Boiled and broiled poultry ; roast veal ; dry bread. Four and one-hall hours : Boiled fresh pork ; boiled mutton. Five hours : Mush rooms ; nuts. Six hours : Fried eels ; stale corned beef. Fluids are absorbed comparatively quickly by the body. See also the articles on BREAD ; FRUITS ; MEAT ; VEGETABLES ; etc.