In considering the value of a dietary we must remember that it is not the food which is actually eaten which nourishes the patient but only those portions of it which are digested and absorbed. By prolonged boiling with sulphuric acid and water sawdust may be converted into grape sugar, which is a most useful nutrient, but the digestive powers of a man are quite insufficient to convert the cellulose, of which sawdust consists, into soluble sugar which can be absorbed, and any one feeding upon sawdust alone would certainly die of starvation, although the quantity consumed might contain as much carbon, hydrogen, oxygen and nitrogen as would supply a sufficient dietary provided only they could be utilized.
1 A number of other typical dietaries are given by Dr. Hutchison in his excellent book on Food, in which he shows not only the equivalents of various kinds of food but their actual cost.
Various definitions have been given to distinguish man from the lower animals. Plato's definition of "a biped without feathers" was turned into ridicule by Diogenes, who plucked a fowl and then let it loose to walk through the streets of Athens, and the definition of man as a speaking animal does not distinguish him from other animals, many of whom have a language of their own intelligible to each other, although we can understand it as little as we can the clucks of a Hottentot. The definition of man as "a cooking animal" is perhaps the best that has ever been given, for man is the only animal that uses fire. By the process of cooking a number of vegetable products that would be too hard to be readily digested in their native state are softened and made available for food. Protein foods are rendered more palatable and more digestible by cooking and noxious microbes with which the food may be infected are killed by the process so that they cannot set up an injurious kind of fermentation after they have entered the digestive canal. The alterations produced by the cooking of food are partly mechanical and partly chemical. The grains of the starch which is the most important constituent of carbo-hydrate food are broken up, so that they are more readily digested by the digestive juices, and they are also partly converted into soluble dextrine. The albuminous matters are coagulated, and this process not only renders them more easily disintegrated by the grinding action of the teeth but renders them more easily soluble in the juices of the stomach, so that the white of an egg is much more easily digested by the gastric juice when boiled than when raw. It must be remembered, however, in regard to this particular substance, that subdivision is of the utmost importance in digestion, and thus raw white of egg when beaten up may be mechanically so subdivided as to be more soluble than the same substance when hard boiled and swallowed in lumps. One peculiarity of starch is that when thrown into hot water it is apt to form lumps, in whose interior the starch is apt to remain unaltered, but if it be mixed into a thin paste with cold water which is afterwards stirred into boiling water an even mixture is produced. This rule not only holds good for the making of starch paste, but for the baking of bread and the boiling of puddings. In boiling meat the process varies according to the result which is desired. If the soup is the more important, then the meat must be put into cold water and gradually brought up to the boiling point. In this way time is afforded for all the flavouring ingredients of the meat to soak out before the outside layer becomes coagulated and therefore more or less impermeable. If, on the other hand, it is desirable to retain all the flavour in the boiled meat it must be put into boiling water so as to coagulate the surface and retain the flavouring matters as much as possible in the interior. In some parts of the Continent oxen are much used for ploughing and are slaughtered for food only when they are too old to work. The meat is therefore tough and requires prolonged boiling, in which case the soup which is made from it is more important than the meat itself which is afterwards served up with some piquant sauce to render it less insipid and more palatable. On the other hand, when young mutton is boiled in this country for the sake of the meat rather than that of the soup it is put at once into boiling water and thus its flavour is retained.
While cooking is, on the whole, an advantage and renders most foods more palatable and more digestible, there are instances in which the reverse occurs. For example, oysters contain a large quantity of a digestive ferment, enough to digest themselves when they are taken into the stomach, but when they are cooked, not only is this ferment destroyed, but the flesh of the oyster becomes considerably tougher, and so cooked oysters are, on the whole, less digestible than raw. Many plants also contain digestive ferments. This is notably in the case of the pineapple, which is now largely used for making pre-digested foods, and fruits in the process of ripening also contain ferments.
Fresh milk is said to contain no less than seven ferments, some of which split up sugar, others digest proteins and others digest fat. When milk is boiled, the activity of these ferments is destroyed, and some authors have considered that, in consequence of the change in the milk thus produced, boiled milk is apt to produce a form of scurvy. On the other hand, as I have already mentioned, thorough cooking destroys all the microbes that may be present in food and thus not only prevents danger from their causing putrefaction in the intestine, but also prevents the spread of disease by a specific virus such as that of enteric fever.
Pawlow showed that the pancreas pours out a secretion whose digestive properties vary with the food supplied. If the animal is fed on flesh the pancreatic juice becomes rich in trypsin; if the diet be starchy the proportion of amylopsin in the juice becomes greater. Allan Macfadyen and I showed that bacteria have a similar power of adapting the ferment they excrete to the soil on which they grow. But the power of adaptation is more rapid in the animal body than in bacteria, so that it is possible by rapid changes in diet from a completely protein to a completely farinaceous diet, and vice versa, to starve out bacteria, and this is sometimes useful as in the diarrhoea of infants.