Depending upon either or both of these sources of stimulation, the
digestive juices of the body are regulated in quantity, and what is
much more worthy of note, in their actual chemical composition. Thus
it will be readily seen how far-reaching in its effect upon scientific
dietetic treatment is the knowledge of the influence of various foods,
quantities, and combinations.
[Sidenote: Comparative digestibility of foods]
Professor Palloff's discoveries throw some very important light on the
comparative digestibility of foods. The former method of estimating
the digestibility of food was first to analyze the food, and then to
analyze the intestinal residue, and subtract the undigested remnant of
each particular class of food from the amount originally eaten. By such
means it was possible to show that certain foods were, say 80 or 90 per
cent digestible, as the case might be. By this method no allowance was
made for the amount of nutrition or material that was consumed by the
body in the digestion of these particular foods. According to these
investigations, milk and meat were about equally digestible. It was not
known that the digestion of milk requires only a small fraction of the
energy that is necessary to digest meat, or proteids from vegetable
sources. Thus it is obvious that when it is desirable to get a large
amount of available nitrogen into the system, with as little expenditure
of energy as possible, milk is a food par excellence. This is very
logical inasmuch as the sole purpose of milk is food for animal life.
[Sidenote: Comparative acidity and energy required in digestion]
The amount of acidity in gastric juice that must be secreted for the
digestion of meat is much in excess of that required for a given amount
of vegetable food. The amount of acidity required is greatest for milk,
second for meat, and least for bread. The digestive energy required is
greatest for bread, second for meat, and least for milk. From this we
learn that starchy foods are unsuitable for those who are afflicted with
hyperchlorhydria or supersecretion of hydrochloric acid, as the excess
of acid prevents their digestion by neutralizing the alkali of the
intestines.
[Sidenote: Insalivation of starchy foods and meats]
The saliva secreted when nitrogenous food is eaten does not contain as
much ptyalin as that secreted when starchy food is consumed; for this
reason the thorough insalivation of starchy foods is much more important
than that of meat, milk, and eggs. Some authorities have recently
advised that people should not chew meat at all, but should swallow
it as do carnivorous animals. This advice, however, is not altogether
sound. In the first place, man is not a carnivorous animal, and the
gastric juice of the human stomach does not act as rapidly on flesh
foods as does the gastric juice of meat-eating animals, but if meat be
taken into the human stomach, either in large or in small quantities,
decomposition may take place before digestion has proceeded far enough
to prevent the action of micro-organisms.
[Sidenote: Mental influence upon digestive fluids]
The mental influence upon the secretion of digestive fluids may
originate from thought, or may be brought about reflexively by the
sight, or by the smell of food. All are familiar with the experience of
having one's mouth water at the sight of a particularly appetizing dish.
Many of us have undergone the same experience by merely thinking of some
particular food of which we are fond.
[Sidenote: Digestive juices vary with different foods]
Scientific investigation has shown that the secretion of saliva is
only an example of what takes place in the other digestive organs.
The experiments of the ingenious Russian scientist, heretofore
mentioned, prove that the act of tasting and of swallowing food was
the chief factor in determining the secretion of the juices from the
stomach-walls. In a series of operations upon dogs, performed by skilled
surgeons, certain interesting facts were observed. The esophagus was
severed and made to open externally so that the food swallowed did
not pass into the stomach. The secretion of gastric juice was then
determined in the case of different foods which were taken into the
dog's mouth and swallowed, but which did not reach the stomach. Not only
did this act of pretended feeding start a flow of gastric juice, but the
juice secreted in the case of different foods was especially adapted to
the particular food, according to the general principle which we have
already discussed.
These facts emphasize several important considerations regarding our
diet:
1 We should eat slowly and get the whole taste out of food by
thorough mastication, because taste largely controls the secretion
of the digestive fluid