This theory, if true, goes far toward explaining the more rapid stimulation of coffee.
The statement has sometimes been made that milk or cream causes the coffee liquid to become coagulated when it comes into contact with the acids of the stomach. This is true, but does not carry with it the inference that indigestibility accompanies this coagulation. Milk and cream, upon reaching the stomach, are coagulated by the gastric juice; but the casein product formed is not indigestible. These liquids, when added to coffee, are partially acted upon by the small acid content of the brew, so that the gastric juice action is not so pronounced, for the coagulation was started before ingestion, and the coagulable constituent, casein, is more dilute in the cup as consumed than it is in milk. Accordingly, the particles formed by it in the stomach will be relatively smaller and more quickly and easily digested than milk per se. It has been observed that coffee containing milk or cream is not as stimulating as black coffee. The writer believes that this is probably due to mechanical inclusion of caffein in the casein and fat particles, and also to some adsorption of the alkaloid by them. This would materially retard the absorption of the caffein by the body, spread the action over a longer period of time, and hence decrease the maximum stimulation attained.
In a few instances, a small fraction of one percent of coffee users, there is a certain type of distress, localized chiefly in the alimentary tract, caused by coffee, which can not be blamed upon the much-maligned caffein. The irritating elements may be generally classified as compounds formed upon the addition of cream or milk to the coffee liquor, volatile constituents, and products formed by hydrolysis of the fibrous part of the grounds. It may be generally postulated that the main causation of this discomfort is due to substances formed in the incorrect brewing of coffee, the effect of which is accentuated by the addition of cream or milk, when the condition of individual idiosyncrasy is present.
Without enlarging upon his reason, Lorand[218] concludes that neither tea nor coffee is advisable for weak stomachs. Nalpasse,[219] however, believes that coffee taken after meals makes the digestion more perfect and more rapid, augmenting the secretions, and that it agrees equally well with people inclined to embonpoint and heavy eaters whose digestion is slow and difficult. Thompson[220] also observes that coffee drunk in moderation is a mild stimulant to gastric digestion.
Eder[221] reported, as the result of an inquiry into the action of coffee on the activity of the stomachs of ruminants, that coffee infusions produced a transitory increase in the number and intensity of the movements of the paunch, but that the influence exercised was very irregular.
An elaborate investigation of the action of tea and coffee on digestion in the stomach was made by Fraser,[222] in which he found that both retard peptic digestion, the former to a greater degree than the latter. The digestion of white of egg, ham, salt beef, and roast beef was much less affected than that of lamb, fowl, or bread. Coffee seemed actually to aid the digestion of egg and ham. He attributed the retarding effect to the tannic acid of the tea and the volatile constituents of the coffee—the caffein itself favoring digestion rather than otherwise. Tea increased the production of gas in all but salt foods, whereas coffee did not. Coffee is, therefore, to be preferred in cases of flatulent dyspepsia.
Hutchinson, in his Food and Dietetics, opines:
As regards the practical inferences to be drawn from experiences and observations, it may be said that in health the disturbance of digestion produced by the infused beverages (tea and coffee) is negligible. Roberts, indeed, goes so far as to suggest that the slight slowing of digestion which they produce may be favored rather than otherwise, as tending to compensate for too rapid digestibility which refinements of manufacture and preparation have made characteristic of modern foods.
Regarding increase in secretory activity, Moore and Allanston[223] report that in their experience meat extracts, tea, caffein solution, and coffee call forth a greater gastric secretion than does water, while with milk the flow of gastric juice seems to be retarded. Cushing[224] and others support this statement. This action is partially explained by Voit on the grounds that all tasty foods increase gastric secretion, the action being partly psychological; but Cushing observed the same effects upon introducing coffee directly into the stomachs of animals.
In general, a moderate amount of coffee stimulates appetite, improves digestion and relieves the sense of plenitude in the stomach. It increases intestinal peristalsis, acts as a mild laxative, and slightly stimulates secretion of bile. Excessive use, however, profoundly disturbs digestive function, and promotes constipation and hemorrhoids.[225] There is much evidence to support the view that "neither tea, coffee, nor chicory in dilute solutions has any deleterious action on the digestive ferments, although in strong solutions such an action may be manifest."[226] After conducting exhaustive experiments with various types of coffee, Lehmann[227] concluded that ordinary coffee is without effect on the digestion of the majority of sound persons, and may be used with impunity.