The physiological effect upon the system of tea, coffee, and cocoa has been much discussed. Of the three, cocoa seems to have much less influence eithe; in retarding digestion or as a stimulant, though there is reason to think that it is not without stimulating effects.
Tea has a marked influence in lessening the action of the saliva, while both tea and coffee retard digestion, the latter to a less extent than the former. This effect seems due to the tannic acid and the volatile oil. The caffeine itself favors digestion. Both tea and coffee act as stimulants because of the caffeine present. It is this that causes them to be so effective in lessening the feeling of fatigue. Strong coffee is a powerful antidote to narcotics, and is often used where a heart stimulant is needed. Coffee and tea may, because of the tannic acid and other astringent substances present, prove irritants to the mucous membrane of the stomach. This action is greater it the stomach is empty. The stimulating effect also is greater if taken upon an empty stomach.
The effects of coffee and tea seem to be influenced largely by the personal equation, and quite opposite results are produced in different persons by them; while in most people they tend to produce wakefulness, in others they are conducive to sleep. Some people can use one freely and must refrain completely from the other.
The general conclusion from experiment and observation seems to be that, taken in moderate quantities and at suitable times, they are not injurious to the healthy adult, but that those of a feeble digestion, or who are nervous, should use them in exceedingly small quantities, if at all. Of the two, coffee seems to have the least harmful effect in the majority of cases.
On the market at present there are a large number of coffee substitutes. Some of them undoubtedly are true cereal drinks, and may be used as such, though when a large amount of food value is attributed to them on this account, one cannot help wondering how the insoluble substances of the wheat grain can so largely be present in the drink made from the treatment of wheat kernels in water. Some of the so-called cereal coffees are said to derive their flavor from the volatile oils produced in the roasting of coffee, while others actually contain coffee.