The fruit juices are readily absorbed and carried at once to the liver, where the sodium, magnesium, and potassium are released and the acids oxidized and changed to carbonates. They increase the alkalinity of the blood. These alkalis are soon eliminated through the kidneys, which accounts for the diuretic effect of fruits.
The seeds in the small fruits are not digested, but they serve the purpose of increasing intestinal peristalsis and of assisting the movement of the contents of the intestines. The skin and the fibre of fruits also assist the intestines in this way, just as the fibre in vegetables does. Fruits may be classified into acid and sweet fruits. Under acid fruits are the citrous group—lemons, limes, grape fruit, oranges, cranberries, gooseberries, whortleberries, pineapples, currants, and rhubarb—if rhubarb is to be classed as a fruit.
There has been a commonly accepted theory that where a blood test shows evidence of too much uric acid, acid fruits are to be avoided, but the reverse is true. It has been fully demonstrated that the use of acid fruits increases the alkalinity,—or neutralizes the acids in the blood.
In case of an excess of hydrochloric acid in the stomach, lemon, or citrous fruits are valuable about half an hour before a meal as they decrease the secretion of the hydrochloric acid into the stomach. Where hydrochloric acid is limited, acids are given after a meal to supplement the deficient amount.
The sweetening of acid fruits does not detract from the value of the acids or of organic salts.
All acid fruits stimulate the action of the kidneys and the skin,—particularly lemons, limes, grape fruit, and oranges, and wherever the kidneys and skin are not sufficiently active, these fruits should be eaten freely.
It is difficult to make a decided distinction between sweet and acid fruits. The best guide is in the amount of sugar required to make them palatable. Some species of cherry are distinctly sour, while others are sweet. The same is true of apples, peaches, plums, etc.
Under sweet or bland fruits are pears, raspberries, grapes, bananas, blackberries, blueberries, melons, apricots, and some peaches, apples, and plums.
The large majority of fruits do not contain sufficient sugar to make them valuable for nourishment. Their chief value is in their appetizing flavor, and in the acids, and salts. Dates, figs, prunes, and dried grapes (raisins) are exceptions. As will be noted by the following table, these fruits contain a large amount of carbohydrates in the form of sugar. The larger amount of protein in these sweet fruits is largely in the seeds and, as the seeds are not digested, they have no real food value to the individual.
Figs and prunes are laxative,—probably the laxative effect of figs is due to the seeds, and of prunes to the salts and acids. However, prunes are free from tannic acid.