Mineral matter occurs not only in the teeth and bones, but in every tissue of the body and in all the fluids. It is necessary in all the vital processes. The principal mineral elements in the body are calcium, magnesium, iron, sodium, potassium, phosphorus, chlorine, iodine, fluorine, silicon, and sulphur. These occur as compounds, forming both mineral and organic salts. Unlike carbohydrates, fats, and proteins, mineral salts are not changed in digestion nor are they oxidized, and so they do not furnish the body with energy. When organic matter is burned, these salts remain unconsumed as the ash.
Mineral matter is present in all the digestive juices and plays its part in the digestion and absorption of foods. Mineral matter is dissolved in the blood and regulates its specific gravity and its alkalinity. It is found in all tissues, where it is concerned in metabolism.* Mineral matter, too, probably stimulates the contractions which cause the heart to beat.
Since man excretes every day a large amount of mineral matter, this loss must be replaced. The necessary amount of mineral matter is found in an ordinary mixed diet. Common salt, sodium chloride, is the only mineral which is added to food, but it is probable that there would be enough of this furnished in the food of a mixed diet. In fact, it is possible that large quantities of salt are really bad for us rather than helpful. People, like the Esquimaux, who are unaccustomed to its use easily detect the addition of an exceedingly small amount and dislike it, but those who are habituated to its use crave it. Salt seems to accentuate flavor.
* Metabolism includes all the processes which food undergoes after it is digested and absorbed and before it is excreted.
In the metabolism of proteins, mineral acids are formed which must be neutralized by such basic substances as sodium, potassium, calcium, and magnesium, in order to preserve the alkalinity of the blood. As these basic elements are abundant in vegetables and fruits, it is necessary that the diet should contain liberal amounts of both and not consist too largely of meat. The modern diet is likely not to be well balanced in this respect, because it consists of liberal amounts of meat, fats, and sugar, all of which contain insignificant amounts of mineral salts.
At first thought it seems strange that fruits should be added to prevent over-acidity when fruits themselves are acid; but the acids present are organic acids, such as citric, malic, tartaric, and oxalic. Some of these are decomposition products of starch and are oxidized by the body to produce energy and are then given off as carbon dioxide and water, just as starch is metabolized. Mineral acids cannot be oxidized in the body and must be neutralized into salts and then excreted, and that is why they require basic elements with which to unite to become salts.
It is important to remember the need for generous amounts of vegetables and fruit in the diet, for these often seem expensive materials in comparison with the amount of energy and of building material which they contain, and the poor are tempted to leave them entirely out of their rations.