MINERAL salts are of very great importance in nutrition. This will readily be understood if we bear in mind that analysis of the tissues yields 5 per cent, of ash. Death occurs in a few weeks if salts are cut out of the diet. The two most important salts are calcium phosphate, which enters largely into the composition of bones, and sodium chloride, which occurs in all the tissues and fluids of the body. Mineral substances are of great value as tissue-builders, and indirectly are sources of energy. We have no very precise knowledge as to the exact amount required daily, but we know that an ordinary mixed diet contains sufficient for the body requirements. Salts enter the blood in organic combination, and much importance attaches to the special form in which the different mineral substances are combined with the other food constituents. They pass into the blood as a rule unchanged by digestion. The chief mineral constituents are calcium, sodium, potassium, magnesium, and iron; phosphorus, chlorine, and sulphur; traces of silica, iodine, fluorine.
The chief uses of the salts may be summarised as follows: -
1. They enter into the composition of the tissues, e.g. the bones and teeth.
2. They subserve special functions, such as the iron in haemoglobin, calcium in coagulation of the blood, and sodium chloride in the production of hydrochloric acid in gastric digestion.
3. They exercise a powerful influence on the chemical composition of the blood, this being effected by their influence on processes of secretion and excretion.
4. They control the rate of absorption by osmosis.
The order in which foods stand as regards richness in salts is as follows: -
Foods rich in lime are milk, eggs, cereals (especially rice), vegetables (especially asparagus and spinach), and drinking-water, when that is of the hard variety. Foods poor in lime are meat, fish, potatoes, and fruits. An adequate supply of foods rich in lime is essential, and more especially during the growing period. Deficiency of lime leads to softness of the bones, and changes of a rachitic-like nature. Excess of lime salts in the diet is supposed to predispose to renal and vesical calculi. Reference may here be made to oxalic acid, which is present in food as oxalate of lime. It occurs specially in tea, coffee, rhubarb, spinach, and pepper. Cereal foods contain little; this knowledge is of value in the treatment of calculus.
Salts of sodium are specially required for the fluids of the body, salts of potassium for the cellular constituents, notably the red-blood corpuscles and muscles. Vegetable foods are, as a rule, rich in potassium salts; animal foods, on the other hand, have a larger proportion of sodium salts. Green vegetables and fruits are the chief sources of these salts, these foods being valuable on account of their containing vegetable acids with which the minerals are combined, the presence of the salts tending to maintain a proper degree of alkalinity of the blood. A deficiency of alkaline salts in foods leads to scurvy. Sodium chloride is the most important salt. It supplies chlorine for the production of the hydrochloric acid of the gastric juice, it stimulates appetite, it promotes renal secretion, and it induces thirst, which is of value in encouraging the drinking of fluid. On an average more than 10 grammes are taken in the daily dietary, this representing an amount which is far in excess of the actual body requirements. This excess of salt above the real requirements is more particularly observed in people who live largely on vegetarian foods. There is an ample supply of salt in a mixed diet without adding salt as an extra. There is no certain evidence that the excess of salt ordinarily taken is on the one hand of special value, or on the other injurious. The therapeutic effects of a salt-free diet are described on p. 416.
Beef, eggs, oatmeal, and lentils have a relatively large amount of iron; milk and its derivatives are poor in iron. A prolonged milk regime leads to anemia in some people. Iron exists in food in organic combination. A knowledge of the amount of iron in the food is of little practical value, since we cannot influence the state of the blood as regards iron by that means alone. Iron is of value in chalybeate waters.
Meat and vegetables are rich in phosphorus. It occurs usually in organic form, and to a less extent in inorganic form as phosphates of the alkalis and earths. It is especially valuable for growing children.
Foods may be classified according to the reaction of the ash left after incineration, into acid, neutral, or alkaline foods. Foods which give an acid reaction include meat, oats, barley, wheat, eggs, and rice; an alkaline reaction is given by milk, peas, and beans, potatoes, lemon and orange juice; animal fats and sugar have a neutral reaction. According to Wright, a deficiency of foods which yield an alkaline ash is a cause of scurvy.