The use of the fats in the body is essentially the same as that of the starches and sugars.  Weight for weight they are more valuable than the carbohydrates as sources of energy, but the latter are more easily digested, and more easily oxidized in the body.  An important use of fatty foods is for the maintenance of the bodily heat.  The inhabitants of Arctic regions are thus enabled, by large use of the fat and oil from the animals they devour, to endure safely the severe cold.  Then there is reason to believe that fat helps the digestion of other foods, for it is found that the body is better nourished when the fats are used as food.  When more fat is consumed than is required to keep up the bodily heat and to yield working power, the excess is stored up in various parts of the body, making a sort of reserve fuel, which may be drawn upon at any future time.

105.  Saline or Mineral Foods.  All food contains, besides the substances having potential energy, as described, certain saline matters.  Water and salts are not usually considered foods, but the results of scientific research, as well as the experience of life, show that these substances are absolutely necessary to the body.  The principal mineral foods are salt, lime, iron, magnesia, phosphorus, potash, and water.  Except common salt and water, these substances are usually taken only in combination with other foods.

These saline matters are essential to health, and when not present in due proportion nutrition is disturbed.  If a dog be fed on food freed from all salines, but otherwise containing proper nutrients, he soon suffers from weakness, after a time amounting to paralysis, and often dies in convulsions.

About 200 grains of common salt are required daily by an adult, but a large proportion of this is in our food.  Phosphate of lime is obtained from milk and meats, and carbonate of lime from the hard water we drink.  Both are required for the bones and teeth.  The salts of potash, which assist in purifying the blood, are obtained from vegetables and fruits.  An iron salt is found in most foods, and sulphur in the yolk of eggs.

106.  Water.  Water is of use chiefly as a solvent, and while not strictly a food, is necessary to life.  It enters into the construction of every tissue and is constantly being removed from the body by every channel of waste[17].

As a solvent water aids digestion, and as it forms about 80 per cent of the blood, it serves as a carrier of nutrient material to all the tissues of the body.

Important Articles of Diet.

107.  Milk.  The value of milk as a food cannot be overestimated.  It affords nourishment in a very simple, convenient, and perfect form.  It is the sole food provided for the young of all animals which nourish their young.  It is an ideal food containing, in excellent proportions, all the four elements necessary for growth and health in earlier youth.

[Table:  Composition of Food Materials.  Careful analyses have been made of the different articles of food, mostly of the raw, or uncooked foods.  As might be expected, the analyses on record differ more or less in the percentages assigned to the various constituents, but the following table will give a fair idea of the fundamental nutritive value of the more common foods: