=Nutritive Ratio.=—As we said before, the “nutritive ratio” of a food means the ratio of its flesh-forming constituents—proteins—to its heat-producing nutrients—carbohydrates and fats. Cow’s milk, containing 3.25% protein, 4% fat and 5% milk-sugar, has a nutritive ratio of 1 : 4.3, i. e., 1 part of protein to 4.3 parts of heat-giving nutrients, counting the fat equal to 2¼ of carbohydrates (multiplying the 4% fat by 2¼ makes 9, added to the 5% of sugar, makes 14; 3.25 to 14 equals 1 to 4.3). Skim milk, containing 3.4% protein, 0.2% fat, and 5.1% sugar, has a ratio of 1 : 1.6 (3.4 : 5.45). Mother’s milk, containing 2% protein, 4% fat, and 6.5% carbohydrates, has a ratio of 1 : 7.75. To substitute cow’s milk for mother’s, it must therefore be “modified.” =Calories.=—Another way of comparing various foods than by the nutritive ratio is by measuring their “fuel value” or energy-producing capacity. The amount of heat required to raise the temperature of one kilogram of water 1° C., or one pound 4° F., is called a calorie. By extensive feeding experiments the caloric value of the various nutrient groups has been estimated as follows: Protein at 1820 calories per pound[9] Fats at 4040 calories per pound Carbohydrates at 1820 calories per pound Measured by this rule the fuel value of: Whole milk is 310 calories per pound Skim milk is 160 calories per pound Full cream 1885 calories per cheese is pound Butter is 3410 calories per pound Compared with other food, milk, although not suited to act as the sole food of adults, is one of the best and cheapest articles of diet, and should be much more extensively used. The following table, compiled by specialists of the U. S. Department of Agriculture, shows the quantities of various foods needed to supply as much protein or energy as 1 quart of milk: ┌────────────────────────────┬────────────────────────────┐ │ Protein │ Energy │ ├────────────────────────────┼────────────────────────────┤ │1 quart of milk is equal to—│1 quart of milk is equal to—│ │ 7 ounces of sirloin steak │ 11.3 ounces of sirloin │ │ 6 ounces of round steak │ steak │ │ 4.3 eggs │ 14.9 ounces of round steak│ │ 8.6 ounces of fowl │ 9 eggs │ │ │ 14.5 ounces of fowl │ └────────────────────────────┴────────────────────────────┘ Another method of comparison is shown by the table below, in which the relative value of certain foods as economical sources of protein is given: