NUTRIENTS
All foods contain certain groups of nutrients which may be classified according to various functions in nutrition and their chemical composition.
Protein.—The protein group of nutrients contains nitrogen and is necessary for building up the tissues of the body, the muscles and the tendons which also contain nitrogen. Only by this group can tissues wasted by constant wear and tear be rebuilt. Proteins are the flesh-forming group. To some degree the proteins or albuminoids are also active in producing fat in the body, but the other groups of nutrients, especially the fats, also contribute.
Fats and Carbohydrates.—Another important function of food is to produce and maintain the animal heat. The main sources of this necessity are the fats and the carbohydrates, so called because they consist of the element carbon combined with oxygen and hydrogen, the last two in the exact proportion in which they are combined in water. All of these three groups furnish the fuel, so to speak, for the body, but they are not equal in this respect. Pound for pound, when burned in the body, the fats yield 2¼ times as much heat as protein or the carbohydrates.
Mineral Matters.—Finally there are in all foods the mineral matters, a group containing a number of salts which are indispensable because they are constituents of every part of the body. Phosphate of lime, for instance, makes up one-half of the substance of the bones, and the sulphates and chlorides of potash and soda, iron, etc., are everywhere present in smaller quantities. No food in which any of them is lacking is complete.
Relation.—The value of a food depends largely upon the relation of one group of nutrients to another. Proper nutrition can only be obtained when a sufficient amount of flesh-forming as well as of heat-producing nutrients are present, when the “nutritive ratio” is properly balanced for the particular purpose in view, be it the growth of the child, the maintenance of the body, the restoration of matter consumed by labor of body or brain, or the supply of heat to make up for cold surroundings.
Milk contains all of these groups of nutrients. The protein is represented in milk by the casein and albumin, the fats by the butter-fat, the carbohydrates by milk-sugar, and the mineral matters by the milk-ash. Human milk contains them in a perfect proportion for infants, and for all purposes of nutrition cow’s milk may be used to make up a “balanced ration,” if not alone, then in connection with other food.
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 cheese is | 1885 calories per 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— 7 ounces of sirloin steak 6 ounces of round steak 4.3 eggs 8.6 ounces of fowl | 1 quart of milk is equal to— 11.3 ounces of sirloin steak 14.9 ounces of round steak 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:
Milk at— | Is as cheap as sirloin steak at— | Or eggs at— |
|---|---|---|
| 7 cents a quart | 16.3 cents a pound | 17.6 cents a dozen |
| 8 cents a quart | 18.6 cents a pound | 20.1 cents a dozen |
| 9 cents a quart | 21.0 cents a pound | 22.6 cents a dozen |
| 10 cents a quart | 23.3 cents a pound | 25.1 cents a dozen |
| 12 cents a quart | 27.9 cents a pound | 30.2 cents a dozen |
| 15 cents a quart | 34.9 cents a pound | 37.7 cents a dozen |
According to this table, if milk is selling at 10 cents a quart, sirloin steak must sell as low as 23.3 cents a pound, and eggs at 25.1 cents a dozen to supply protein at equal cost.
To Supply Energy at Equal Cost
When milk is— | Sirloin steak must not be more than— | And eggs not more than— |
|---|---|---|
| 7 cents a quart | 9.9 cents a pound | 9.3 cents a dozen |
| 8 cents a quart | 11.3 cents a pound | 10.6 cents a dozen |
| 9 cents a quart | 12.8 cents a pound | 11.9 cents a dozen |
| 10 cents a quart | 14.2 cents a pound | 13.2 cents a dozen |
| 12 cents a quart | 17.0 cents a pound | 15.9 cents a dozen |
| 15 cents a quart | 21.3 cents a pound | 19.8 cents a dozen |
Fallacy of Theoretical Valuation.—While the contents of protein and the ratio between digestible protein and fats and carbohydrates on one hand, and the fuel or energy value on the other, have long been the only recognized measures for food values, they are admittedly quite inadequate and insufficient and although they are a great help when used with discrimination in making up food rations, they are often abused by persons who do not take their fallacies into consideration.
Dr. E. V. McCollum
“Something Unknown.”—Recent investigations by Dr. F. G. Hopkins, of Cambridge, England, and Dr. E. V. McCollum, formerly of Wisconsin, now of the Johns Hopkins University, have proven conclusively that one food ingredient cannot always be substituted for another with impunity even though the most searching chemical analysis shows them both alike in contents and digestibility. There is “Something Unknown” in certain foods—“Vitamines” some call it—essential especially in promoting the growth of the young, which our present knowledge of chemistry cannot explain. In Bulletin No. 17 of the Wisconsin Experiment Station experiments with the feeding of rats are described which show how butter-fat could not be replaced in the ration by vegetable fats of apparently the same composition and digestibility without disastrous results, and similar conditions have been found in regard to other foodstuffs, proteins as well as fats. The yolk of eggs and butter-fat contain this unknown something which is absolutely essential for the growth of the child and which is missing in most substitutes, especially in lard and vegetable fats.
The rat on the left got five per cent of cottonseed oil and the one on the right got instead one and a half per cent of butter-fat, otherwise their rations were alike. These results are typical for any ration made up of purified foodstuffs with butter-fat in them as compared with any fat of plant origin. The plant fats lack an unknown something without which growth cannot proceed.
The above illustration is from the work of McCollum and Davis at the Wisconsin Experiment Station.
Realizing the fallacy of the old rules for making up rations for the feeding of farm animals, Professor Evvard of Iowa is trying the reliability of the instincts of animals as a guide to the proper selection of the most favorable combinations and proportions of food ingredients.[[10]]
We mention these experiments as a warning against placing too great reliance on the caloric theory or the relation of nutrients in making up food rations. We have yet much to learn and the good housewife trying to cook according to scientific rules will do well not to neglect the palatability of the food, but to watch the “instinct” which causes the child or the adult to reject or approve of, and enjoy, the food, which in most cases is a better guide than calories or protein contents, or the ration between the various groups of nutrients.