All the carbohydrates and fats which are digested and burned in the body give off, so far as is known, the same amounts of energy under these circumstances that they do in the calorimeter. Proteins, however, are not completely oxidized in the body, and so furnish it with a somewhat smaller amount of heat than the calorimeter would indicate. Allowing for losses in digestion, it has been determined that every ounce of either protein or carbohydrate eaten supplies the body with 113 calories of heat; fats have greater fuel value and give 255 calories for every ounce.
The total amount of energy which must be supplied per person per day is, obviously, affected by the size of the person whose temperature must be maintained, as well as by the amount of energy used up in work. Atwater, for the United States Department of Agriculture, determined the following amounts as necessary for the man or woman of average size. The woman is allowed less because she averages less in weight.
Man with hard muscular work .......
Man with moderately active work......
Man at sedentary or the woman with moderately ac-tive work.............
Man without muscular exercise or the woman at light to moderate work ..........
4150 calories 3400 calories
How much protein is necessary daily is still a matter of controversy, but it is well established that the old notion that much meat or protein is necessary for strength is false. Some put the figure higher than others, but 300 to 400 calories a day from protein is probably a safe amount for the average person.
In stating the composition of food, sometimes the percentages of the various ingredients are given. Another way of expressing it is to state the number of calories of heat given by the carbohydrates, fats, and proteins in an amount of food which furnishes a total of one hundred calories of energy. This amount is known as the "stand-ard portion." The second method is perhaps less confusing, especially when comparisons between foods are to be made.
For example, suppose a boiled potato and white flour are to be compared. The percentage composition of the two is as follows:
With this great difference in the percentage of water present, it is evident that the percentages of the other ingredients cannot be compared directly.
Compare this with the number of calories furnished by the ingredients in the hundred-calorie portions.
Weight op 100-Calorie Portion
Boiled potato . .
3 2/3 OZ.
Wheat flour . .
It is at once evident that they are not so unlike in their nutritive elements, but that about four times as much weight of potato must be had to provide the same amount of energy.
If potatoes cost five cents for three pounds (1 2/3 c per pound) and flour is five cents a pound, it is easy to calculate that four pounds of potatoes would cost six and two-thirds cents, while a pound of white flour would furnish the same amount of fuel value for five cents.
Compare milk with flour.
The difference in the percentage of water present in the two makes it hard to decide which really furnishes the more protein, or fat, or carbohydrate.
Weight of 100-Calorie Portion
It may be seen at once, by the second table, that over five times as much milk is necessary to equal the total calories in flour, but that the milk will furnish almost twice as many calories from protein. If milk costs four cents a pint (about a pound), it is evidently much more expensive both as a source of energy and as a source of protein.
Such comparisons are of great interest, for food value and cost, curiously, have no connection with each other. Some of the government pamphlets show by graphs the amount of fuel value which can be purchased in different foods for a given amount of money, but the price of foods changes so rapidly that these quickly become antiquated.
In addition to the recognized food principles, there seems to be another class of substances present in foods, which are known as vitamines. These are present in natural foodstuffs in only small amounts, but they seem necessary, some for nutrition and others for growth. Fruits, vegetables, and milk are our best sources for these substances, so far as is known at present. The whole subject of vitamines is under investigation, and it is to be hoped that before long we shall be able to speak more definitely concerning them. However, they furnish an excellent additional reason for the inclusion of fruits and vegetables in our diet.
In all these discussions, it should be remembered, no account has been taken of mineral matter. Sherman says that of the mineral elements, only calcium, iron, and phosphorus need to be considered, if the diet be sufficient and balanced. (See the Appendix.)
Since many of our servings of food are about a hundred-calorie portion or bear a simple relation to it, it is convenient to use such a table in estimating the fuel value of a meal. A table of this kind will be found in the Appendix.
U. S. Dept. of Agriculture. Office of Exp. Sta. Bulletin No. 28. "The Chemical Composition of American Food Materials."
1. Describe Swiss chard. Why is it not commonly found in the market?
2. List leaves which may be substituted for spinach as "greens."
3. Can the outer leaves of lettuce, not desirable for salad, be used in this way?
4. If you had peas that you were not going to use for a couple of days, would they keep better cooked or raw? How about potatoes?
5. Why must all green vegetables be washed before using, even though they look clean?
6. Make a list of vegetables classified as to composition.
7. What vegetables and fruits contain more water than milk does?