This section is from the book "Elements Of The Theory And Practice Of Cookery", by Mary E. Williams. Also available from Amazon: Elements Of The Theory And Practice Of Cookery; A Textbook Of Domestic Science For Use In Schools.
We eat not merely to keep our bodies alive, but to make them as fit as possible to serve us in all the activities of life. Food requirements vary with age, work, climate, and other conditions. Attempts have been made to fix dietary standards, that is, to determine the exact amounts of each class of foodstuffs required daily by a person of given age, weight, and habit of life. As a result of these and other dietary studies, certain facts are known which should guide us in selecting our own diet or in providing a diet for others.
A calorie is the amount of heat required to raise 1 kilogramme of water 1° Centigrade, or 1 pound of water about 4° Fahrenheit, or if expressed as work, the energy required to raise a 1-pound weight 3087 feet.
Protein and carbohydrate have about the same fuel value; fat has a fuel value a little more than twice as great as either protein or carbohydrate. The food eaten each day, often termed the daily ration, should include carbohydrate, protein, and fat. As fuel these foodstuffs can to a certain extent take the place of one another; but the fuel value of all three together must be sufficient to furnish the required heat and energy. The amount of protein not used for fuel, but for the repair, and in young people for the growth, of living tissue, is also measured in calories.
The fuel value or energy value of all foods in common use has been ascertained by burning weighed portions of them and measuring exactly the amount of heat given off by each. Tables have been prepared stating these fuel values in calories. Other tables have been made showing just how much by weight and by measure of any kind of food is required to furnish 100 calories. With the aid of these tables and scales to weigh food, we can learn to calculate the fuel value of a dish or of a meal, and to plan a meal or a dietary for a day or a week that shall provide one person or a family with the number of calories to meet its needs. Of course, people of different ages and degrees of activity require different numbers of calories, a grown person more than a child, but a child more in proportion to its weight than a grown person. These varying requirements have been worked out and tabulated.