[Illustration: Table of food values. Determine the percentage of water in codfish, loin of beef, milk, potatoes. Percentage of refuse in leg of mutton, codfish, eggs, and potatoes. What _is_ the refuse in each case? Find three foods containing a high percentage of protein; of fat; of carbohydrate. Find some food in which the proportions of protein, fat, and carbohydrate are combined in a good proportion.] Fuel Values of Nutrients.--In experiments performed by Professor Atwater and others, and in the appended tables, the value of food as a source of energy is stated in heat units called _Calories_. _A Calorie is the amount of heat required to raise the temperature of one kilogram of water from zero to one degree Centigrade._ This is about equivalent to raising one pound four degrees Fahrenheit. The fuel value of different foods may be computed in a definite manner. This is done by burning a given portion of a food (say one gram) in the apparatus known as a _calorimeter_. By this means may be determined the number of degrees the temperature of a given amount of water is raised during the process of burning. It has thus been found that a gram of fat will liberate 9.3 Calories of heat, while a gram of starch or sugar only about 4 Calories. The burning value of fat is, therefore, over twice that of carbohydrates. In a similar manner protein has been shown to have about the same fuel value as carbohydrates, _i.e._ 4 Calories to a gram.[38] Footnote 37 and 38: W. O. Atwater, _Principles of Nutrition and Nutritive Value of Food_, U. S. Department of Agriculture, 1902. The Relation of Work to Diet.--It has been shown experimentally that a man doing hard, muscular work needs more food than a person doing light work. The mere exercise gives the individual a hearty appetite; he eats more and needs more of all kinds of food than a man or boy doing light work. Especially is it true that the person of sedentary habits, who does brain work, should be careful to eat less food and food that will digest easily. His protein food should also be reduced. Rich or hearty foods may be left for the man who is doing hard manual labor out of doors, for any extra work put on the digestive organs takes away just so much from the ability of the brain to do its work. [Illustration: Foods of plant origin. Select 5 foods containing a high percentage of protein, 5 with a high percentage of carbohydrates, 5 with a high percentage of water. Do vegetable foods contain much fat? Which of the above-mentioned foods have the highest burning value?] [Illustration: Foods largely of animal origin. Compare with the previous chart with reference to amount of protein, carbohydrate, and fat in foods. Compare the burning value of plant and animal foods. Compare the relative percentage of water in both kinds of foods.] [Illustration: The composition of milk.]