A Civic Biology, Presented in Problems

by George W. Hunter

Phosphorus is a necessary substance for the making of living matter, milk, eggs, meat, whole wheat, and dried peas and beans containing small amounts of it. Iron also is an extremely important mineral, for it is used in the building of red blood cells. Meats, eggs, peas and beans, spinach, and prunes, are foods containing some iron.

Some other salts, compounds of calcium, magnesium, potassium, and phosphorus, have been recently found to aid the body in many of its most important functions. The beating of the heart, the contraction of muscles, and the ability of the nerves to do their work appear to be due to the presence of minute quantities of these salts in the body.

Uses of Nutrients.—The following table sums up the uses of nutrients to man:[36]—

Protein	Forms tissue
White of eggs (albumen), curd of milk (casein), leanmeat, gluten of wheat, etc.	(muscles, tendon, and probably fat)
Fats	Form fatty tissue.
Fat of meat, butter, olive oil, oils of corn andwheat, etc.		All serve as fuel and yield energy in formof heat and muscular strength.
Carbohydrates	Transformed into fat.
Sugar, starch, etc.
Mineral matters (ash)	Aid in forming bone,
Phosphates of lime, potash, soda, etc.	assist in digestion, aid in absorption and in other wayshelp the body parts do their work.
Water used as a vehicle to carrynutrients, and enters into the composition of living matter.

Common Foods contain the Nutrients.—We have already found in our plant study that various plant foods are rich in different nutrients, carbohydrates forming the chief nutrient in the foods we call cereals, breads, cake, fleshy fruits, sugars, jellies, and the like. Fats and oils are most largely found in nuts and some grains. Animal foods are our chief supply of protein. White of egg and lean meat are almost pure protein and water. Proteins are most abundant, as we should expect, in those plants which are richly supplied with nitrogen; peas and beans, and in grains and nuts. Fats, which are melted into oils at the temperature of the body, are represented by the fat in meats, bacon, pork, lard, butter, and vegetable oils.

Water.—Water is, as we have seen, a valuable part of food. It makes up a very high percentage of fresh fruits and vegetables; it is also present in milk and eggs, less abundant in meats and fish, and is lowest in dried foods and nuts. The amount of water in a given food is often a decided factor in the cost of the given food, as can easily be seen by reference to the chart on page [283].

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.

Refuse.—Some foods bought in the market may contain a certain unusable portion. This we call refuse. Examples of refuse are bones in meat, shells of eggs or of shellfish, the covering of plant cells which form the skins of potatoes or other vegetables. The amount of refuse present also plays an important part in the values of foods for the table. The table[37] on page [276] gives the percentages of organic nutrients, water, and refuse present in some common foods.

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]