644 PHYSIOLOGICAL CHEMISTRY. 



to 2 pounds of bread, 10 to 14 ounces of lean beef, 2 to 3 ounces of 

 butter, with 2 quarts of water. 



Digestibility. In providing a diet, it must be borne in mind that 

 the digestibility of a food is more a measure of its nutritive value than 

 its elementary composition. Different foods show great differences 

 in the rapidity and completeness with which they are absorbed. 

 Thus eggs, fresh meat, white bread, and butter are absorbed and 

 assimilated more readily than pork, rye bread, potatoes, green vege- 

 tables, and bacon. 



By digestibility of food many different conditions are, or may be, implied. 

 Some of these, as the ease with which a certain food is digested, the time re- 

 quired for the process, the influence of different substances and conditions on 

 digestion, and the effects on health and comfort, are so dependent upon indi- 

 vidual peculiarities, that no definite rule for the measurement of food-digesti- 

 bility can be established. Fortunately, the most important factor, viz., the 

 amount digested, can be determined accurately by experiment. The method 

 consists in analyzing and weighing both the food consumed and the feces 

 excreted, the difference being taken as the amount digested. 



In general it can be said that animal protein is easily and completely di- 

 gested, while protein of vegetable food is less so. Thus, of the protein contained 

 in potatoes, whole wheat, and rye flour, one-fourth, or more, may escape diges- 

 tion, and thus be rendered useless as nourishment. About 5 per cent, of fats 

 escape digestion, while carbohydrates are, in general, completely digested, 

 cellulose being the only exception. 



In adjusting a diet, it is important to provide sufficient protein for the build- 

 ing and repair of tissue, and enough of other materials to furnish the body 

 with heat and energy for the work to be performed. A proper diet for a grown 

 person doing moderate work should provide about 3500 calories of energy with 

 a nutritive ratio of from 1 : 4 to 1 : 6. 



The nutritive ratio is the ratio of the protein to the sum of all the other 

 nutritive ingredients. The fuel value of fats is two and a quarter times that 

 of proteins and carbohydrates, that of the two latter being considered to be 

 alike. In calculating the nutritive ratio, the quantity of fats is multiplied by 

 2.25, and the product added to the weight of carbohydrates. The sum divided 

 by the weight of protein gives the nutritive ratio. 



If less protein be administered than is needed for repair, although a sufficient 

 number of calories be provided, more nitrogen will be excreted in the urine 

 than is contained in the food. When the protein is given in sufficient quantity 

 to replace the worn tissues, sufficient calories being also provided for, a nitroge- 

 nous equilibrium is established i. e., the nitrogen excreted equals the nitrogen 

 administered. Should more protein than is necessary be administered, with 

 sufficient calories, then more nitrogen is excreted and thereby the equilibrium, 

 as far as nitrogen is concerned, is rapidly re-established. 



During the periods of growth and convalescence from acute disease the pro- 

 teins can be increased in the body by increase of protein food. The nitrogenous 

 equilibrium is then less rapidly re-established, as nitrogenous matter is utilized 



