500 APPLIED BIOLOGY 



for possible future use, and (3) that if excess protein goes 

 to cells they use the necessary amount for making new pro- 

 toplasm and disintegrate into excretions the remainder, 

 which may serve as a source of energy. In other words, 

 at least one-half of 200 grams of protein might furnish heat 

 or muscular energy as other foods do. 



423. Why limit Protein Food? Since protein is neces- 

 sary for repair and growth, and may also supply energy, we 

 may properly ask why physiologists so often recommend a 

 limited protein diet, e.g., 60 grams per day. There are 

 several answers : 



(1) Protein as a source of energy is physiologically waste- 

 ful, for the human body can obtain only about 4 calories 

 per gram, which in the chemist's calorimeter shows 5.6 

 calories. This is because nitrogen excretion leaves the body 

 incompletely oxidized. 



(2) Protein for energy-supply forces the body to handle 

 the useless nitrogen which it contains, for energy comes from 

 oxidation of the carbon and hydrogen. A hard-working man 

 getting about 2880 calories from a daily diet of 120 grams of 

 protein, 100 of fat, and 375 of carbohydrates would have to 

 eat about 720 grams of protein daily to get the same amount 

 of energy if he took food other than clear lean meat. This 

 would require the kidneys to excrete 200 extra grams of 

 urea which would come from the nitrogen content of the 

 extra 600 grams of protein. Such an excess of nitrogen 

 excretions is injurious and tends to cause disease of kidneys 

 and other organs. Hence it is better to get the necessary 

 energy from 100 grams of fats and 375 of carbohydrates 

 (100 X 9 + 375 X 4 = 2400 calories) rather than from 600 of 

 extra protein (600 X 4 = 2400 calories). 



(3) Protein is not economical in a pecuniary sense. It 

 costs much more than common carbohydrates (starch in 

 vegetables, or even sugar) which have the same energy value, 

 namely, 4 calories per gram ; and clear lean meat is much more 



