NUTRITION 117 



while carbohydrates furnish energy for work, but there is, in 

 this respect', no such fundamental difference between them. 



As nutrition on an exclusively proteid diet is at least 

 theoretically possible, we can express the amount of energy- 

 yielding foodstuffs necessary for .nutrition in terms of the 

 amount of proteids needed. For a man of 70 kg body 

 weight, the amount of proteids must be about 700 grams a 

 day. 



A part of this food must be proteid, it being absolutely 

 necessary. This amounts to about 70 g per day. It has 

 been observed that the body can get along with a much 

 smaller quantity of proteids (40 g per day), but such experi- 

 ments only lasted for a very short time, hence it is a ques- 

 tion whether the body can be nourished for a great length 

 of time with such a small amount of proteid. 



After we have supplied the absolutely necessary proteid, 

 the remainder of the food needed can be taken in the form 

 of proteid, gelatin, fat, or carbohydrate or as a mixture of 

 these substances. The proportion in which the foods can, 

 in this case, replace each other is based upon the law of 

 isodynamics, which states that such quantities of combustible 

 foodstuffs as furnish the same amount of energy during their 

 physiological combustion are equivalent (see Chapter XIII). 

 In round numbers the following quantities furnish the| 

 same amount of energy: 2.3 g proteid = I g fat = 2.3 gn 

 carbohydrates. 



For an exact application of the law of isodynamics in the prac- 

 tical study of nutrition it must be borne in mind that, in meta- 

 bolism, the proteids behave differently from the fats and 

 carbohydrates (see Chapter XII). 



As a diet, the following is necessary: 



Proteid. Fat. Carbohydrates. 



Resting man 100 g 60 g 400 g 



" woman 90 g 40 g 350 g 



Working man 130 g 100 g 500 g 



The absolute amount needed by old people and children 

 is less. But if the food necessary for each kilogram of body 



