376 HUMAN PHYSIOLOGY 



sparers. Their use in diminishing the consumption of pro- 

 teids is, however, strictly limited. 



Fat Diet. Fats, like carbohydrates, will not support life, 

 because they cannot be used for building up protoplasm, but, 

 like carbohydrates, they are a source of energy, and they have 

 more than twice the energy value of proteids or of carbo- 

 hydrates (p. 315). They are thus proteid sparers. But 

 experiment has shown that, in spite of their higher energy 

 value, they have not the same power as carbohydrates of 

 sparing proteids, since greater work is required in their 

 digestion and absorption. 



A knowledge of the part played by proteids, carbohydrates, 

 and fats in the animal body is the groundwork of the study 

 of Dietetics. 



DIETETICS. 



The great essentials of a diet capable of maintaining health 

 are : 



1st. That it should supply the energy required. 



2nd. That it should contain sufficient proteids to make 

 good the waste of these substances. 



3rd. That it should be capable of digestion, absorption, 

 and assimilation. 



I. The energy requirements vary with the mode of life and 

 with the age and size of the individual. 



Size. Other things being equal, a large man requires more 

 energy and more proteid than a small man. For this reason 

 the energy requirements are sometimes stated as per unit of 

 weight, but it is more convenient to take as the standard an 

 adult man of average weight, say 65 kgs. It must further 

 be remembered that the smaller the animal the greater the 

 surface in proportion to its weight ; and hence the greater 

 the loss of heat per unit of weight. For this reason alone 

 small animals and children require more energy per unit of 

 weight than larger animals or older people. 



Age. In children the metabolism is more active than in 

 adults. They are more constantly in motion, and they re- 

 quire energy and material to build up their tissues, and, as 



