REAL NUTRITIVE VALUE OF FOODSTUFIS 35 



Thus complete oxidation or combustion of the substance 

 is brought about, and the heat given off by a known 

 quantity of it raises the temperature of the water, and 

 so is determined. It is found by this means that equal 

 quantities of fat, protein, and carbo-hydrate, on com- 

 bustion produce heat in the proportion of 2*3, 1*25, and 

 1, respectively. Proteid matter really produces more 

 heat than this in a calorimeter, but a deduction has 

 been made equivalent to the heat-producing power of the 

 urea excreted. In practice these comparative heat values 

 are found to be true so far as the maintenance diet of 

 an animal is concerned, but for labour purposes proteid 

 and carbo-hydrate are found to be of similar value, so 

 that the proportion becomes 2*3 : 1 : 1. 



It has already been explained (Chapter 11.) that 

 amides, although allied to proteins in composition, are 

 unable to build up the nitrogenous tissues of the body, 

 but can supply heat and energy on oxidation. Experi- 

 ments have determined their heat value to be 06 when 

 compared with a value of 1 for starch, so that in a 

 food containing a large percentage of amides this heat 

 value must be added to that of the other heat-forming 

 constituents. 



The digestible part of the '' crude fibre " is really 

 cellulose, and has a heat value practically the same as 

 starch. From this, however, a deduction has to be 

 made for reasons stated below (p. 36). 



But there is another factor of supreme importance to 

 consider before we can really fix the value of a food — 

 namely, its dig-estibility. We have already seen that 

 in the process of digestion, protein, fats, and carbo- 

 hydrates, are dealt with separately, and in any one food 

 they are digested in quite different proportions. Thus 



