THE CHEMICAL COMPOSITION OF FOODS 11 



repeated contraction and continued work the muscle 

 tissue, itself protein, gets worn away, and more protein 

 has to be extracted from the food to be actually built up 

 into the muscle tissue. In other words, protein, sugar, 

 and fat from the food can all act as fuel for the muscle 

 engine, but only protein can be used to repair and rebuild 

 worn parts. Thus the body demands a certain amount 

 of protein, which amount is replaceable by no other food 

 substances. Moreover, in a young growing animal, where 

 muscle formation is rapidly going on, the proportion of 

 protein to the whole diet needs to be much greater than 

 at any other time. Similarly, horses doing the fastest 

 work with the greatest taxing of the muscles require a 

 higher proportion of protein than those on slow work. 

 For these reasons, proteins are the most important, but, 

 at the same time, the most costly constituents of a diet. 

 The Amides are generally incapable of rebuilding 

 muscular tissue, but act as heat-producers, and are 

 therefore in line with the other foods — carbo-hydrates 

 and fats — in that they supply a certain amount of energy 

 after assimilation. They are made up of the four 

 elements, carbon, hydrogen, oxygen, and nitrogen, but 

 differ from proteins in that they contain no sulphur or 

 phosphorus, and that their constituent elements are 

 united in simpler combination than in the proteins. 

 Amides are formed in the plant substance as a stage 

 in the manufacture of proteins from simpler inorganic 

 substances. Thus young growing plants contain a larger 

 proportion of their nitrogenous matter in the form of 

 amides than mature ripe plants, a fact well illustrated 

 by the following analyses of hay. These further show 

 the importance, in an accurate calculation of the nutri- 

 tive value of certain foods, of estimating the nitrogenous 



