DIETETICS 157 



waste. The tissues which during the period of scarcity had 

 reduced their oxidation to a minimum become more active at 

 the first hint of returning plenty. 



This last experiment illustrates a general law. An increase 

 of proteid food within certain limits increases the metabolic 

 activity of the tissues provokes them to extravagance. It 

 is possible, by adding protein to a mixed diet which sufficed 

 for the maintenance of body-weight and nitrogenous equili- 

 brium, to bring about a nitrogen deficit and to reduce the 

 body-weight. Or, if the body is gaining in weight, owing to 

 the accumulation of fat, the substitution of protein for carbo- 

 hydrate (weight for weight, since their caloric values are the 

 same) will lead to its reduction. It is difficult to avoid the use 

 of fanciful language in accounting for these results. The 

 animal economy is like an over-careful housekeeper, who, when 

 meat is scarce, doles out porridge also with a thrifty hand. 

 When meat is plentiful she is prodigal with every article of 

 diet. Protein is the most costly of foods. Any indication 

 that it is scarce leads to a shutting-down of activity. On the 

 other hand, no other food is so readily absorbed (unless the 

 digestive organs be protein-sick) ; none is so quickly incor- 

 porated in the bioplasm ; none is so easy to decompose. When 

 fed with protein the machinery hums. The insatiable appe- 

 tite for beef and eggs which overtakes a man of sedentary 

 habits after a long morning in a boat or on a bicycle does not 

 indicate that his muscular tissue is suffering from wear and 

 tear. It does not prove that he is setting free energy by 

 oxidizing proteid food. It shows that he is asking certain 

 tissues which are accustomed to a quiet life to exhibit pro- 

 digious energy. They will not shake off their customary sloth 

 unless he stimulates them with sumptuous fare. At the end 

 of a week he finds that proteins are not the best fuel for steady 

 work. If he consumes sufficient to supply all the energy 

 needed by his muscles, he is hampered by a quantity of nitro- 

 genous residues which have to be reduced to urea and elimi- 

 nated by the kidneys. He goes back approximately to his 

 old regimen, so far as proteins are concerned, and consumes 

 more carbohydrates for the supply of the force which his 

 increased muscular activity demands. 



It is possible to live on meat alone, but the quantity required 



