Perfectly trustworthy evidence is at hand showing that the needs of the body for potential energy can be fully met, and indeed are more advantageously met, by the non-nitrogenous foods, carbohydrates and fats. The energy of muscle work, as we have seen, comes preferably from the breaking down of non-nitrogenous material, so that there is no special call for proteid in connection with increased muscular activity. In fact, it would appear that the need for proteid food by man is limited to the requirements of growth and development, reinforced by the amount called for in that form of tissue exchange which we have emphasized under the term “endogenous proteid metabolism,” or true tissue metabolism. To be sure, there must be a certain reserve of proteid, available in case of emergency, but this is easily established without resorting to excessive feeding. The peculiar position which proteid foods occupy in man’s dietary naturally make them the central figure, around which the other foods are grouped. No other form of food can take the place of proteid; a certain amount is needed each day to make good the loss of tissue material broken down in endogenous katabolism, and consequently our choice and combination of the varied articles of diet made use of should be regulated by the amount of proteid they contain. But while proteid foods occupy this commanding position, it is not necessary or desirable that they should exceed the other foodstuffs in amount, or indeed approach them in quantity. We must be ever mindful of the fact, so many times expressed, that proteid does not undergo complete oxidation in the body to simple gaseous products like the non-nitrogenous foods, but that there is left behind a residue of non-combustible matter--solid oxidation products--which are not so easily disposed of. In the forceful language of another, “The combustion of proteid within the organism yields a solid ash which must be raked down by the liver and thrown out by the kidneys. Now when this task gets to be over-laborious, the laborers are likely to go on strike. The grate, then, is not properly raked; clinkers form, and slowly the smothered fire glows dull and dies” (Curtis).