"Looked at in the manner above represented, muscular work, like heat, in opposition to Liebig's theory, is derivable from the oxidation of non-nitrogenous as well as nitrogenous matter, and Professor Frankland's tables show that 55 lbs. of fatty matter will furnish the same amount of power as is obtainable from 13 lbs. of flour, 15 lbs. of sugar, 35 lbs. of lean beef, and 5 lbs. of potatoes. Traube even inverted the proposition of Liebig, and asserted in the most decided manner that the substances by the oxidation of which force is generated in the muscles are not the albuminous constituents of the tissues, but non-nitrogenous principles, viz., either fats or carbo-hydrates.

"According to the foregoing table, wherein is mentioned the cost of the various articles of food required to be consumed to accomplish a given amount of work, it appears - viewing these articles purely in their capacity as force-producing agents by oxidation - that the same amount of work is obtainable from oatmeal costing 3 1/2d.; flour, 3 3/4d.; bread, 4 3/4d.; and beef fat, 5 1/2d.; as from beef costing 3s. 6 1/2d. and isinglass, £1 2s. 0 1/2d.

Taking all the facts as at present revealed into consideration, we appear to be warranted in adopting the following terms of expression. It is in the first place admitted on all hands that food is the source from which muscular power is derived, and hence the supply of food should be in proportion to the amount of work that is performed. It was formerly thought that food must be converted into muscular tissue before it could be available for the performance of work, which involved the origin of work from nitrogenous alimentary matter. The effect of recent investigation, however, is to show that it is not to an oxidation of muscular tissue that we are to look for the force produced. The muscles appear to stand in the position of instruments for effecting the conversion of the chemical energy evolved by the oxidation of combustible matter into working power. Fats and carbo-hydrates can furnish the combustible matter required, and, under ordinary circumstances, probably do largely, if not chiefly, supply it. Nitrogenous matter can do so likewise, but it has to undergo a preparatory metamorphosis for effecting the separation of nitrogen in a suitable form for elimination.

As thus considered the non-nitrogenous alimentary principles appear to possess a higher dietetic value than the nitrogenous, and when regarded solely in relation to capacity for force production, there is no doubt they in reality do so. But there is a further point to be looked at. The physical development and maintenance of the body must be likewise taken into account, and for this it is nitrogenous alimentary matter only that can supply what is needed. Wherever vital operations are going on there exists the need for nitrogenous matter. It is, indeed, through the instrumentality of nitrogenous matter that the operations of life occur. The tissues which form the instrument of living action require to be constructed, in the first instance, and next, to be constantly renovated, to compensate for the loss by deterioration which is continually going on. Thus, a demand for nitrogenous alimentary matter is created quite apart from direct contribution to force production; and, further, not only is nitrogenous matter required for the construction and repair of the tissues, but likewise to form a constituent of the secretions, for all secretions which possess active properties owe them to the presence of a nitrogenous principle. Here then is an additional demand for nitrogenous matter, and it is to be remarked that as increased work leads to an increased development of the tissues employed and thereby an increased appropriation of nitrogenous matter, so it calls for an increased production of secretions in consequence of the larger amount of food that has to be prepared for consumption. In this way, theoretically, without contributing in a direct manner to force production, the performance of work may be looked upon as necessitating a proportionate supply of nitrogenous alimentary matter.

Practically, it is found that hard work is best performed under a liberal supply of nitrogen-containing food. The reason probably is that it leads to a better nourished condition of the muscles and of the body generally. Under the use of animal food, which is characterised by its richness in nitrogenous matter, the muscles, it is affirmed, are observed to be firmer and richer in solid constituents than under subsistence upon food of a vegetable nature.....As albuminous food produces firm muscles, so exercise makes them red.

To sum up - science intimates that a liberal supply of nitrogenous matter is necessary to produce and maintain muscles in a good condition for work, and - the result of experience is to confirm it.

I have been speaking of food considered in relation to the performance of work, but it would be unphilosophical to look at it only in this light. The question should be viewed under a broader aspect; and the point really for the physiologist to discuss is under what combination of alimentary principles the highest state of development, both mental and physical, is attainable. If regarded as living for the mere performance of work, and looked at economically, man, it may be said, would bear an unfavourable comparison with a machine set in motion by steam. Mechanical work is under no form so costly as under that produced by muscular agency, and particularly by that of man. It has been calculated, it is true, that whilst through the medium of the animal system, one-fifth of the power stored up in the food consumed is realisable as external mechanical work, the amount realisable from fuel is only one-tenth in the case of even the best constructed steam-engine, the remainder being dissipated or lost as heat. Thus far, the animal machine is more economical of its force than the machine of artificial construction; but on the other hand, the fuel (food) consumed in the former is very much more costly than that consumed in the latter. From this consideration human labour can never compete in economy with steam, and hence, as suggested by Donders, the worse use to make of a man is to employ him exclusively in mechanical work - a proposition which harmonises with the increasing introduction of machinery in our advancing age of civilisation.

Letheby (Cantor Lectures on Food) on the subject of the comparative costliness of food and fuel, says, 'Taking a steam engine of one horse power (that is a power of raising 33,000 lbs. a foot high per minute) it will require two horses in reality to do the same work for ten hours a day, or twenty-four men; and the cost would be l0d. for the steam engine, 8s. 4d. for the two horses, and just £2 sterling for the twenty-four men.'

From what has preceded we may conclude that, with a supply of nitrogenous matter sufficient for the thorough development and subsequent maintenance of the body in good condition, the best materials for the production of working power, as well as heat, are the non-nitrogenous principles, and that of these the fats are more effective than the others." ("A Treatise on Food," etc., by F. W. Pavy, M.D., 1874.)