Much stress is ordinarily laid upon the importance of a large intake of proteid food whenever the body is called upon to perform severe, or long-continued, muscular work; but in view of what has been stated it may be questioned whether there is any real physiological justification for such custom. The pedestrian Weston,1 who in 1884 walked 50 miles a day for 100 consecutive days, was found by Blyth during a period of five days to consume in his food 37.2 grams of nitrogen a day, while he excreted only 35.3 grams, leaving a balance of 1.9 grams of nitrogen per day apparently stored in the body. His daily food during this period was composed of 262 grams of proteid, 64.6 grams of fat, and 799 grams of carbohydrate, with an estimated fuel value of 4850 calories. Yet he performed this large amount of work daily, and still laid by a certain amount of proteid on a ration, the energy value of which would not ordinarily be considered high for the muscular work to be done. Fourteen years prior to this, Weston, while in New York, was carefully studied by Dr. Flint during a period of 15 days, on 5 of which he walked a total of 317 miles. His diet was essentially a proteid diet, consisting principally of beef extract, oatmeal gruel, and raw eggs. Nitrogen intake and output were carefully compared during the days of rest and during the days of work, with the results tabulated.

1 This and the following account of Weston are taken from Bulletin No. 98, U. 8. Department of Agriculture, Office of Experiment Stations. The effect of severe and prolonged muscular work on food consumption, digestion, and metabolism. By W. O. Atwater and H. C. Sherman, p. 18.

In this case it will be noted that the daily ration was comparatively small, and, further, that during the working period the subject consumed much less proteid than on the resting days. Moreover, when we remember that the total energy value of his diet must have been quite small, it is not at all strange that in the laborious task of walking 62 miles a day he should have temporarily drawn upon his store of body proteid to the extent of 62.5 grams, or 10 grams of nitrogen a day. Such experiences, however, do not by any means constitute proof that in excessive muscular work there is need for the consumption of correspondingly increased quantities of proteid food, or that the energy of muscular work comes preferably from the breaking down of proteid material. Carbohydrate and fat unquestionably take precedence over pmtcid in this respect, and we tuny accept, us settled the view that in all practical ways carbohydrate and fat stand on an equal footing as sources of muscular energy. Less clear, perhaps, is the question as to how these two radically different types of organic material are utilized by the muscle. It has been a favorite belief among some physiologists that the contracting muscle makes use of only one substance as the direct source of its energy, and that this substance is the sugar dextrose. This view would seemingly imply that fat and proteid -must undergo alteration prior to their utilization by the muscle; that, possibly, the carbon of the fat and proteid is transformed into sugar before the muscle can make use of it. So far as fat is concerned, this view is not supported by the facts available, since experiments show that the heat and energy liberated in the utilization of a given amount of fat in muscle work are in harmony with the energy value of the fat; in other words, the fat is apparently burned,, or oxidized, directly, without undergoing previous transformation into any form of carbohydrate; or, if transformation does occur, under some conditions, it must take place within the muscle and without loss of energy. The practical significance of these facts is at once apparent, for if fat, in order to he available as a source of muscle energy, must first undergo conversion into sugar, it would be far more economical from a physiological standpoint to replace the fat of the diet with carbohydrate in any attempt to provide suitable nourishment for the working muscle. We may safely conclude, however, that fat and carbohydrate, as previously suggested, are in reality both capable of direct metabolism by the muscular tissue, and that each is of value as a source of muscular energy in proportion to its heat of combustion, yielding substantially the same proportion of its potential energy in the form of mechanical work.

Regarding the utilization of proteid as a source of energy by the muscle, there are many grounds for believing that here the body has to deal with certain alterations, before the proteid can be made available. We may indeed conjecture the transformation of a non-nitrogenous portion of the proteid molecule into carbohydrate as a necessary step in its utilization for muscle work. It is certainly true that in the ordinary katabolic processes, through which proteid passes, there is a tendency for the nitrogen-containing portion to be quickly split, off and eliminated, leaving a carbonaceous residue which may represent as much as SO per cent of the total energy of the original proteid. This so-called carbon moiety of the proteid molecule is apparently much less rapidly oxidized than the nitrogenous portion, and may indeed bo temporarily stored in the body, in the form of fat or carbohydrate.1

1 See Leo Laogitein: Die Kohlehydratbildung nus Eiweiss. Ergebninc der Physiologie, Band 3. Enter Theil, p. 460.

See also, Luthje: Zur Frage der Zuckerbildung aua Eiwcibb. Archiv furd. geaammte Phynulogie, Band 106, p. 160.