Muscular work is much the most important of the factors which raise the food requirements of adults above the basal rate necessary for mere maintenance.

Accurate measurements by means of the calorimeter have shown that the average total metabolism of a man sitting still is about 100 Calories per hour; while the same man working actively increases his metabolism up to about 300 Calories per hour; and a well-trained man working at about his maximum capacity metabolizes material enough to liberate 600 Calories per hour, i.e. his metabolism may be six times as active during the hours actually spent in such work as when he is at rest. If during 24 hours a man works as hard as this for 8 hours and spends 2 hours in such light exercise as going to and from work, his food requirement for the day will be somewhat over 6000 Calories, or three times the maintenance requirement. Thus, work may increase the day's metabolism as much as 200 per cent, whereas liberal feeding at the end of a fast was found to increase the metabolism only 22.5 per cent, or one ninth as much. Only a few exceptional occupations, such as that of lumbermen, for example, involve such heavy work as to cause a metabolism of 6000 Calories per day. More often the man who works 8 hours a day at manual labor will increase his metabolism by 1000 to 2000 Calories above what is needed for maintenance at rest, making his total food requirement 3000 to 4000 Calories.

1 Journal of Biological Chemistry, Vol. 18, page 141 (July, 1914).

* Proceedings of the National Academy of Sciences, Vol. 1, pages 105-109.

Voit estimated the food requirement of a "moderate worker" at about 3050 Calories; and Atwater, in adapting this standard to American conditions, increased the allowance to 3400 - 3500 Calories in the belief that the American works more rapidly and therefore with a greater expenditure of energy. The mistake is often made of supposing that these estimates were intended for every one who leads an active life, whereas they really contemplate a long day of manual labor, for Voit's definition of "moderate worker" was a man laboring 9 or 10 hours a day at an occupation such as that of a carpenter, mason, or joiner.

The amount of energy spent during 24 hours by a sedentary worker will depend not only upon the number of hours which he devotes to exercise, but especially upon the kind of exercise chosen. Lusk estimates that an average-sized man sleeping 8 hours, sitting 14 hours, and walking 2 hours spends about 2500 Calories; whereas if he spends 2 hours in vigorous exercise instead of walking, his total energy output rises to about 3000 Calories.

The importance of muscular activity as the chief factor governing the energy expenditure and food requirement of healthy adults calls for a careful quantitative study of its effect upon metabolism.