We may say then that experimental investigation in a calorimeter, and experience, teach that, as we should expect from the principle of the conservation of energy, the muscular work done by any individual cannot be increased over any long period without an increase of food, and that the amount of food added must always be very much greater than the heat equivalent of the actual work done. In ordinary individuals about one-fifth of the added food will be furnished as work, the rest being dissipated as heat; under special circumstances probably a greater efficiency may be attained. When work is carried on under adverse conditions of climate and is exhausting, so that tired muscles are called upon to contract, no doubt the efficiency is much lowered, and this is probably why such an extraordinary supply of food was consumed by the lumbermen. Whatever may be said later about the need for protein, there can be no question as to the greater caloric requirements of workers. This is recognized in the regulations for the army rations of various nations in peace and in war, as is shown in the following table. We must remember, however, that exposure to cold, as on active service, is an additional factor calling for an increase in the food supply. It will be seen that the peace ration is raised by at least 1,000 calories in war time.
Great Britain -
1Actual diet obtained....
Revue de Science de l'Entendance Militaire, Aug., 1907, vol. 20.
We have seen above that the requirements of a moderately active man may be taken as 3,000 calories. The usual rations in the navy slightly exceed this. In severe weather, or on night duty, the value is raised to about 4,400 calories. It appears then that ordinary severe labour is met by the addition of about 1,000 calories to the diet. This amount is contained in | lb. of bread and 2 oz. of meat, or in 1/2 lb. of bread and 3 oz. of cheese. When an extraordinary amount of work and endurance is required, the caloric value of the food must be further increased.
We have seen that in cold climates, owing to a greater loss of heat, more food will be necessary, other things being equal, than in the temperate zone, and in the temperate zone more than in the tropics. The difference, however, in civilized races under ordinary conditions is far less than might be expected, and this is because the heating of houses indoors and the thickness of the clothing out of doors is so arranged that most of the surface of the body is kept at about the same temperature, so that as Rubner has said, we all live in a temperature of about 90° Fahr., which is that of the air between the skin and the clothes. In very hot weather the appetite diminishes and less food is taken, but weight is often lost, showing that the needs of the body are not entirely met by the food. The performance of work needs the same supply of food in hot and cold climates. When the worker is so exposed to cold, or to wet and cold, that the clothes cannot prevent great loss of heat from the skin, then much more food will be required. This is a partial explanation of the large quantities of food, amounting to 7,000 calories, taken by the Maine lumbermen referred to on p. 135. In arctic regions those foods which have the greatest heat value, that is, the fats, are . especially serviceable.
1 Calculated by Pembrey and Parker.