Even in adults the extent of surface should be considered with the diet. Stout people have a relatively smaller surface than thin, and this is one reason why they often have a smaller appetite. Another reason is that fat is an inert tissue and is not undergoing active metabolism and does not need nourishing as do the active tissues. A fat man, therefore, having relatively a smaller amount of active tissue, and a smaller loss of heat from the surface than a thin man, requires less food for each pound of weight.
Growing animals naturally require more food to form the tissue which is being developed from day to day. We have already mentioned that the younger the animal the greater is the rapidity of growth. Heubner found that the rate of growth in a baby in the first three months of life was seven times that from the sixth to the ninth month. No weight is added unless the food supplied is above a certain amount, which is seldom less than 70 calories per kilogramme, or 1 1/2 oz. of cow's milk to the pound per day, and is, as stated in the last section, usually much more. It is true that instances have been recorded in which infants have grown naturally with this supply, but they are uncommon. The excess of food above this figure must be greater than the weight to be put on, for the actual amounts of food retained each day form only a small proportion of the extra food taken.
In adults increasing in weight there is a definite limit to the amount which can be laid on daily, with the exception that after long periods of fasting and in convalescence from wasting diseases a more rapid increase may be for a time maintained.
In the ordinary life of adults the amount of food needed depends more closely upon the muscular activity than upon any other factor. Certain muscles are always active, such as those of the circulatory and respiratory apparatus; the tone of the voluntary muscles is being constantly maintained, except in sleep, and during digestion the muscles as well as the glands of the alimentary canal are working; all this work is usually designated as "internal work," and it is essential; a man lying in bed upon a minimum diet would expend the energy of his food upon the performance of these functions, and the heat manifested would be employed in maintaining his temperature. Individuals who are moving about in the ordinary course of life and employing their skeletal muscles naturally use more energy and require more food. Thus Atwater found in his admirable observations that a subject confined in the small calorimetric chamber, even when not doing any definite work, used 80 per cent more energy than when lying asleep; a demonstration in figures of the recuperative powers of sleep. Slight unnoticed movements are responsible for a considerable expenditure of energy; it has been found by Johannsen that by sitting perfectly still and controlling such movements the oxidation as measured by the carbon dioxide expired could be reduced by nearly a third. The taking of food itself causes a definite expenditure on the part of the glands and muscles of digestion. In a resting man it has been found that the excretion of carbon dioxide is over 20 per cent greater when food is taken than upon a fasting day. That this is due to gastric and intestinal activity is supported by the observation that a similar increase follows the administration of purgatives. A meal of protein is followed by a greater and more prolonged rise of oxidation than one of fat or carbo-hydrate, and this is in accordance with the fact that protein takes longer to digest.
A much greater increase in the oxidation processes is caused by the exercise of the voluntary muscles and is followed by a corresponding demand for more food. The relation of work to the diet of man is of great practical importance, and many experimental studies have been made upon it. We shall here discuss the subject so far as it bears upon the total amount of food, and shall have occasion later to consider the constituents of which this food should be composed. It is clear that unless the tissues are to be drawn upon additional energy must be furnished to supply that which is manifested as mechanical work. In Atwater's experiments a man working a stationary bicycle against resistance evolved as much as 1,000 calories in a couple of hours. The same subject working moderately hard evolved 500 calories in that time, and when resting in the night 150 only. The point of importance in fixing a dietary for stringent conditions, as for soldiers on active service, is to determine how much more food need be given. The following figures compiled from bulletins 98 (1901) and 149 (1904) issued from the Office of Experiment Stations of the United States Department of Agriculture, shows the heat value taken in the food by various men doing severe muscular work.
Cals. per day.
Brickmakers, New England.....
Lumbermen, Maine, U.S.A., working in winter and sometimes in cold water.
Bicyclist (Albert) in six days' endurance contest......
Bicyclist (Miller) in six days' endurance contest (winner)....
Ordinary standard for man with light to moerate work....
Ordinary standard for man with sedentary occupation.....
The amounts of food taken by some of these workers are enormous. We cannot, however, conclude that they are excessive unless the food were not absorbed, or the subjects were putting on weight, and neither in the Maine lumbermen nor in the bicyclists was this the case. These quantities are far in excess of those required by ordinary workmen, and they show strikingly how extraordinary demands must be met by extraordinary supplies. It is surprising that the bicyclists Albert and Miller, who rode for six days an average of 18 1/2 and 20 hours respectively, did not take more food, but their nitrogen balance showed that there was a considerable daily loss of protein, and it is probable that their stores of body-fat were called upon. That the bicyclists and the rowing men should come low down on the list may be partly due to the fact that their work was done in spells, the body recuperating in between; it also suggests that the body may be able to act much more efficiently as a work producer when one definite series of muscular movements is repeated over and over again, as in bicycling or rowing. According to Atwater's experiments an ordinary individual doing muscular work does not yield more than about 20 per cent of the energy expended as actual work. That is to say, in working a bicycle 100 calories are expended in the body in order to get the mechanical equivalent of 20 calories of heat actually delivered on the pedal. The most economical oil engine yet devised is said to yield 33 per cent of its energy as work; ordinary good oil and steam engines have an efficiency of 15 to 22 per cent. Zuntz has concluded on experimental grounds that men, horses, and dogs have an efficiency of 35 per cent and Kellner and Wolff found the same for the horse, a figure much higher than Atwater's. It is clear that if by training the efficiency of a man could be increased from 20 per cent to a much higher figure he would need proportionally less food. This would be more likely to take place where only one series of movements was involved. The football team in the above series took much more food than the oarsmen or bicyclists. Success in rowing or bicycling depends on learning to use certain muscles so as to get the greatest possible mechanical advantage from the levers represented by the oar and the pedal, and upon being able to keep these muscles supplied with blood. In playing football, as in hard labouring employments, most of the muscles of the body are called upon and in various combination with other muscles, and in such a case we should not expect them to work with the same neatness and economy.