From the fact that in animals of the same species, but of different size the heat production is proportional to the surface.

In a child 2 years old weighing 25 pounds the energy metabolism is approximately half as great as in an adult of six times this weight, i.e. the energy expenditure per unit of weight is three times as great for the young child as for the resting man, and while for the man the expenditure may be taken as a measure of the requirement, in the case of the child an additional allowance must be made to provide the material retained in the body for growth. In studies of infants 7 to 9 months old, Rubner and Heubner found a storage of 12.2 per cent of the energy value of the food consumed, and Camerer found a storage of 15 per cent of the energy and 40 per cent of the protein of the diet.

The following data from Tigerstedt illustrate the relative intensity of metabolism at different ages:

According to these observations the metabolism per unit of weight is greatest in infancy and declines steadily with increasing size; but calculated per unit of surface it is distinctly less in infancy than in children of 10 years, probably because the infant sleeps a greater proportion of the time and the tension (tonus) of its muscles is not yet fully developed.

As between children and adults the energy metabolism is more nearly proportional to the surface than to the weight; but among children of about the same age the energy requirement may be computed on the basis of weight about as well as on the basis of surface.

Murlin and Bailey estimate from their own observations, and the earlier ones of Benedict and Talbot, that the energy requirement of the newborn baby kept comfortably warm and sleeping quietly may be placed tentatively between 1.7 and 2.0 Calories per kilogram per hour, the lower figure for a very fat (10 lb.) child and the higher for a thin (6 lb.) child. According to these authors even vigorous crying does not raise this figure more than 40 per cent. Benedict and Talbot in their later publication l give measurements of minimum heat production of 94 newborn infants (2 hours to 6 days old) which range from 1.33 to 2.17 Calories, averaging 1.75 Calories per kilogram per hour. "Maximum" energy metabolism, chiefly due to vigorous crying, was also observed in 93 of these cases and found to average 65 per cent above the resting value, while in several instances (10 out of 93) "crying and extreme restlessness" resulted in energy expenditure more than double that of the same infant at rest.

With the development of the musculature and of muscular tonus, the energy expenditure of the normal infant increases for a time even more rapidly than his body weight, so that at from 2 months to 1 year of age the expenditure of energy while sleeping averages 2.7 Calories per kilogram per hour (average of Howland's, Benedict and Talbot's, and Murlin and Hoobler's data as summarized by the latter). During the waking hours the rate of expenditure is of course materially higher, and in calculating food requirements allowance must be made for growth and for the possibility of losses through imperfect utilization of the food. In order to provide adequately for all contingencies and support the rapid growth which is normal at this age, it is estimated that a vigorous child will require during the greater part of the first year about 100 Calories of food per kilogram of his body weight per day. But in cases of artificial feeding, since the digestive tract must be gradually educated to handle the milk of a different species, it will often be necessary to feed much less than 100 Calories per kilogram per day at first, perhaps for several months, and only very gradually increase the food allowance.

1 Physiology of the New Born Infant, Publication No. 233, Carnegie Institution of Washington, 1915.

From the end of the first year until growth is completed the food requirement increases, but not so rapidly as does the body weight, so that while the allowance of food becomes larger per day it becomes smaller per kilogram. On the latter basis the energy requirement at the different ages may be estimated approximately as follows:

Children who are very active or growing very rapidly may require even more food than the table just given suggests. Such cases are perhaps most frequently found among boys between 10 and 15 years of age. DuBois finds in boys 12 and 13 years old an average basal metabolism (complete rest and almost complete fasting) of 1.76 Calories per kilogram per hour, or about 75 per cent above that of healthy adults.* Assuming average activity for boys of this age the energy expenditure during 24 hours would probably amount to 60 to 70 Calories per kilogram and as this is a period of rapid growth the requirement would be materially higher than the rate of expenditure.

Assuming average size at the different ages the allowances in Calories per day become about as follows: †

* Per unit of surface the basal energy metabolism of these boys was about 25 per cent higher than that of healthy men.

† See also the more detailed table of energy allowances for children in Chapter, XIV.