The requirements for an adult's nutrition under the most varying circumstances are definitely known. The amount of food that is necessary to keep him in a state of equilibrium when at rest, when exercising moderately and when undergoing the most violent exertions, has been determined with scientific accuracy. These determinations may be repeated as often as wished, and, provided they are properly made, will give uniform results. They are laborious and time-consuming, but yield trustworthy data because, with the assent and the cooperation of the adult subject, the conditions can be absolutely controlled.
It is not so with children or infants. There is no reason to believe that in them the laws of nutrition differ in any marked way from those of the adult, but the control of conditions so as to make our results trustworthy is extremely difficult, and we are able to obtain, at best, only a part
HOWLAND J. THE FUNDAMENTAL REQUIREMENTS OF AN INFANT'S NUTRITION. Am J Dis Child. 1911;II(1):49-54. doi:10.1001/archpedi.1911.04100070056006