~Age and Weight.~--As has already been stated, there are certain points to be kept in mind in attempting to provide an adequate diet for the human machine; _First_, the _age_ and _weight_. The gain during infancy should be steady--an allowance of 40 calories per pound of body weight to cover the energy requirements and 4 protein calories per pound to cover the nitrogen needs. During the second and third years the energy requirements will be covered by 30 to 40 calories per pound and the nitrogen needs by 3 to 4 protein calories per pound. From the fifth to the eighth year the nitrogen needs continue to be covered by 3 to 4 protein calories per pound and the energy requirements by 35 to 37 calories per pound during the fifth year; 32 to 34 calories per pound during the seventh year. After the body has reached its full development its requirements will be met if sufficient protein is provided to cover its maintenance needs and if the energy calories are regulated according to the amount of exercise taken, keeping in mind that the energy requirements of a man at rest (sitting) will be about 2,000 calories per day and that exercise, especially that taken in the open air, raises the energy needs of the body. ~Daily Gain.~--In estimating the relative daily gain in body weight of children of different ages, Mendel[64] gives the following table: TABLE In the first month, about 1.00 per cent At the middle of the first year 0.30 per cent At the end of the first year 0.15 per cent At fifth year 0.03 per cent Maximum in later years for boys 0.07 per cent Maximum in later years for girls 0.04 per cent ~Retention of Nitrogen in Infancy.~--When the baby is gaining in weight and strength there is a retention of both nitrogen and salts, and when the baby is not gaining there may be a loss of both of these bodies; when one is retained in the body the other is apt to be retained.[65] NORMAL INFANT FEEDING Much has been written in the past few years on the care and feeding of infants and children. This is well, since statistics show an alarming increase in the rate of infant mortality during the early years of life, and anything which can be done to check this lamentable and often avoidable waste of valuable life should be resorted to with care and attention. ~Food for Infants.~--The natural food of all young mammals is the milk of their own mother. The rate of growth and development differs in every species; the calf, for example, doubles birth weight much more quickly than does the baby of the same age. However, the milk of the cow, which meets the needs of the calf perfectly, falls short of meeting the requirements of the infant, whose rate of growth is not nearly so rapid. For this reason if for no other, it would be advisable to give the baby its natural food rather than to attempt a substitute which is, at best, a poor one.