Kinds and amounts of food are both important to health. They act together in securing health. Both must change somewhat under different conditions of life if food is to aid a body in living and working effectively. The life of the body itself requires food; the work of the body does, too. The age of the person, the size, sex, health; the work; the climate, season, all affect both the kinds and amounts of food needed. The location and circumstances affect the food-supply of families.

Amounts and kinds of food must not only provide adequately for the body-needs but must supply foods that can be used under the conditions prevailing. A child is learning to eat; an adult is using food to work; the aged are losing the ability to use food. The food-need of the adult of very active physical life differs from that needed for less muscular exertion, mainly in the energy-supply necessary. For much manual work much heat-energy is needed; for a life of little physical activity more digestion food-aid is required; for age and childhood easily digested food is essential. But the power to digest food is going from the aged and coming to the child. The aged are becoming increasingly inactive, with tissues that are worn, not developing as are a child's. The aged have decreased need for energy food for work, but somewhat increased need for body-heat and body-repair.

Building foods, heat-energy, and digestion foods are all needed always. Which foods are preferable under different conditions, and why they are, has been discussed. Because most foods contain some of all constituents that build, give energy, and aid digestion, a very limited food-supply will keep alive those restricted to it. But for vigor and health the food-supply needs to be plentiful, varied, wholesome, and the diet selected in accord with the food-needs of those it feeds.