Diets made in the chemical laboratory from mixtures of pure (isolated) protein, fat, carbohydrate, and ash to satisfy all the requirements which we have so far mentioned, do not behave alike when fed to animals. The kind of protein is important as well as the amount. This is shown by experiments in which only one protein is fed at a time. On some, the animals will not thrive. On others, adult animals do very well, but the young ones become stunted like the one shown on page 296. Milk has been found to contain proteins on which young animals can thrive. But even in diets containing the protein from milk, young animals do not develop normally unless the salts of milk are added too. No perfect substitute for milk has ever been found. During the first year of life, a child lives on it almost exclusively; for the first five years it should be considered the most important article in the diet; and throughout the period of growth it should be freely used if children are to become vigorous men and women. If not liked as a beverage, it can be used in cocoa, or cereal coffee, in soups, puddings, and other dishes. Considering what milk may save in the way of more expensive protein foods, such as eggs and meat, and of ash-supplying foods like fruits and vegetables, it is to be regarded as a cheap food. It is possible to get the proper amounts of fuel and protein from white bread and meat, but such a diet is poorly balanced as to ash constituents and especially lacks calcium. It would need to be balanced by adding some fruit or vegetable and even then would not contain as much calcium as is best for growing people. A diet of bread and milk, on the other hand, is so nearly perfectly balanced (supplying fuel, protein, and ash constituents in suitable amounts) that it can be taken exclusively for a long time. Whole wheat bread and milk would be even better, because the whole wheat would supply more iron, in which white bread and milk are not rich. The addition of fruits and vegetables to the bread and milk diet would also be an advantage - partly for the same reason.
Other foods especially valuable for growth are eggs and cereals from whole grains. Children should acquire the habit of eating fruits and green vegetables of all kinds, for when they are older and likely to take less milk and cereals, the fruits and vegetables supply important ash constituents and also help to prevent constipation.
The foods good for children are also good for adults, but the latter can keep their bodies in good repair with less protein and ash in proportion to body weight than are required during growth, and many kinds of protein serve for repair. If there are not enough milk and eggs to go around, adults can take meat, nuts, peas, beans and bread for protein, and trust to these and fruit and vegetables for ash. When the body has been wasted by sickness, however, a return to the foods of growth, especially a diet of milk and eggs, is best for building it up again.