The heat and energy are not alone for muscular activity in exercise or movement. It must be kept in mind also that the body is a busy workshop, or chemical laboratory, and heat and energy are needed in the constant metabolism of tearing down and rebuilding tissue and in the work of digestion and elimination. In this chapter, a few points given in the preceding pages are repeated for emphasis. The proteins represented in purest form in lean meat build tissue, and the carbonaceous foods, starches, sugars, and fats, supply the heat and energy. An excess of proteins, that is, more than is needed for building and repair, is also used for heat and energy. The waste products of the nitrogenous foods are broken down into carbon dioxid, sulphates, phosphates, and other nitrogenous compounds, and excreted through the kidneys, skin, and the bile, while the waste product of carbonaceous foods is largely carbon dioxid and is excreted mostly through the lungs. Since the foods richest in protein are the most expensive, those who wish to keep down the cost of living, should provide, at most, no more protein than the system requires. Expensive meats may be eliminated and proteins be supplied by eggs, milk, legumes, and nuts. The fundamental thing is to decide on the amount of protein—two to four ounces, nearly a quarter of a pound a day—and then select a dietary which shall provide this and also supply heat and energy sufficient for the day. If the diet is to include meat, a goodly proportion of protein will be furnished in the lean meat. This will vary greatly with the different cuts of meat, as shown in Table IV, page 54. If, as often happens, one does not care for fats, then the starches and sugars must provide the heat. If much sugar is eaten less starches and fats are needed. The normally healthy individual is more liable to take too much protein than too little, even though he abstain from meat. If the diet is to include meat, it will be of less bulk, because the protein is more condensed; for the same reason, if it includes animal products of eggs and milk and a fair proportion of legumes, it will be less bulky than a vegetable diet. Yet unless vegetables, fruits, and plenty of water are consumed one must guard against constipation. This point is important for busy people who eat their meals in a hurry and proceed at once to active mental work. Those who engage in physical labor are much more likely to take a complete rest for a half-hour, or an hour, after eating. The _thinkers_ seldom rest, at least after a midday meal, and those who worry seldom relax the mental force during any waking hour; their brains are as active as those of mental workers. Every housewife, to intelligently select the daily menus for her family, needs a thorough knowledge of dietetics. She must understand the chemistry of food that she may know food values.