We have still another group of food substances containing the three elements of carbon, hydrogen, and oxygen, though they are not carbohydrates. These are the fats. In them we find a larger proportion of hydrogen than we do of oxygen, which gives them the power to unite with oxygen to a greater extent than can the starches or sugars. This means that they can burn with greater heat, and are therefore the best heat-producing foods we have. We have fats of both animal and vegetable origin. Because of their great value as food, they should be used with care and thought. We have learned much from the war, and perhaps not the least of the lessons has been the disastrous effect of a shortage of fats upon the health of a nation. It is to be hoped that we have learned this lesson well, and that it will not be necessary in the future to call in the plumber so often to cut out the waste fat from the kitchen drain-pipe. The whole subject of fats is taken up in chapter XII (Sugar).