THE leading principle in the use of food is that we should eat to live, but not live to eat. It is certain that more people die from eating too much than too little. It is wonderful to consider how little food animals, or human beings, can exist upon for a long time and remain in good health; and it is certain that the foundations of many diseases are laid by excessive eating. It must be borne in mind that the elaboration and assimilation of a large quantity of food requires the activity, or even hyperactivity, of several of our most important organs, upon the condition of which our length of life depends. And here we may repeat the statement that has been made so often in the course of this book, that overactivity of an organ may be followed by its exhaustion. By laying too great a burden upon an organ, and continually overworking it without giving it any rest for recuperation, we are burning the candle at both ends, and rapidly exhausting the vitality of such important organs as the liver, kidneys, pancreas, stomach, and intestines, not forgetting those important glands, the thyroid and parathyroid, which take a great share in the destruction of poisonous products formed in our bodies from the end-products of food. We have just mentioned that with very little food animals, or human beings, can live for a long time. But prolonged underfeeding may be quite as dangerous as overfeeding. In starvation the resistance against infectious diseases, and especially tuberculosis, is diminished. This disease, the most common of all maladies, is found most often in underfed people, especially if they live in confined localities with little air and no sunshine. Moderation in food, as in everything else, is the only way to attain a happy old age. The Romans had a very appropriate proverb which ran: “Omne quod est nimium, vertitur in vitium,” “Everything in excess becomes a vice.” Much depends upon the quality of the food we take, for some foods are of very little nutritive value; and even of the most nutritive food, some parts may pass out as waste products, because those organs which elaborate and assimilate food may be partially, or wholly, changed by disease, and so unable to fulfill the work for which they are destined. Therefore, given a moderate amount of food, the condition of the body and the maintenance of strength will depend mainly upon the nutritive value of that food. An exact method of estimating the nutritive value of food has been shown by Frankland, Stohmann, Danilewsky, and Rubner.