The tendency of those who have investigated the relation of the diet to disease, as in the experiments under discussion, has been to tacitly assume that a diet which does not lead to clinically recognizable symptoms of malnutrition is essentially an adequate one. The tendency during recent years, while knowledge of the properties of foods and the physiological effects of faulty diets has been rapidly accumulating, has been to appreciate the existence of many grades of malnutrition. It is becoming more and more evident that even slight departure from the optimal in the composition of the food, may lead to states of nutritional instability which become contributing factors to physical breakdown when hygienic factors are unfavorable, or when infectious processes are operating. Even under the most favorable living conditions, however, there can be little doubt that under such circumstances the body tissues lose their vitality and the span of life tends to shorten.