We have reached the conclusion that it is not possible to state the amount of any food factor which constitutes the minimum or the optimum, or an excess, without taking into consideration the biological values of each of the other factors in the diet. The minimum amount of any dietary factor of constant quality, which will just serve to maintain apparently normal well-being, will fail to suffice when another factor is modified so as to be less satisfactory, and after such a modification of the diet, increasing either of the two factors under consideration will suffice to protect the body from harm in some measure. There is a principle of relativity which is of fundamental importance in estimating the value of a diet as a whole. The diet must be considered as made up of a number of factors, and an effort made to adjust each factor at the optimum. Long observation of the type of experiments here described, serves to reveal when animals are brought into a "twilight zone" of nutritional instability, but in which there is no external evidence that they are not in the optimum condition.