If we, like rats, are capable of learning to select diets that are rich in the vitamins and minerals we need why are dietary deficiencies so prevalent in our society? One reason is that, in order to maximize profits, manufacturers produce foods with the tastes that we prefer but with most of the essential nutrients extracted from them. The second reason is illustrated by the classic study by Harris and associates When thiamine-deficient rats were offered two new diets, one with amine and without, almost all of them learned to eat the complete diet and avoid the deficient one. However, when they were offered ten new diets, only one of which contained the badly needed thiamine, few developed a preference for the complete diet. The number of different substances consumed each day by most people industrialized societies is immense, and this makes it difficult, if not impossible, for their bodies to learn which foods are beneficial and which are not.