Duncan was a subject in a clinical study that was designed to determine the effects of certain diets on health. While participating in the study, he had to live in a special medical unit at the university hospital. He wasn't permitted to eat anything other than the synthetic formula diet provided by the researchers. The formula contained vitamins, choline, minerals, animal protein, purified water, and corn starch. Within a few weeks, Duncan developed patches of scaly skin on his arms and legs and bald spots on his head. When _____ were added to his formula, the signs of the nutritional deficiency disappeared.