Ever heard of lutein - If you have consider yourself well-informed. There is no Recommended Daily Allowance. But a study of monkeys fed a laboratory diet, presumably containing all necessary nutrients, found that they got macular degeneration eight years earlier than monkeys fed ordinary foods. The missing nutrient appears to be lutein, which is found in green leafy vegetables such as spinach. More info here .
One more indication that our knowledge of nutrition is incomplete even at the simplest (single nutrient) level.
Thanks to Martha Neuringer, who was one of the first researchers to study the brain effects of omega-3s.