Niacin

Nicotinic acid or niacin could cure pellagra by being converted to nicotinamide in the body. This discovery highlighted the importance of niacin as a member of the vitamin B family in preventing vitamin-deficiency diseases. It turned out that other vitamins also worked because they were essential parts of coenzymes that could not be manufactured in the body.
 Conrad Arnold Elvehjem United States 1937 Niacin

Nicotinic acid or niacin could cure pellagra by being converted to nicotinamide in the body. This discovery highlighted the importance of niacin as a member of the vitamin B family in preventing vitamin-deficiency diseases. It turned out that other vitamins also worked because they were essential parts of coenzymes that could not be manufactured in the body.