“Levels above 2,400 IU are terra incognita,” Dr. Garland said. The lowest level of vitamin D associated with toxicity is 3,800 IU, according to the National Academy of Sciences. Some scientists have proposed daily intakes of 4,000 IU. In any case, supplements and vitamin-fortified foods, such as cereals and grains, will be necessary for most people to maintain optimal levels of vitamin D, he noted.