While studies show that 2000 to 10,000 IUs per day of vitamin D may be needed for optimal health, most Americans get less than 400 IUs per day. Vitamin-D 2000 IU™ is designed for those who get 30 minutes of daily exposure to sunlight (without sunblock). Those who do not may wish to use Vitamin-D 5000™ instead. This product may be used with any multiple that does not supply more than 4000 IUs of vitamin D per day.*