The RDA of Vitamin D3 is a joke. I tell people they need to be aiming for 5000 iu per day and they continue to babble on about the RDA of 400 iu on multi vits for the prevention of Rickets. In today's society its not really Rickets we need to worry about, its CANCER!
Just take a look at this incidence prevention graph:
https://www.grassrootshealth.org/_dow...art_101608.pdf
Notice how the increase in serum 25(OH)D effects disease incidence! You arent liable to get anywhere near optimum 25(OH)D levels on a paltry 400 iu per day as RDA on multi vits.