[cperciva]

My understanding is that 600 IU/day is a "we have lots of evidence that getting less than this is bad" whereas 1500 IU/day is "we know this is completely safe, and there's some evidence that it's better than 600 IU/day". Rather like vitamin C, there's some debate about how much is optimal, but you need to take truly astounding amounts in order to cause adverse effects.