I was on 2,000iu/day last year. When I also used a UVB+IR lamp, my serum 25(OH)D rose from 73nmol/L to 115nmol/L and I felt great.
When I took 2,000iu/day and stopped using the UVB+IR lamp, my serum 25(OH)D fell to 70nmol/L and I felt like crap.
2,000iu/day (10 x RDA) on its own just ain't enough (for me, anyway). Cheers, Nige.