I'started taking 4,000iu of D about a year ago when my first D test showed my D levels were only at 20. After a few months of taking supplements I got the level up to 50ish. My doc told me anything over 40 is good and I should cut my D intake down to 2,000iu to maintain that level. A few months on 2,000iu and my levels were back down to 30. So I went back up to 4,000iu daily. After many more months on 4,000iu my latest test shows my D levels are only up to 38. Not impressive. Now, reading around this site, my doc's suggestion that "anything over 40 is good" seems low. It seems the consesus around here is that optimal D levels are more like 80-100. At the rate I'm going that seems an impossible goal. I think I'm going to bump up to 6,000iu, but is that enough? Should I get really aggressive and go for 10,000? This is so frustrating. Why is it so darn difficult to get D levels up?