I increased mine to 4000 IU / day with the advent of COVID-19 (so did Anthony Fauci, MD and many others) and I continue to this day. This study only supplemented with 2000 IU which is not very much. It remains to be seen if a higher dose would provide more benefit, but it is interesting finding nonetheless.

Doctor to check for anemia. I was tired all the time. I'd wake up tired and thought that low iron might be the culprit. She did that test and a vitamin D test. My iron was fine but I was deficient in vitamin D. I started taking supplements and about a month later my energy level was back to normal. Truly amazing what a difference it made.