There is research showing that the figure of 400 IU is based on a statistical error [1] (that's the original article, there's plenty more if you search).
Most articles I have read recommend somewhere between 2000 and 5000 IUs. Some recommend higher.
The body can produce up to 25000 IU/day in very sunny places on very pale skin. 10000IU/day is considered the safe upper limit. Whether or not that's a good idea long term is a different matter.
The body can produce over 250,000 IU in short order delivered as 25,000 IU a day as a controlled release through the skin. If you want this, the best idea is to spend 5 minutes in your underwear in a tanning bed. Dietary supplements can work, but it takes months.
It actually isn’t. 400 IU is based on preventing disease. To obtain optimal health though, dosages should usually be much higher. Obviously this differs from person to person, but 2000 IU should lead to optimal vitamin D levels. This is an interesting video on the topic: https://www.youtube.com/watch?v=jeLC_b5gDUM
My MD prescribed me 5000 IU / day after a blood panel came back with me on the low end of healthy.
Same. He wanted to do it after asking if I worked in an office, and the results came back pretty low.