Hi Radar, I am taking 400 iu of vitamin D a day to supplement my levels. Various advice exists on the optimal amount. The NHS recommends 10mcg or 400 iu a day in Autumn and Winter as most of us should be able to get ample vitamin D from sun exposure (which causes our bodies to make Vitamin D) from April to September inclusive. It doesn't take very much sun exposure to get enough each day.
Like a lot of things health issues can arise if you have too little vitamin D or too much Vitamin D.
I feel taking 400 iu per day is enough as I don't want to get too much.
The link below gives more advice.
NHS Advice on vitamin D
Best wishes, Ian