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

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. 1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4210929/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4210929/