Citation needed.The "RIGHT DOSE" is not universal - depends on your age, how much body fat you're carrying, how much high fructose corn syrup you consume, if you take cholesterol lowering medication, your skin colour, etc.
One thing for sure is 400iu for a fully grown adult is pretty much useless for most adults living in the UK from now until April 2021 when the UVB Radiation Index is somewhere between 0 to 2.