It is not how much you choose to take; it is how much your circulating blood level is. If you are seriously below the norm, you DO need supplements.
I question this study until I read more. I know people who take Vitamin D and can’t get their levels anywhere near normal. That means they are using what they are ingesting.
Since Vitamin D is stored in fat, it is not until the fat stores are saturated that one can get into trouble; the circulating blood level can then rise and dramatically. That’s why I’d never recommend messing with taking fat soluble vitamins—like D—unless they are being measured and yes, by a good lab.
F
My Nephrologist actually measures my level once a year and I have come up from below normal to mid-range on 4000 - 5000iu a day and I live where we have sunshine 300 days a year... yet I was below normal.