Prepare to be disappointed.
Nobody knows exactly.
The one size fits all approach won’t work here. Everyone is different thus only a personalized approach will do.
Not even the authorities can agree upon how much one should take.
What can we expect then for mere mortals?
If you look at the label it says the RDA recommends 600 IU of D3. On the other hand American Geriatrics Society recommends 4,000 IU for adults. It’s more than a 6x difference.
It’s not an easy question to answer. To have an accurate picture you need to know how much vitamin D3 is in your blood.
There are some who don’t want to get tested. For them there are some ball park figures. You can measure the amount of Vitamin D to take by body weight. For this you can use this calculator.
During winter time you might need to up your intake because there’s no UVB radiation to create. Also during this season we don’t show much skin either.
Some people go out enough in the summertime so they don’t need supplementation. For this to happen you need to be out between 10am-3pm. The amount of UVB you’re exposed to depends also by the location on the globe. The further you are from the Equator the more you have to be out in the sun.
I personally take more than 4,000 IU a day.
If you don’t want to do much research then stick with American Geriatrics Society recommendation.
There’s a small difference between D2 and D3. D2 is created by plants and our body can and will transform it to D3. Even nature can’t achieve 100% efficiency. Vitamin D2 is less effective than its counterpart D3. Thus if you are taking D2 you need to take more.
According to Dr. James Dowd the conversion rate from D2 to D3 is about 30%. This means you will need to take 3x more to have the same intake with D3. D2 is mostly popular with vegans.
The concise answer for the question would be if you want to just get by than take 600 IU a day.
If you want to thrive aim for 4,000 IU or higher