[Trick to remember: we were 40 years in the midbar[dessert]
There are very different opinions, below one opinion.
Many vitamin D tests are unreliable, alas and when the same blood sample is given to different laboratories the test result sometimes may show big. discrepancies
Different countries recommend different levels of vitamin D for their citizens, but on average the recommended amounts are as follows: Children under 1 should take 10 micrograms a day, kids and teens between the ages of 1 and 18 should take 15, and adults should consume 20 to 100 depending on their BMI.
multpluy by 40 100 mcg = 4000 IU