Professor Rose Anne Kenny:

That is very variable depending on how low the level was, as well as on the other conditions associated such as darker skin, obesity and other health comorbidities. That should probably be discussed with one's doctor.

Generally speaking, for a person in the community, I would say double what we are recommending for two months. In other words, one should take 2,000 international units a day for two months and then reduce it to 1,000. That would be a reasonable recommendation.