I wondered about that also. But I don't think it is a short term problem. As I understand it Vitamin D3 levels do not change rapidly. It seems our body has the capacity to keep levels up for a period of time.

I have never gotten a blood test for vitamin D but it would be interesting to do so. My thought for my last blood test was I was going to take a supplement anyway so probably would not change my habits. I think the lower end recommended is like a level of 25 but in reality a lot of doctors now say a level of 50 or above is optimal. I forget where actual deficiency comes in but I think it is down under 12. Supposedly more people in the US are low vitamin D than the number with sufficient levels. Most are sub-optimal.

Ivor Cummins has reviewed the research and has a wealth of information on the subject.

https://www.youtube.com/results?search_query=ivor+cummins+vitamin+d

John