In Florida, in the sunshine state, more or less the same amount of people are vitamin D deficient as northern states now that we protect our skin so much from the sun. Here in FL most of us use sunscreen with SPF every day. The whitest I have ever been is while I lived in FL. No tan, no D. SPF blocks D absorption, and so does clothing and keeping yourself shaded. Again, no tan, no D. UVB rays give us our tans and give us the D, and that is exactly what suntan lotion blocks, and all the cosmetics we use that have sun protection block UVB also.
Traditionally, we see diseases like MS cluster in the low sunlight states in the upper midwest, but it is observed all over the country to some extent of course. I wonder if more and more it is turning up in warm states now that we protect ourselves from the sun so much. I really think the D has saved me, or at least pushed far into the future, some sort of serious neuromuscular problem.
There are conflicting studies regarding D, but I know many many people who notice a big difference if they are taking a therapeutic dose. I know two people who went from limping to not limping anymore, because of the pain in their leg or foot.
However, if someone is feeling fine, I agree with @Judi, you aren’t likely to feel a difference. I think vitamin D deficiency taxes the body over time. So, women like me who have pritected their skin for 15 years might start to have troubles, while the vitamin D deficient 22 year old may not be feeling the affects yet. I think it might partly explain why so many women have so many problems, including autoimmune problems. This is all my own guessing, there is no concrete scientific proof.
Make sure to have your blood calcium level tested not too long after starting (they should check your D level in a few months anyway to see how you are doing). High blood calcium is serious, it is a cardiac risk, and risk to other organs as well. Some doctors don’t check it, I guess they don’t read the package insert or something for the D they are prescribing.