Hi Lisa, I have never had my D level checked but started taking a larger dosage on the recommendation of a naturapathic doctor as well as my chiropractor.  The recommended daily does is only 400 but I've done research and listened to other experts (including Dr. Oz on Oprah one day  who recommend taking 4,000 IUD a day rather than that real lose dosage.  I would like to think that it helps keep me feeling less weak and less numb but I have no concrete proof other than the fact that I do feel less weak and less numb.    I also have a friend who does energy work and she recommended trying to get outside for 10 minutes and sit in the sun with your eyes closed but so that the sunlight can seep in through your eyelids.  She claims it gets into your blood stream much faster through the eyes.  But important to keep your eyes closed because you could damage your eyes if you have them consistantly open while looking at the sun.   So of all the supliments, I am a big fan of D!   Kathy