I had a recent chance to see the effects of sun-tanning on Vitamin D levels. One of my patients had a Vitamin D level of 67 when I first tested her a couple of years ago. This was the highest level I have ever seen but not toxic. She spent 30 minutes in a tanning booth five days a week at that time. Now she does not have the time to visit the tanning booth more than one or two times per week. Her blood level now is 39 - definitely too low and she will have to start daily Vitamin D supplements.
If you need blood testing and your doctor will not order it, here is a website to look at. This site will test you once you join for a subscription fee of $40. This is a bargain!
https://www.grassrootshealth.net/ - $40 to join
Dr. Mercola's article on Vitamin D was linked to a new product he has found - a Vitamin D spray which is supposed to enhance absorption. However, if you work out the cost of this product to take 4,000-6,000 units per day it would be prohibitive for many of us.