One doctor made one superfulous claim with questionable data and the supplement industry used him to sell billions in vitamin D. Here’s more:
https://news.immunologic.org/p/vitamin-d-can-we-finally-stop-beating
The wellness industry is a convergence of logical fallacies, lack of regulatory oversight, financial motives, and preying on the desire to do what’s best for our health. It brings in $5.6 Trillion every year and is wildly unregulated. Products range from relatively benign (vitamin supplements) to the seriously harmful (IV infusions, ozone enemas, urotherapy). Many utilize a common tactic in pseudoscience: make claims based on a grain of truth, and this will give you immediate credibility.
Vitamin D is a poster child of this. Yes, it’s an essential micronutrient. Yes, it is critical for key cellular processes. Yes, deficiencies can cause issues like rickets.