@copyman
Yes indeed...I was a Director of Clinical and Anatomical Laboratory in my career. It was a HUGE deal when physicians started ordering Vitamin D on their patients about 12-15 years ago. A few medical journal articles started showing up with reports on just what Vitamin D was all about...the rest is history. The number of that test being ordered went through the roof after 2010 or so.
When I transitioned to the industry side of the laboratory testing and worked for two different companies over 15 years, the Vitamin D assay was the #1 priority for all lab instrument vendors to develop and market...it is and remains a huge money maker for those companies., so much so, that insurance companies and Medicare started re-thinking their willingness to reimburse for the test...it was costing them millions of dollars per year.
Milk companies were aware of Vitamin D deficiency across the board many years ago, thus "Fortified with Vitamin D" or "Vitamin D Fortified" started showing up on every carton of milk to help develop string health bones in kids (to avoid Rickets). Problem with that is, while it helped kids, teenagers, young adults, the now fully-grown adults transitioned away from milk and start drinking sodas, coffee, energy drinks, and Gatorade type drinks. The only way they could hope to have normal Vitamin D levels, was/is to be out in the sunlight every day for 30 minutes or more, so the underlying tissue layers of their skin could start producing the Vitamin D needed. It's funny how far medicine and nutritional science has come in just 40 years. When I considered a career in biochemistry, focusing on nutritional chemistry, I went to a technical book store and bought a book on vitamins. This was about 1977. The chapter on Vitamin D basically had no information. It was described as the "quiet"/"silent" vitamin whose functions were not well known as yet. We now know that Vitamin D is necessary for a strong immune system, and keeps muscles and brain cells healthy and functioning at optimal levels. That is why the push to consume more fish started in the 1980's and beyond. Fish like Salmon and sardines are high in Vitamin D.
We have come a long way since then in all of Laboratory science and testing. In fact, the first actual laboratory instruments capable of running multiple assays and several different assays were only first-developed in the mid/late 1970's. That is what got me to shift my career focus from what had become cancer research (after biochemistry/nutritional science) to clinical lab. The clinical lab industry exploded in growth in the 1980's, 1990's, and beyond. I satisfied my interest in biochemistry and many other areas of medicine, including cancer, by becoming a Clinical Lab Scientist.