I am not a Doctor, (but I play one in certain, direct to video movies – stage name Doctor Big Dong Ramser) but I did train as a scientist (Nuclear Physics, not Medicine, Biology or Chemistry) and I want to share what I have seen in my own research on vitamin D.
Few foods provide a significant source of D. However, humans are capable of producing all the D they need from exposure to sunlight. Unfortunately, in far Northern and far Southern latitudes, the ability to make this is significantly reduced. At the beginning of the industrial revolution, there was an explosion of rickets in Northern Europe and America. Children who previously had played outside in the sun, had moved to cities with high rise building with small windows, no porches and dark alleys. Eventually the treatment of supplementing D in some food products, especially milk, virtually eliminated this problem.
However, underlying vitamin D deficiencies remain. Rickets is only the tip of the iceberg for severely low levels. D is also essential in a whole range of functions in the body. Studies are showing that D may have a role in preventing heart disease, cancer, and diabetes. Most topical is an emerging role in boosting the immune system to prevent or lessen upper respiratory infections (Convid-19?)
Unfortunately, many people still have low levels of D. Simply a glass of D milk a day is not enough to support a healthy level (25(OH)D level of 50 nmol ⁄L - Vieth R. What is the optimal vitamin D status for health? Prog Biophys Mol Biol 2006; 92: 26–32.). Sun exposure in the summer will give you that if you spend time outdoors. However, in the winter, living above 37°N or below 37°S will give you almost no D even with long periods outdoors. Add to this that many now avoid the sun due to skin cancer worries. A study of Australian Dermatologists in the winter showed that for the study group, the average serum 25(OH)D was34.4 nmol ⁄L (range 5–84). Only six (13%) of the dermatologists had serum 25(OH)D levels > 50 nmol ⁄L but seven (15%) had serum 25(OH)D levels <20 nmol ⁄L. Note that sub 20 is regarded as clinically deficient and in children is very likely to lead to Rickets.
https://www.researchgate.net/publication/24034150_The_vitamin_D_status_of_Australian_dermatologists
While the research is relatively new and conclusions are still evolving, it seems that those who do not work outside should increase their vitamin D by taking supplements. Note that there is such a thing as D toxicity with mega doses. (Interestingly, it seems that the body self regulates D manufacture from sun and rarely enters a toxic level solely from sun exposure.)
I take and I would recommend to others 25-50 mcg (1,000-2,000 IU) daily (daily is important) of D3 (preferred form) supplements. This is less than a third of the amount regarded as risking toxicity. This is especially valuable for the elderly (who absorb D less efficiently) and those living in far Northern (or Southern hemisphere) areas such as Canada, Britain, Scandinavia, Baltics, Northern Russia.
I think the advice is even more important today when people may be socially distancing by staying inside more.