The human body can get vitamin D in two ways: diet or sun exposure. In fact, vitamin D is the only vitamin the body can actually synthesize in response to sun exposure, and therefore does not necessarily need to be provided from the diet or supplements.
If you cannot get sufficient vitamin D from your diet, you can follow recommendations for adequate sunlight exposure. How much sun exposure you need is determined by your skin type and the UV index, which depends on your location, time of day, and season of the year.
TABLE 1. MINUTES OF SUNLIGHT EXPOSURE FOR SUFFICIENT VITAMIN D
SKIN REACTION TO SUNLIGHT UV INDEX 3-5 6-7 8-10 11+ Skin type I: Always burn, never tan 10-15 min. 5-10 min. 2-8 min. 1-5 min. Skin type II: Burn easily, rarely tan 15-20 min. 10-15 min. 5-10 min. 2-8 min. Skin type III: Occasionally burn, slowly tan 20-30 min. 15-20 min. 10-15 min. 5-10 min. Skin type IV: Rarely burn, rapidly tan 30-40 min. 20-30 min. 15-20 min. 10-15 min. Skin type V & VI: Very rarely burn, always dark 40-60 min. 30-40 min. 20-30 min. 15-20 min.
The recommendations in the above table assume that you are exposed to this amount of sunlight at least 3 times per week, wearing a swimsuit or shorts and a T-shirt, and have not applied sunscreen. If you wear sunscreen, your whole body is covered with clothing, or the UV index is 2 or lower, you will not get enough vitamin D.link
Further reading: Calculate the amount of sun you need using the UV index. If you supplement with vitamin D3, best to also take vitamin K2 with it. Magnesium and zinc are also helpful. Be sure to do your own research and consult with your doctor, though my doc (who is a good doc) does not know much about this. Also, blood levels of vitamin D should be measured to accurately gauge either sun exposure or supplementation or both. For some reason, you may have to push to get your doc to write the order for this test. Since serum vitamin D levels are a very strong marker for immune health and our ability to avoid serious covid, it is hard to understand why the medical system does not promote vitamin D and blood tests for it. ABN