The myth is that you can get enough vitamin D if you spend enough time in the sun (besides getting sun damage.)
Could you get enough vitamin D from sun exposure? The quick answer is yes if you are in the sun long enough, perhaps 20 minutes five days a week or more, and did not wash with soap for 72 hours after every exposure…How many do that? How many would want to do that and lose friends and their job? It takes 72 hours of not washing with soap for your skin to synthesis vitamin D from the sun working with the oils on your skin. This only happens in primitive countries like some places in Africa or in the Islands of the Pacific and south Pacific islands like Fiji where the natives living in primitive situations don’t use soap.
So you need to take a vitamin D3 tablet or enough cod liver oil (milk contains D2 and milk products don’t agree with many people.) Besides D2 does not work as well as D3 (you need about 2x the amount to equal D3, and milk products excrete more calcium than they carry due to the milk protein and the phosphorous it contains.
I recommend 5,000 units of D3 for normal sized adults and less for young children.