[igl]

Hard to believe. We have fortified foods and it should be rather hard to catch a nutrition deficiency, even if your diet solely consists of cheeseburgers (This probably differs from country to country).

Vitamin D can be found in Fish, Egg and it's even added to Milk.

[sn9]

Our skin stops producing Vitamin D once it's synthesized about 10,000 IU.

The RDA is only 400 IU.

There's mounting evidence that a regular intake in the thousands of IU (or synthesis from sun bathing, depending on your skin tone, latitude, and cloud cover) has health benefits beyond keeping you out of the hospital.

Examine.com has a comprehensive summary of the current research: https://examine.com/supplements/Vitamin+D/