Myth # 2. I get enough from the sun.
Not likely, says Dr. Ting, since sun exposure is hard to quantify and our skin doesn’t absorb vitamin D as easily as we age. Healthy adults should aim to get the RDA (recommended dietary allowance) of 600-800 International Units (IU) from fortified foods like milk and cereal. Ask your doctor what dose is best for you.
Myth # 3. I sit by a window all day, so my levels are probably fine.
Actually, you won’t absorb any D if you’re behind glass.
http://www.healthmonitor.com/your-health/written-article/4-myths-about-vitamin-d
For modern first world humans you are absolutely right. The absorbsion mechanism evolved with scant body covering in all but cold weather, little bathing and removing skin oils. It has also been found that Vitamin D is involved in depression, especially the winter kind called SAD. For that 1,000 to 2,000 IUs may be needed. Make sure your doctor actually knows about these new findings when asking advice. I worked in a medical school. Doctors don’t learn much about nutrition.