Many Americans have inadequate levels of Vitamin D, a vitamin that's essential to our health. Known as the "sunshine vitamin," your body manufactures Vitamin D when you are exposed to sunlight. It is also available through the diet via fish, fish oil, egg yolks, cheese and beef liver -- and supplements. Vitamin D is essential for strong bones, a healthy cardiovascular system and mental cognition. Sunscreen blocks the production of Vitamin D through the skin. Some doctors, including Dr. Frank Lipman on Gwyneth Paltrow's site GOOP, make the controversial...