This is a tricky one. Vitamin D is an important part of your immune system. It also builds strong bones and, as a result, is particularly important for women and aging adults hoping to avoid osteoporosis. It's not easy to find in foods, but it is produced by spending time in the sun. Sunscreen, however -- which is intended to protect you from too much sun exposure -- also minimizes the amount of vitamin D produced. That's why vitamin D supplements are so helpful. You don't want skin cancer, and you don't want brittle bones and a weakened immune system, so what you do want is vitamin D in a supplement form [source: Powell].