Only vitamin D can be manufactured by the body. All others must be derived from the diet. Lack of sufficient vitamin intake causes a wide range of health problems and dysfunctions. Unfortunately, these RDAs give only the bare minimum required to ward off deficiency diseases such as rickets, beri-beri, scurvy, and night blindness. What they do not account for are the amounts needed to maintain maximum health.
All vitamin supplements work best when taken along with food.
Typically, oil-soluble vitamins should be taken before meals and water soluble vitamins should be taken after meals. Vitamin A is a pale yellow primary alcohol derived from carotene. The body obtains vitamin A in two ways. One is by manufacturing it from carotene, a vitamin precursor found in such vegetables as carrots, broccoli, squash, spinach, kale, and sweet potatoes. In animal form, vitamin A is found in milk, butter, cheese, egg yolk, liver, and fish-liver oil.