Interestingly, the researchers said, beta carotene in foods like carrots and spinach is still reliably linked to fighting some cancers. The amount of beta carotene in most multi-vitamins is not harmful, although the new research suggests it may not be beneficial, says Charles H. Hennekens, M.D., the Harvard Medical School researcher who headed one of the beta carotene studies. It can neither substitute for a good diet nor compensate for a bad one. Vitamins are complex organic micro-nutrients, which is a fancy way of saying that they work at the microscopic level to help your body turn the food you eat into energy. They have no calories themselves, and with a few exceptions, they're not manufactured inside your body.
Some vitamins help in biochemical transformations, such as absorbing oxygen in the lungs.
Minerals are naturally occurring inorganic elements your body needs to function. Minerals allow the body to build some structures and help trigger some bodily reactions. Iron, for example, is a key to the production of red blood cells. Calcium is used to build bone and tissues. Zinc is essential for nerve transmission.