They're quickly eliminated and therefore not stored in the body. Thus, they need to be replaced regularly. Other vitamins, such as A, D, E and K, are fat-soluble and stored in the body. These vitamins are more likely to cause toxicity problems if taken in excessive doses. A healthy, well-balanced diet should provide many of the vitamins and minerals a person needs.
Few people are able to maintain such a diet, however.
Even if they can, it's difficult to be certain that fruits and vegetables haven't been grown in mineral-depleted soil or lost much of their vitamin potency while being stored or prepared. Many other factors, including emotional stress, physical exertion, infection, disease, use of prescription medications, use of illicit drugs and alcohol, trauma and aging, can interfere with adequate absorption of vitamins and necessitate a higher vitamin intake. Vitamins should be kept in a cool, dry, dark place. Kept too long, they can lose potency and should be discarded. Manufacturers aren't required to prove the substances' safety or efficacy.