DEAR DR. ROACH: Why do multivitamins invariably provide some ingredients with more than 100%, and some with less than 100%, of daily requirement? Why aren’t all ingredients 100%? — D.H.
ANSWER: 100% of the RDA is intended to provide the nutrient requirements for about 98% of the population. Most people will get what they need from food. Many North Americans feel that more vitamins are better, or take a vitamin pill as an insurance policy against an inadequate diet. Both of these have been reasonable hypotheses, but many studies have been done, and have not shown any convincing benefit of taking vitamins in absence of a clear medical reason for deficiency.
Vitamin manufacturers put in large amounts mostly for marketing: People are more likely to buy the pill that has 1,000% of B12, for example, since it must be better than the one with 500%. In fact, your body will simply excrete more B12 if you take more, so the supplement with 1,000% of a water-soluble vitamin like B12 is not any better for you.