Conventional wisdom has long held that people don't need to supplement their diet if they consume one that meets the RDAs-- recommended dietary allowances -- and other guidelines for vitamin and mineral intakes developed by health agencies of the federal government. Problem is, overwhelming evidence shows that most people don't come close to meeting those nutritional requirements. Surveys report that only nine percent of Americans eat five daily servings of fresh fruits and vegetables -- the amount recommended for obtaining a minimum level of nutrients believed necessary to prevent illness. And average daily calcium consumption in the United States is estimated to be far below the recommended amount.

According to a review of national data by experts at the University of California, Berkeley, people often make food choices that are nutritionally poor: For example, they are more likely to select French fries than broccoli as a vegetable serving, and will opt for a soft drink rather than a glass of skim milk as a beverage. Do you do that? Not only may these and other foods contribute too much fat and sugar to your diet, but they can also result in less-than-optimal intakes of vitamins, minerals, and disease-fighting phytochemicals. The result: Many American diets contain half the recommended amounts of magnesium and folic acid. Also, vitamins A, C, and B6, as well as iron and zinc, are other nutrients that surveys show are at notably low levels in the American diet.