How close are the Recommended Dietary Allowances (RDAs) and ULs?

There is no standard or definable mathematical relationship between the RDA and the UL. For some nutrients, the two values are widely separated (for example, the RDA for vitamin B₆ for adult women is 1.3 mg/day, whereas the UL is 100 mg/day). In some cases the two standards cannot be compared directly because the UL is to be applied only to sources of the nutrient that are not naturally in foods (e.g., the UL for magnesium is only for intake from supplements).

Will we find out in a few years that the RDA and Adequate Intake (AI) are too low and that higher nutrient intakes are better to prevent specific diseases such as cancer?

As our ability to study the chronic effects of various levels of nutrient intakes on humans improves, our knowledge of the relationships between single nutrients and disease prevention will improve. As a result, suggested desirable intake may increase or decrease. Higher nutrient intakes may not be found to be better. In some clinical intervention trials, high doses of β-carotene being studied for cancer prevention were reported to actually increase the risk of lung cancer in long-term current smokers. This demonstrates that it is difficult to speculate about even the direction of an effect when an individual consumes high doses of a nutrient (those that greatly exceed the amounts found in foods).