The Dietary Reference Intakes (DRI) were introduced in 1993 as a way to expound on the outdated RDA (Recommended Daily Allowance) of vitamins.
The DRI includes four subcategories, as follows:
RDA (now as the Recommended Dietary Allowance) - This is figured to be the daily average intake level of a micronutrient required to meet the need of 97-98% of the healthy people of any given life stage & gender group. It is not a goal for planning or assessing a diet, but a goal for intake.
Adequate Intake Level/AI - Based on experimentally determined or observed estimates of the vitamin intake of healthy people that is assumed to be adequate. Used when EAR cannot be determined.
Tolerable Upper Intake Level/UI - Estimated maximum level at which a nutrient is thought to be safe. Increased risk goes up as ingestion goes above this level.
Estimated Adequate Intake (EAR) - This is the average daily nutrient intake level estimates to meet the requirement of 50% of the individuals in a particular life stage/ gender group. It is population weighed and is considered to be the most accurate reflection of food-nutrient needs of a group, excluding young children and pregnant/lactating women, whose needs differ greatly.
In closing it is important to note that these are reference values for planning & assessing diets for a healthy population. The RDA & AI levels should keep a healthy person from developing symptoms of a deficiency, but they are not intended to replenish undernourished people nor may they be adequate for people in states of dis-ease.
I hope this clears some thing up. For more information on the requirements and sources of each vitamin, stay tuned for upcoming articles, or follow the link provided. Thank You!