People do not all need the same amount of a nutrient, so a single recommended number can never be the whole story. The Dietary Reference Intakes answer this by describing the spread of requirements in a group and then placing four different markers on it. Drag an intake across the curve below and watch what each marker is actually for.
Vitamin C · men 19 y and older
Usual intake—
Requirement met for—
Risk of inadequacy—
Position—
EAR is the middle. RDA is the middle plus a margin. AI is what we use when there is no middle to find. UL is the ceiling. Every one of these is a population reference value used to plan and evaluate diets — none of them is a prescription, and none of them is a measurement of any individual person’s need.
The requirement curvedrag the line, or tap a standard below
Usual daily intake
Every value names the life-stage and sex group it belongs to. A DRI without its population attached is not a usable number — the same nutrient can differ two-fold between groups.