To derive an RDA from this minimum amount, some “safety factors” are added. The more important of these is probably the fact that there is a 20-30% (or more) range of individual variation around the average requirement for most nutrients. The safety factor added will result in an amount that covers the majority e.g. 97.5% of the population. Therefore, the RDA can be defined as “the average amount of a nutrient which should be provided per head of a group of people if the needs of practically all (i.e. 97.5 %) healthy members of that group are to be met”. It is important to note that an RDA applies to a group of people, not to the individual.


Whereas most of the DVs currently in use are based on the RDAs from 1968 (9) and a few from 1989 (12) as required by regulation (13), there are now substantially more data on human requirements for nutrients than existed 2–4 decades ago. The new RDAs for many nutrients are lower than those from 1968, reflecting the DRI approach to specifically setting them based on 2 standard deviations above the average requirement estimate, as opposed to methods used in 1968 where often the minimum amount required for all subjects tested was determined and a safety factor added (9).