The use of recommended daily allowances to assess dietary adequacy.

Using the recommended daily allowance (RDA) to assess dietary adequacy implies making a direct comparison between RDA and some measure of dietary intake. The value and usefulness of doing so must clearly depend on the limitations of each. By definition the RDA are intended to cover ‘the average amount of the nutrient, which should be provided per head in a group of people if the needs of practically all members of a group are to be met’. Philosophically and statistically speaking the RDA are set theoretically at two standard deviations above the mean requirement of the group. Thus the RDA as published in the summary table of the report by the Department of Health and Social Security (DHSS) (1979) have limitations for the assessment of dietary adequacy. If the mean intake of a group is above the RDA we can expect that only a few individuals with particularly high requirements and particularly low intakes might be deficient. However, if the mean intake is below the RDA, we do not know what interpretation to put on the value. If we are to assess the probability of deficiency in a group, we need three pieces of information: ( I ) a good measure of the mean intake of that group and the range of intakes, (2) a good measure of the mean requirement and the range of requirements, (3) a knowledge of the correlation between individual intakes and requirements. These are now discussed in turn.