The Manual offers these instructions to interviewers:

International units are defined by the Office of Dietary Supplements at the National Institutes of Health as "a measurement used to measure the activity of some vitamins and other biological substances (such as enzymes and hormones)." (http://ods.od.nih.gov/factsheets/showterm.aspx?tID=296)

It is possible to use both VITYVITEUN and VITYVITEPILS to determine the number of IU of vitamin E respondents were taking, on days they took vitamin E. The related variables VITAMINEMO and VITAMINEDMO report the frequency of respondents' use of vitamin E in number of months and number of days per month, respectively.

Analysts interested in the units of vitamin E taken may wish to consult the following variables, which report similar information for other vitamins/minerals (all only available for 1987). Users should note that vitamin A is also measured in international units (a measure of biological activity), but vitamin C and calcium are measured in milligrams (a measure of quantity). Further, a direct comparison of IU across substances is not possible. That is, 1,000 IU of vitamin E is not equivalent to 1,000 IU of vitamin A.

• VITYVITAUN Vitamin A

• VITYVITCMG Vitamin C

• VITYCALCMG Calcium

For more information on the full range of variables related to vitamins and minerals, see VITANY.