In 1987 and 1992, interviewers asked, "During the past 12 months, did you take any vitamin C?" In 1992, this question and others dealing with the use of particular vitamin/mineral supplements were prefaced with the instruction, "The following questions are about individual vitamins and minerals. Do not include the multi-vitamins you already told me about." The 1987 survey included a similar instruction, which interviewers were told to read if there had been an affirmative response in VITAMULTV (i.e., if the respondent had taken multi-vitamins in the past 12 months).

In 2000, interviewers asked, "During the past 12 months, did you take any vitamin C?" Interviewers were instructed to "read if necessary," "Do not include any vitamin C in the multi-vitamins you told me about. Do not include vitamin C fortified drinks."

In 2002, interviewers asked, "During the past 12 months, did you use high dose or megavitamin therapy for your own health or treatment?" Respondents with an affirmative response (i.e., a "Yes" response in VITYR) were shown a flash card containing a list of vitamins and minerals. They were asked, "During the past 12 months, did you take any of the following vitamins in high dose? You may choose more than one."

The other such supplements, along with their availability, are:

• Beta carotene (VITABETA, 2005)

• Calcium (VITACALC, 1987, 1992, 2000, 2002, 2005)

• Vitamin A (only) (VITAMINA, 1987, 1992, 2000)

• Vitamin E (VITAMINE, 1987, 1992, 2000, 2002, 2005)

• Multi-vitamin (VITAMULTV, 1987, 1992, 2000, 2002, 2005)

• Selenium (VITASELN, 2002, 2005)

• Coenzyme Q-10 (VITYCOEN, 2002)

• DHEA (VITYDHE, 2002)

• Vitamins A and/or D (VITYVITAD, 2002)

• Vitamin B complex (VITYVITB, 2002)

• Zinc (VITYZINC, 2002)

In 1987, 1992 and 2000, information was also collected on how long respondents had taken vitamin C. Respondents who gave an affirmative response to the question associated with VITAMINC were asked to report the number of months they had taken vitamin C supplements (VITAMINCMO).

In 1987 only, respondents were asked to report the number of vitamin C pills they took per day (VITYVITCPILS) and the number of milligrams of vitamin C per pill (VITYVITCMG).

In 2000 only, respondents who had taken vitamin C during at least one month in the past 12 months were asked how often they had taken vitamin C in those months (VITAMINCNO), responding as desired in terms of days per week or days per month (VITAMINCTP).

The National Center for Health Statistics recoded data from VITAMINCNO and VITAMINCTP to create two new variables with consistent time units: number of days per week took vitamin C during the relevant months of the past 12 months (VITAMINCDWK), and number of days per month took vitamin C during the relevant months of the past 12 months (VITAMINCDMO). VITAMINCDMO is also available for 1987 and 1992, when it was asked as a direct question (i.e., "About how many days per month did you take vitamin C?").

For more information on the full range of variables dealing with vitamins, see VITANY and VITYR.

The 2002 Field Representative's Manual defines such therapy as "the use of vitamins in excess of the Recommended Daily Allowances (RDA) established by the National Academy of Sciences, Food and Nutrition Board."

Thus, an affirmative response to VITAMINC in 2002 indicates that the sample adult took vitamin C in high doses. A negative response to VITAMINC in 2002 indicates that the respondent did not take high doses of vitamin C, but does not indicate that he/she took no vitamin C at all in the past 12 months. By contrast, affirmative responses to VITAMINC in 1992 or 2000 indicate that the respondent took any amount of vitamin C in the past 12 months.

RDAs vary by age group, sex, and pregnancy/lactation status. In 1987, the highest RDA of any group for vitamin C was 60 milligrams. Thus, any respondent in 1987 taking more than that amount of vitamin C in a day would have been considered to be using "high dose or megavitamin therapy" by the above definition offered in the 2002 Field Representative's Manual. (However, by 2002, the Dietary Reference Intake for vitamin C had been revised to 90 milligrams per day.)

Researchers who wish to combine data on vitamin C intake from 1987 with that from 2002 should exercise caution because there is more precision available in the variables from 1987 than in the variables from 2002. In the former year, respondents only provided the dosage amount of vitamins and minerals if they volunteered to get their container of supplements and read the dosage of vitamin C to the interviewer. In 2002, by contrast, "high dose or megavitamin therapy" was user defined to the extent that, although a definition of such therapy was provided in the Field Representative's Manual, the definition was not routinely shared with respondents. Further, respondents were not routinely reading the dosages of the supplements they were taking from a container in 2002.