Comparison of Two Instruments for Quantifying Intake of Vitamin and Mineral Supplements: A Brief Questionnaire versus Three 24-Hour Recalls

American Journal of Epidemiology Copyright © 2002 by the Johns Hopkins Bloomberg School of Public Health All rights reserved Vol. 156, No. 7 Printed in U.S.A. DOI: 10.1093/aje/kwf097 Comparison of Two Instruments for Quantifying Intake of Vitamin and Mineral Supplements: A Brief Questionnaire versus Three 24-Hour Recalls Suzanne P. Murphy1, Lynne R. Wilkens1, Jean H. Hankin1, Janet A. Foote1, Kristine R. Monroe2, Brian E. Henderson2, and Laurence N. Kolonel1 1 Cancer Etiology Program, Cancer Research Center of Hawaii, University of Hawaii, Honolulu, HI. Department of Preventive Medicine, School of Medicine, University of Southern California, Los Angeles, CA. Received for publication November 30, 2001; accepted for publication May 24, 2002. Although methods of collecting food intake data have been studied intensively, there have been fewer investigations into the collection of supplement intake data. Use of eight types of vitamin and mineral supplements was reported between 1994 and 1997 by 2,377 subjects participating in a calibration substudy of the Hawaii-Los Angeles Multiethnic Cohort Study. Subjects gave information on supplement use as part of a dietary questionnaire (administered twice) and during three 24-hour dietary recalls. Multivitamins were the most commonly used supplements (55% of the subjects), followed by vitamin C (40%), vitamin E (33%), and calcium (29%). Vitamin A, β-carotene, selenium, and iron supplements were each used by fewer than 10% of the subjects. Weighted kappa statistics for agreement between the recalls and the questionnaire across six categories of frequency of use ranged from 0.74 for vitamin E to 0.16 for vitamin A and were generally higher for frequently used supplements. The reproducibility of questionnaire responses at two time points varied from 0.64 to 0.39. In comparison with three recalls, a brief questionnaire can accurately and reproducibly capture data on supplement use for frequently consumed products, but it may perform less well for products used less often or more intermittently. data collection; dietary supplements; epidemiologic methods; ethnic groups; nutrition surveys; questionnaires; vitamins In 1997, it was estimated that consumers would spend $11.8 billion that year on dietary supplements, with an expected growth rate of 10–14 percent over the ensuing 3 years (1). National survey data confirm widespread use of these supplements; in the Third National Health and Nutrition Examination Survey (1988–1994), approximately 40 percent of Americans aged 2 months or older took a dietary supplement during the month prior to interview (2). Use may be even higher in certain population groups; for example, 81 percent of women at risk for breast cancer recurrence reported taking a dietary supplement at least once in a 4-day period (3). When a substantial proportion of a study population is taking dietary supplements, total intake of vitamins and minerals of interest cannot be determined unless supplement use, as well as food consumption, is measured. If supplement use is not considered in nutritional analyses, observed asso- ciations between diet and health may be attenuated or even misleading. Although nutritional epidemiologists have urged that accurate data on supplement use be collected (4, 5), there is a paucity of information on both the validity and the reliability of current collection methods. The baseline questionnaire for the Hawaii-Los Angeles Multiethnic Cohort Study contained several questions about the use of vitamin and mineral supplements. Because of the large number of participants, information on brand names was not requested. However, a subsample of participants in a calibration study also completed three 24-hour recalls which included detailed information on supplements consumed and a second questionnaire. In this paper, we present the results of a validation study comparing reported usage from the two instruments and the results of a reproducibility study comparing reported usage over a 2- to 4-year period. Reprint requests to Dr. Suzanne P. Murphy, Cancer Research Center of Hawaii, 1236 Lauhala Street, Suite 407, Honolulu, HI 96813 (e-mail: ). 669 Am J Epidemiol 2002;156:669–675 2 670 Murphy et al. MATERIALS AND METHODS Subjects Questionnaires The dietary questionnaires were self-administered and included questions about the use of eight vitamin and mineral supplements. Subjects were asked to indicate whether any of the following supplements had been used at least weekly during the past year: multivitamins or multivitamins with minerals; vitamin A; vitamin C; vitamin E; βcarotene; calcium; selenium; and iron. If a supplement had been used, subjects were asked to indicate one of five categories of use: 1–3 tablets per week, 4–6 tablets per week, one tablet per day, two tablets per day, or three or more tablets per day. Use of a supplement less often than once a week was defined as never use. For all categories but the multivitamin category, subjects were also asked to indicate the approximate dosage per tablet, choosing from several dose ranges. Nutrient levels in a multivitamin were calculated as composites of levels in the two multivitamin brands most frequently reported in the dietary recalls (Centrum Silver and Centrum Hi Potency (Wyeth Consumer Healthcare, Madison, New Jersey) were reported by 33 percent of persons who gave a brand name). Previous analyses had shown that this default choice minimized errors in intake estimates (8). If a subject did not know the nutrient level of a particular vitamin or mineral supplement consumed, the amount in the lowest amount category was assumed. Recalls Three 24-hour recalls were administered by registered dietitians during telephone interviews. Days of the week were randomly assigned for the interviews in order to obtain a balance of all seven days. Recalls were completed only on days that subjects considered typical of their usual intake. Subjects were asked whether they had used any dietary supplements on the day of the recall. If they had, the type of supplement and the brand name, place of purchase, dosage, number of tablets per dose, and number of tablets taken were Calculation of amounts consumed For the questionnaire data, frequency of use was multiplied by the dosage reported (or assumed, for multivitamin use) to obtain an estimate of the amount consumed daily for each supplement. For the recall data, the daily amount consumed was the average across the 3 days of intake for each product. For both instruments, the total amount consumed per day was then calculated as the sum of the nutrient levels of multivitamins, if reported, plus the nutrient levels of all single vitamin or mineral supplements reported. For the questionnaire, amounts of the B vitamins and zinc reflect only those amounts contained in multivitamins, because intake of these nutrients in other forms was not assessed. To correspond with current recommendations, vitamins A (...truncated)