Comparison of two instruments for quantifying intake of vitamin and mineral supplements: a brief questionnaire versus three 24-hour recalls.

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, beta-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.