A Dietary Supplement Frequency Questionnaire Correctly Ranks Nutrient Intakes in US Older Adults When Compared to a Comprehensive Dietary Supplement Inventory

Background: Dietary supplements are commonly taken by adults in the United States and can contribute substantially to daily nutrient intakes. Short supplement-use questionnaires are often used in dietary surveys, but their accuracy has not been well studied. Objectives: The primary objective was to evaluate the accuracy of a short, self-administered supplement frequency questionnaire (SFQ) relative to a comprehensive 1-y inventory of supplement use. A secondary objective was to compare SFQ responses for participants in the intensive measurement study to those from a control group to investigate a possible research participation effect. Methods: The Supplement Reporting study enrolled 1029 older adults in 2005-2006, with a mean age of 67.8 y, who participated in the Multiethnic Cohort and reported regular use of dietary supplements. Of these, 375 were interviewed quarterly to collect detailed information on types and amounts of dietary supplements used, while 654 served as the control group. All participants completed 2 SFQs, 1 y apart. Results: Agreement between the 2 instruments in use at least weekly ranged from 88% to 97% for 15 of 16 supplement types, with a lower agreement of 74% for vitamin D. The correlations of nutrient intakes from supplements between the 2 instruments were high, ranging from 0.48 to 0.75, except for iron (r = 0.29). However, mean nutrient intakes as reported on the SFQ were higher than intakes from the inventory for most nutrients, sometimes twice as high. Nutrient intakes based on the SFQ were similar for the inventory and control groups, at both baseline and the end of the study. Conclusions: A self-administered short SFQ can be used in large surveys to identify participants who use 16 categories of dietary supplements at least once a week and can correctly rank participant intakes of nutrients. However, the SFQ does not accurately estimate absolute levels of nutrient intakes from supplements.