Validity and reproducibility of an iodine-specific food frequency questionnaire to estimate dietary iodine intake in older Australians
Aim: Iodine deficiency, which has adverse effects on health has re-emerged in Australia. The present study aimed to develop and validate a novel iodine-specific food frequency questionnaire for use in older Australians. Methods: A 49-item food frequency questionnaire that included iodine-rich foods was constructed and administered in 84 men and women aged 60–95 years with normal cognitive function. Dietary iodine intake assessed by the food frequency questionnaire was validated against three repeated 24-hour dietary recalls. Urinary spot iodine concentrations were selected as iodine intake biomarker. Agreement between the two dietary methods was determined using a Bland–Altman plot and intra-class coefficients. Correlations between dietary and urinary iodine were assessed. Forty-three participants repeated the questionnaire after 9 months for reproducibility. Results: Mean iodine intake measured by the food frequency questionnaire and 24-hour dietary recalls did not differ significantly (P= 0.870). The two methods were moderately correlated (r = 0.377; P < 0.05) and the Bland–Altman plots demonstrated an acceptable level of agreement (P= 0.870). Despite an association (r = 0.230; P < 0.05) between urinary iodine concentrations and 24-hour dietary recalls, the food frequency questionnaire was not associated with urinary iodine concentration (r = 0.094; P= 0.40). The method of triads showed coefficients of 0.238 (urinary iodine), 0.953(food frequency questionnaire), 0.396 (24-hour dietary recall) with the unknown true value. Conclusion: A short food frequency questionnaire to assess habitual dietary iodine intake in older Australians has been shown to be valid at the group level with regard to categorising individuals according to their habitual iodine intake. Reproducibility of the food frequency questionnaire remains to be demonstrated.
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