Development and validation of a short questionnaire to assess sodium intake

Objectives: To develop and validate a short food-frequency questionnaire to assess habitual dietary salt intake in South Africans and to allow classification of individuals according to intakes above or below the maximum recommended intake of 6g salt day(-1). Design Cross-sectional validation study in 324 conveniently sampled men and women. Methods: Repeated 24-hour urinary Na values and 24-hour dietary recalls were obtained on three occasions. Food items consumed by >5% of the sample and which contributed >= 50 mg Na serving(-1) were included in the questionnaire in 42 categoties. A scoring system was devised, based on Na content of one index food per category and frequency of consumption. Results: Positive correlations were found between Na content of 35 of the 42 food categories in the questionnaire and total Na intake, calculated from 24-hour recall data. Total Na content of the questionnaire was associated with Na estimations from 24-hour recall data (r = 0.750 P<0.0001; n = 328) and Urinary Na (r = 0.152; P = 0.0105; n = 284). Urinary Na was higher for subjects in fertile 3 than tertile 1 of questionnaire Na content (P< 0.05). Questionnaire Na content of < 2400 and >= 2400 mg day(-1) equated to a reference cut-off score of 48 and corresponded to mean (standard deviation) urinary Na values of 145 (68) and 176 (99) mmol day(-1), respectively (P< 0.05). Sensitivity and specificity against urinary Na >= 100 and < 100 mmol day(-1) was 12.4% and 93.9%, respectively. Conclusion: A 42-item food-frequency questionnaire has been shown to have content-, construct- and criterion-related validity, as well as internal consistency, with regard to categorising individuals according to their habitual salt intake; however, the devised scoring system needs to show improved sensitivity.