Reproducibility and relative validity of an extensive semi-quantitative food frequency questionnaire using dietary records and biochemical markers among Greek schoolteachers

Background. We evaluated the reproducibility and relative validity of a 190-item semi-quantitative food frequency questionnaire (FFQ) to be used in a large prospective study in the Athens area of Greece. Methods. In all, 42 men and 38 women, aged 25-67 years, completed two self-administered semi-quantitative FFQ spaced approximately 1 year apart. Within this 1-year interval, participants visited the study centre monthly and completed an interviewer-administered 24-hour diet recall questionnaire, We also collected two venous blood and three 24-hour urine samples from participants at randomly selected periods during the year between the two administrations of the dietary questionnaire. Results. Mean values for intake of most nutrients assessed by the two FFQ were in good agreement while those assessed by the two different dietary methods were reasonably similar for most nutrients. Intraclass correlation coefficients for energy-adjusted nutrient intakes assessed by questionnaires one year apart averaged 0.57 (range 0.24-0.75) and were not substantially different between genders. Correlation coefficients between the energy-adjusted nutrients measured by repeated 24-hour recalls and the semi-quantitative FFQ ranged from 0.25 for beta-carotene and polyunsaturated fats to >0.50 for saturated fats, cis-linoleic acid, calcium and phosphorus (average: 0.46 for men and 0.39 for women). Reliability and relative validity were very high for alcohol intake. Dietary intakes from the FFQ were significantly correlated to plasma levels of vitamin C, but not beta-carotene or cholesterol. Dietary protein intake assessed by both FFQ was correlated (0.36, 0.30) with average urinary nitrogen excretion levels from three 24-hour urine samples. Conclusions. These data indicate that the semi-quantitative FFQ is reproducible and provides a reasonably reliable measure of intake. Significant correlations between diet and an independent biochemical marker further corroborate the relative validity of our questionnaire in this Greek population.