Validation of a food frequency questionnaire in older South Africans

Objectives. To assess the validity of a 213-item semi-quantified food frequency questionnaire (FFQ) in estimating habitual energy and protein intake in a sample of older South Africans. Repeatability of the FFQ was assessed by comparison of reported intakes after a 6-month period. Design. Cross-sectional analytic study. Methods. Twenty-one subjects were selected from a baseline sample of 200 non-institutionalised subjects aged 65 years and over in Cape Town, who had previously been randomly selected for a nutrition and health survey using a two-stage cluster design. Reported dietary energy and protein intakes, estimated by means of the FFQ method, were compared with 24-hour energy expenditure, measured by the heart-rate monitoring technique and 24-hour urinary nitrogen excretion, respectively. Results. Spearman correlation coefficients for reported energy intake (using the FFQ) versus measured energy expenditure were 0.31 (P = 0.482) and 0.36 (P = 0.345) for men and women respectively. Men tended to underreport energy intake, while women tended systematically to overestimate energy intake by 21% and 25%, respectively. In men, reported protein intake using the FFQ closely matched urinary protein excretion and a good association between the two measures was found (r = 0.62; P = 0.061). In women, no association was found between reported protein intake and urinary nitrogen excretion. The FFQ resulted in a twofold overestimate of protein intake, based on urinary nitrogen excretion. In women, correlations between 6-month repeated measures of energy and protein intake using the FFQ were 0.69 (P = 0.067) and 0.61 (P = 0.053), respectively; however a poor between measure association was found in men. Conclusions. The study findings demonstrate that the semiquantified FFQ method underestimated food energy intake in older men and overestimated both energy and protein intake in older women.