A culture-sensitive quantitative food frequency questionnaire used in an African population: 2. Relative validation by 7-day weighted records and biomarkers.

OBJECTIVE: To determine the relative validity of the culture-sensitive quantitative food frequency questionnaire (QFFQ) developed for the Transition, Health and Urbanisation in South Africa (THUSA) study by 7-day weighed food records, urinary nitrogen excretion and basal metabolic rate (BMR).
DESIGN: A cross-sectional study.
SETTING: A community-based study in a population stratified according to level of urbanization.
SUBJECTS: Residents of the North West Province, South Africa. aged between 15 and 65 years. The weighed food record study comprised 74 participants while 104 participants collected 24-hour urine samples.
METHODS: All participants were interviewed using the QFFQ. For the weighed food record study, participants kept detailed weighed food diaries for seven consecutive days. For the urinary nitrogen study, participants made one 24-hour urine collection. Completeness of the urine collections was checked against 240 rug pan-aminobenzoic acid. BMR was estimated by the Schofield equations.
RESULTS: Spearman rank correlation coefficients between the QFFQ and weighed food record ranged between 0.14 (fibre) and 0.59 (vitamin C). The QFFQ tended to underestimate intakes compared with the weighed records. Quintile distributions were similar for both methods. The correlation between urinary nitrogen excretion and dietary intake was poor. Possible underreporting was identified for 43% of the participants with the QFFQ and 28% with the weighed food record.
CONCLUSIONS: The QFFQ appeared to be a relatively valid instrument for the assessment of dietary intakes of the population of the North West Province. The use of biomarkers in this population was difficult and needs further investigation.