Doctoral Degrees (Human Nutrition)

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Browsing Doctoral Degrees (Human Nutrition) by Author "Venter, Irma"

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    Development and evaluation of a food frequency questionnaire to assess daily total flavonoid intake using a rooibos intervention study model
    (Stellenbosch : Stellenbosch University, 2013-03) Venter, Irma; Marnewick, Jeanine L.; Herselman, Marietjie G.; Nel, Daan G.; Stellenbosch University. Faculty of Medicine and Health Sciences. Dept. of Interdisciplinary Health Sciences. Human Nutrition.
    ENGLISH ABSTRACT: A comprehensive food frequency questionnaire (FFQ) was developed to assess the daily total flavonoid intake over the past fortnight within a 14-week intervention that consisted of four periods to determine the effect of rooibos consumption on oxidative stress in adults (n=40) at intermediate to high coronary heart disease (CHD) risk. Within the intervention the comprehensive FFQ validity (against six estimated dietary records and biomarkers), reproducibility (on administrations in the washout and control periods six weeks apart as these periods had similar flavonoid intake restrictions) and responsiveness (across the four intervention periods of changed dietary conditions) was evaluated. The baseline period dietary record and FFQ dietary sources found to contribute most to the participants’ daily total flavonoid intake, considering the percentage contribution, and the between-person variation in intake, considering the stepwise multiple regression analysis, formed the food list of the resultant abbreviated FFQ. The validity, reproducibility and responsiveness of the latter were also evaluated within the intervention and its validity (against dietary records) and reproducibility (on re-administration two weeks apart) in an additional group (n=90) being at low and intermediate CHD risk to evaluate its external strength. The validity and reproducibility evaluations of the comprehensive and abbreviated FFQs in the intervention and abbreviated FFQ within the additional group comprised paired difference tests (to establish the ability to estimate group intakes), correlation coefficients (to establish the ability to rank individual participants), category agreement and gross misclassification next to the weighted kappa statistic (to establish the ability to classify the participants into tertiles and quintiles of intake) and Bland-Altman plots (as representation of the limits of agreement between the two dietary assessment methods). Correlation coefficients were also used for biomarker validity evaluations in the baseline period. The repeated measures analysis of variance (ANOVA) (Bonferroni correction) was used for the responsiveness evaluations of the comprehensive and abbreviated FFQs across the intervention periods alongside that of the biomarkers as evidence for the changed dietary conditions. The study demonstrated that the comprehensive FFQ could be modified to a format with a brief food list as few items contributed appreciably to the total flavonoid intake and of which most also contributed to the between-person intake variability. The comprehensive and moreover the abbreviated FFQ in the validity evaluations provided sufficiently accurate daily total flavonoid intake estimates. They could determine the intake at group level in correspondence with that of the dietary records. The participant intakes could additionally be categorized and in particular ranked greatly alike to the dietary record intakes. The Bland-Altman plots revealed proportional bias regarding overestimation at the higher intake level. The reproducibility also appeared to be greatly satisfactory although seasonal fruit exclusions from the abbreviated FFQ food list may hamper its repeated administration. Both FFQs also confirmed the changed total flavonoid intakes across the intervention periods in relation to changes in the expected direction concerning the plasma total polyphenol, conjugated diene and thiobarbituric acid reactive substance concentrations.