Determining the relative validity of a Food Frequency Questionnaire (FFQ) for assessing nutrient intakes against Four-Day Food Diaries (4DFDs) of New Zealand adults following a vegan diet : the thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Nutrition and Dietetics at Massey University, Albany, New Zealand

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Background: In recent years, veganism has gained traction in developed countries. A vegan diet eliminates animal products and animal by-products including milk, yoghurt, cheese, eggs, honey and gelatine. It is often adopted for animal welfare, environmental concern and associated health benefits. As the vegan diet becomes increasingly more common, a validated food frequency questionnaire (FFQ) is needed for future research regarding the health benefits and risks of the vegan diet. To our knowledge the only validated FFQ for the vegan adult population was developed in the USA. This FFQ is unsuitable for use in New Zealand (NZ) due to cultural, ethnic and food supply differences. Aim: This study assesses the relative validity of a semi-quantitative FFQ for assessing nutrients against a four-day diet recall in vegan adults living in Auckland, New Zealand. Methods: As a part of The Vegan Health Research Programme, a convenience sample of adults aged >18 years whom had adopted a vegan diet for a minimum of two years, were recruited for a cross-sectional observational study. Participants (n=167) completed both a four-day food diary (4DFD) and FFQ, which were compared for the relative validity of 31 nutrients. Relative validity was assessed using correlation coefficients, paired t-tests or Wilcoxon signed rank test, cross classification and weighted kappa statistic, linear regression and Bland-Altman plots. All 31 nutrients underwent energy adjustment. Results: Correlation coefficients improved after energy adjustment with values ranging from 0.116 – 0.661. Out of the 31 nutrients, 25 showed improvement in correlation after energy adjustment. Following this, 12 energy-adjusted nutrients were correctly classified (i.e. >50% correctly classified). After energy adjustment, weighted kappa analysis moderately improved the outcomes with values ranging from lowest value 0.149 (caffeine) to highest value 1.10 (vitamin B6 and vitamin E). Of the 31 nutrients assessed, 24 had higher mean intakes in the 4DFD compared to the FFQ. In Bland-Altman and linear regression, the slope of bias was statistically significant (p-value 0.05). For energy adjusted nutrients (n=17), saturated fat (SFA), poly-unsaturated fat (PUFAS), sugars, riboflavin, phosphorus, and iron were non-significant, indicating that the measurement differences between FFQ and 4DFD were not significantly dependent on mean nutrient intakes. Conclusion: Despite nutrients iodine, vitamin B6, total folate, niacin equivalents and niacin showing poor correlations, the FFQ was reasonably accurate at measuring relative validity of the remaining 26 nutrients. However, it is noteworthy the FFQ tended to underreported on most nutrients compared to the 4DFD. This FFQ could be used in future research to assess the relative intake of nutrients of adult vegans living in NZ. However, it should not be used to assess absolute nutrient intakes.
vegan, nutrients, benefits, risks, validation, food frequency questionnaire, food diary