Development and validation of the New Zealand women's healthy diet index : a thesis presented in partial fulfilment of the requirements for the degree of Masters of Science in Nutrition and Dietetics at Massey University, Albany, New Zealand

Abstract

Background: Diet quality indices represent an alternative approach to assessing associations between diet, health, and disease. At present, there is no simple, valid, food-based diet quality index to reflect the adherence of young women to national dietary guidelines in New Zealand Aim: To develop and validate a healthy diet index to assess diet quality and adherence of young women to the Eating and Activity Guidelines for New Zealand Adults (EAGNZA). Method: Dietary information was obtained from young women (n=110) aged 19 – 45 years who completed the New Zealand Women’s Food Frequency Questionnaire (NZWFFQ). Data from the NZWFFQ was used to develop the New Zealand Women’s Healthy Diet Index (NZW-HDI). Participants also completed a four-day weighed food record (FR), which was used to validate the index. Relative validity was evaluated by comparing the NZW-HDI total scores derived from the NZWFFQ against the NZW-HDI derived from the FR. Paired t-tests, Pearson’s correlation coefficients, cross-classification, and weighted kappa were used to assess relative validity. Construct validity of the NZW-HDI was determined using nutrient intakes from the FR and Spearman’s correlation coefficients and linear contrast analysis. Results: Participants achieved an average total score of 76.7 ± 0.9 for the NZW-HDI derived from the NZWFFQ and 75.2 ± 11.1 derived from the FR. For relative validity, a significant correlation between total scores from the NZWFFQ derived NZW-HDI and the FR derived NZW-HDI (r = 0.29; P < 0.05) was found. Cross-classification of participants showed 50% were classified into the same tertile, and 16% were grossly misclassified into opposite tertiles. The weighted ĸ-statistic found both methods had fair agreement in ranking the NZW-HDI total score (ĸ = 0.244). For construct validity, the NZW-HDI total score was significantly and positively related to dietary fibre (0.227), vitamin C (0.214), folate (0.286), and calcium intakes (0.277), and inversely related to intakes of saturated fat (-0.318) and alcohol (-0.236) (all p<0.05). In addition, using linear contrast analysis, higher NZW-HDI scores were associated with higher intakes of vitamin C and folate (P < 0.05). Conclusion: Adherence to the EAGNZA was moderate in this sub-sample of participants. The NZW-HDI was found to have fair relative validity in assessing diet quality. As expected, the total score was positively associated with dietary fibre, vitamin C, folate, and calcium intakes, and inversely related to intakes of saturated fat and alcohol. Future research is required to improve the validity of the NZW-HDI before use in larger study populations.