Abstract
OBJECTIVE: Comparing two dietary cost measurements at the individual level: 'Food Frequency Questionnaire (FFQ) and supermarkets prices' with 'cash-register-receipts-items'. DESIGN: Method comparison study. Reference method: participants collected receipts of food purchases for 28 days; conventional method: participants completed a diet-specific online FFQ. SETTING: A Vegan Israeli Study substudy. PARTICIPANTS: 30 participants were recruited using advertisements on social media. MAIN OUTCOME MEASURE: Average diet cost, energy and nutrients consumption, generated by: (1) items on receipts; (2) online FFQ with supermarkets prices. ANALYSIS: Examining correlations between methods and generating Bland-Altman graphs. RESULTS: Agreement between measurement tools increased when 'eating-away-from-home' dietary costs were omitted from the analysis, from differences of 1453 New Israeli Shekel (NIS)/28 days (414 US$/28 days) higher to 1010 NIS/28-days (288 US$/28days) lower compared with differences of 756 NIS/28 days (215 US$/28 days) higher to 1159 NIS/28 days (330 US$/28 days) lower. Moreover, the Pearson correlation between methods, which was r=0.29 (p=0.13), increased to r=0.52 (p<0.0042). Finally, Pearson correlations between questionnaire-based and receipt-based nutrients were: energy=0.58 (p=0.001); protein=0.46 (p=0.012); fat=0.50 (p=0.005); carbohydrates=0.76 (p<0.001); calcium=0.46 (p=0.012); and iron=0.37 (p=0.049). CONCLUSIONS AND IMPLICATIONS: The dietary cost of the 'FFQ-and-supermarket-prices' method is more strongly correlated and agreeable with the 'cash-register-receipts-items' method when 'eating-away-from-home' items are omitted, indicating that 'eating-away-from-home' costs are poorly estimated when using the standard 'FFQ-and-supermarket-prices' method. Finally, estimating energy, carbohydrates, protein, fat, calcium and iron using 'cash-register-receipts-items' is feasible.