Modelling the cost differential between healthy and current diets: the New Zealand case study

BACKGROUND: Evidence on whether healthy diets are more expensive than current diets is mixed due to lack of robust methodology. The aim of this study was to develop a novel methodology to model the cost differential between healthy and current diets and apply it in New Zealand. METHODS: Prices of common foods were collected from 15 supermarkets, 15 fruit/vegetable stores and from the Food Price Index. The distribution of the cost of two-weekly healthy and current household diets was modelled using a list of commonly consumed foods, a set of min and max quantity/serves constraints for each, and food group and nutrient intakes based on dietary guidelines (healthy diets) or nutrition survey data (current diets). The cost differential between healthy and current diets was modelled for several diet, prices and policy scenarios. Acceptability of resulting meal plans was validated. RESULTS: The average cost of healthy household diets was $27 more expensive than the average cost of current diets, but 25.8% of healthy diets were cheaper than the average cost of current diets. This cost differential could be reduced if fruits and vegetables became exempt from Goods and Services Tax. Healthy diets were cheaper with an allowance for discretionary foods and more expensive when including takeaway meals. For Māori and Pacific households, healthy diets were on average $40 and $60 cheaper than current diets due to large energy intakes. Discretionary foods and takeaway meals contributed 30-40% to the average cost of current diets. CONCLUSION: Healthy New Zealand diets were on average more expensive than current diets, but one-quarter of healthy diets were cheaper than the average cost of current diets. The impact of diet composition, types of prices and policies on the cost differential was substantial. The methodology can be used in other countries to monitor the cost differential between healthy and current household diets. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1186/s12966-018-0648-6) contains supplementary material, which is available to authorized users.