Madam

In a paper published in Public Health Nutrition entitled ‘A cost comparison of more and less nutritious food choices in US supermarkets’, Katz et al. compared the cost of foods from a given food category according to their nutritional quality⁽Reference Katz, Doughty and Njike¹⁾. Foods were classified as ‘more nutritious’ (n 68) or ‘less nutritious’ (n 63) using subjective and objective criteria based on information on the packaging. The authors found that the average price for more nutritious foods did not differ significantly from that of less nutritious foods.

We fully agree with their assertion that ‘improving dietary choices does not invariably cost more’. The possibility of modelling low-cost nutritious diets has been known for a while⁽Reference Stigler²⁾ and is routinely used by the US Department of Agriculture to design the Thrifty Food Plan⁽Reference Carlson, Lino and Fungwe³⁾. We also agree with Katz et al.'s proposal that ‘objective measures of overall nutritional quality might be used for direct comparison of nutrition per dollar’. In fact, we demonstrated the validity of such a measure, based on French data⁽Reference Maillot, Ferguson and Drewnowski⁴⁾.

However, we disagree with their statement that before their study ‘no other study has investigated the price differences of foods within food categories’. A previous study in the UK analysed and compared the contents of energy, fat, minerals and vitamins in branded and ‘economy line’ simple foods (canned tomatoes, orange juice, sliced bread, fresh potatoes and sausages) and found that prices of foods with similar nutrient contents could differ fourfold⁽Reference Cooper and Nelson⁵⁾. Another study conducted in France analysed the relationship between the nutritional quality and price of 220 food products in seventeen different categories⁽Reference Darmon, Caillavet and Joly⁶⁾. Within a given category, branded products cost 2·5 times more than the low-cost products with an equivalent energy and lipid content; they also had a slightly higher (1·3) ingredient quality score, suggesting that the lower price could be due in part to the replacement of higher-quality, higher-cost ingredients (meat, fish, fruit, vegetables) with poorer-quality, cheaper ingredients such as sugars, fats (including hydrogenated or partially hydrogenated vegetable fats), refined starch, salt, polyphosphates, certain food additives, colorants and texturizing agents. Other studies have analysed the difference in price between regular food items and their better-nutritional-quality equivalents (low-fat and/or low-salt and/or more fibre) in Canada⁽Reference Ricciuto, Ip and Tarasuk⁷⁾ and Australia⁽Reference Giskes, Van Lenthe and Brug⁸⁾ and found that the healthy options were significantly more expensive than the regular ones. Yet more recent studies conducted by the French Observatory for Diet Quality (http://www.oqali.fr/oqali/), particularly on dairy products⁽Reference Menard, Dumas and Gillot⁹⁾, show that cheaper products are not always the more energy-dense, saltier or sweeter options.

Too few studies have rigorously examined whether the nutritional quality of foods is associated with their prices within the same category. In addition, these studies have used different methods to estimate the nutritional quality of food and different definitions of what is a food category, perhaps explaining why conflicting results were obtained. As poor people's food choices are highly influenced by food prices, whether or not there is a difference between the nutritional quality food products according to their price is an important issue in the fight against health inequalities.