Background Supermarkets are a major source of food for families. Food choices are influenced by in-store factors including the variety, price, placement and promotion of food products. Recent trends show families are shopping more at discount supermarkets. A greater understanding of the effect of the supermarket environment on dietary quality and socioeconomic disparities in dietary quality is needed. We examined the relationship between mothers’ dietary quality and the in-store environment of the supermarket where they did most of their food shopping, and assessed whether this relationship differed according to socioeconomic position.

Methods A validated food frequency questionnaire (FFQ) was used to generate a standardised dietary quality score for 829 mothers with young children in Hampshire. An in-store survey of the 49 supermarkets in which they shopped assessed the availability, variety, price, quality, promotions, placement and nutrition information of seven healthy and five less healthy foods from the FFQ. These measures were combined to create a standardised ‘healthfulness’ score for each supermarket. The relationship between supermarket healthfulness and mothers’ dietary quality was assessed using linear regression. An interaction term for educational attainment was added to the regression model to determine socioeconomic differences in the relationship.

Results Univariate analysis showed no relationship between dietary quality and the healthfulness of main supermarket (β = 0.19; 95% CI: –0.02, 0.40) for the full sample of mothers. However, the effect of main supermarket’ environment differed according to mothers’ level of educational attainment (interaction p = 0.004). Stratified analyses showed a strong positive relationship between dietary quality and supermarket healthfulness among mothers who left school at 16 years (β = 0.36; 95% CI: 0.10, 0.61) and an inverse association among mothers with degrees (β = –0.54; 95% CI: –1.08, –0.00). After adjustment for confounding factors associations were β = 0.34; 95% CI: 0.11, 0.58 and β = –0.59; 95% CI: –1.19, 0.02 respectively. These associations equate to disadvantaged mothers, who  shopped at less healthful supermarkets, consuming crisps four times more and vegetable dishes four times less each week than more advantaged mothers shopping at similar stores.

Conclusion These findings suggest that mothers of lower socioeconomic position were more susceptible to the effects of less healthful supermarkets than mothers of higher position. The market share of discount supermarkets is increasing, so improving the environment of less healthful supermarkets is important. Expanding policy initiatives, such as the UK Public Health Responsibility Deal, to include cheaper pricing and greater variety of healthy foods, is needed to address dietary inequalities.