Despite the well-established benefits of good nutrition for health, large proportions of the population in many countries do not consume the types or amounts of foods and drinks that are important for leading healthy lives . For instance, more than 80% of Australian adults do not eat the recommended amount of vegetables, and over 40% do not eat enough fruit for good health . High intakes of sugar-sweetened beverages are also a key contributor to obesity risk and associated adverse health outcomes [3, 4]. There is, therefore, strong impetus for promoting increased intakes of fruits and vegetables, and decreasing intakes of sugar-sweetened beverages in the population. However, the most effective strategies for doing so remain unknown.
Initiatives aimed at improving population diet can involve 'downstream', individually-targeted approaches, or 'upstream' structural approaches. There are a number of important reasons to examine the effectiveness of individually-targeted ('downstream') interventions for improving diet. For example, our previous work has demonstrated that the strongest correlates of women's fruit and vegetable intakes were not upstream structural factors, but rather intrapersonal factors such as nutrition knowledge and health considerations . Observational studies have identified a number of other potential intrapersonal determinants of eating behaviours, including confidence or skills in meal planning/preparation/cooking, and perceived financial costs of healthy eating [6, 7]. Difficulty with budgeting is often reported as a key barrier to healthy eating [8, 9]. Collectively, such findings suggest that improving individuals' skills in planning, budgeting for, procuring and preparing healthy foods may be important goals of healthy eating interventions.
Increasingly, however, it is recognised that traditional 'education'-based nutrition promotion strategies that rely solely on individual responsibility are unlikely to be effective in the absence of broader structural supports. Fiscal policies, such as taxations or subsidies for certain foods or beverages, represent one such structural support that has recently received considerable attention. Recent global increases in the costs of foods, by as much as 75% in recent years , attributable to factors such as drought, rising oil prices, increased demand for certain crops such as corn for biofuel production, and declining world-food stockpiles , have placed increased food-related financial strain on individuals across a range of socioeconomic circumstances. There is strong evidence that prices influence food consumption choices . 'Upstream' fiscal intervention approaches such as reducing the prices of healthier foods in relation to less healthy alternatives are hence potentially valuable strategies for promoting healthier eating amongst large sectors of the population. In Australia and internationally, fiscal food policies have been advocated as a means of promoting healthy eating and reducing obesity and associated health outcomes [1, 13, 14] but there remains little empirical evidence of their effectiveness in populations .
Previous skill-based and price reduction nutrition interventions
While there is now a body of observational data indicating the likely influences on eating behaviours, there remains a paucity of robust intervention research about the most effective means of changing behaviours and promoting healthy eating. This is particularly the case amongst persons experiencing socioeconomic disadvantage, who are at high risk of nutrition-related diseases [16–18]. The existing evidence on the effectiveness of skills-based or price reduction approaches to improving diet has focused primarily on fruit and vegetable consumption. Two reviews have reported on the effectiveness of different behavioural approaches, including skill-building, in increasing fruit and vegetable intakes amongst adults [19, 20]. Those reviews highlighted that behavioural interventions show promise in increasing the quantity and/or variety of fruit and vegetable intakes. They also identified a number of common elements to effective interventions, including goal-setting, providing skills to achieve goals, provision of recipes and motivational newsletters. Printed information appeared to be an effective, more feasible and less expensive alternative to face-to-face or telephone contact. An important gap identified in the reviews was the fact that no studies had at that stage been conducted outside of the United States or Europe; and few provided evidence for the efficacy of interventions in low-income or socioeconomically disadvantaged individuals.
In the Australian context, one skill-based nutrition promotion program, 'Food Cent$', trialled an innovative intervention approach aimed primarily at increasing food budgeting skills to support people with limited budgets to allocate money to healthier foods . The intervention emphasised value for money by comparing foods on a cost per kilogram basis and provided resources for participants to develop budgeting, cooking and shopping skills. While this program showed positive changes in cooking, shopping and eating behaviours, all outcome measures were self-reported, and there was no control group, which limited conclusions about the intervention's effectiveness.
In terms of existing price reduction approaches, one review  of price-related nutrition interventions concluded that while price reduction strategies show considerable promise as effective approaches to promoting healthy eating, most of the existing research has focused on relatively contained settings, such as schools or worksites, and there is a need for further research on the effectiveness of such strategies in the broader community, such as through supermarkets. There is also a need to consider the effect of such fiscal strategies relative to, and in tandem with, other promising approaches, such as skill-building strategies.
Only one previous published study, the SHOP trial in New Zealand , has investigated the effectiveness of individually-targeted nutrition education in conjunction with price reduction strategies in promoting healthy eating in a real-world setting, using a randomised controlled trial design. That study found a significant and sustained effect of price discounts on food purchasing, but no impact of the education strategies on food purchasing or nutrient intakes. However, the education component employed in that study primarily comprised tailored messages suggesting substitution of unhealthy foods with specific healthier products. It was not strongly based on formal behaviour change theories or strategies, and although it did provide recipes, it did not address food budgeting, purchasing, or preparation skills. In addition, the SHOP study population was generally well-educated, and generalizability to a lower-educated or more disadvantaged population is unknown.
Notably, none of the above reviews or studies reported on the mediators or mechanisms of dietary change resulting from interventions. An understanding of these mediators is important for highlighting the most successful intervention elements and how they operate to change behaviour. Similarly, there remains very little evidence on the cost-effectiveness of intervention approaches to promoting healthy eating. With limited resources available for public health, it is increasingly important to understand the most effective specific intervention components, their combination effects, and the 'real world' implementation opportunities to establish whether interventions would represent good 'value-for-money'
This paper describes the protocol for the Supermarket Healthy Eating for Life (SHELf) study, a randomised controlled trial that will build on an important intersectoral partnership with Coles Supermarkets, a major national supermarket chain, and the National Heart Foundation of Australia. Supermarkets are a major controller of food access, pricing and affordability . Supermarkets and grocery stores accounted for 64% ($75 billion) of the total food retail in Australia in 2007-8 . Coles Supermarkets are the second-largest grocery chain in Australia, with around 740 stores nationally. Coles Supermarkets have a store loyalty program called FlyBuys. Shoppers who sign up to FlyBuys are given a credit card style membership card which can be scanned every time a purchase above five Australian dollars is made at a participating FlyBuys business. This allows members to collect points which can then be exchanged for rewards. The National Heart Foundation of Australia is a not-for-profit non-government organization whose mission is to reduce suffering and death from heart, stroke and blood vessel disease in Australia, with nutrition promotion a key focus.
SHELf aims to address the gaps in the existing literature identified above. It tests the effectiveness and cost-effectiveness of a skill-building intervention, a price reduction intervention, and a combined skill-building and price reduction intervention, against a control condition, in promoting purchasing of fruits and vegetables, reducing purchasing of sugar-sweetened soft drinks, and increasing purchasing of low-joule soft drinks/water amongst women (of both high and low socioeconomic status). Secondary aims are to test the impact of the intervention on increasing self-efficacy for, and perceived affordability of, healthy eating, and to examine the contribution of self-efficacy and perceived affordability as mediators of changes in purchasing and consumption behaviours resulting from the intervention. The SHELf study will build upon and extend the SHOP study by drawing on two theoretical frameworks (Social Ecology Theory  and Social Cognitive Theory ), and by incorporating a skills-based intervention component (rather than education alone), utilizing strategies shown to be feasible and effective in the Australian context in the Food Cent$ study.
The study tests the key null hypotheses that, at the end of the three-month intervention, and at the six-month follow-up, there will be no differences in:
fruit and vegetable purchasing or consumption;
purchasing or consumption of sugar-sweetened high-joule soft drinks versus low-joule soft-drinks/water;
the proposed mediators, self-efficacy and perceived affordability of healthy eating;
the costs to society,
between the skill-building intervention participants and the controls; the price reduction intervention participants and the controls; or the combined skill-building and price reduction intervention participants and controls.