Diets within planetary boundaries: What is the potential of dietary change alone?

Dietary change is often presented as a key factor in the transition toward healthy sustainable diets. However, the evidence base is limited as most studies are based on a single environmental aspect, such as climate impact, contrived dietary scenarios that don't reflect the full array of foods typically eaten, or dietary patterns that are not nutritionally adequate. In this study, a weighted environmental impact (EI) score was developed using a planetary boundary-based distance-to-target approach. The included environmental aspects were climate footprint, water-scarcity footprint, and cropland-scarcity footprint. To enable sensitivity analysis, three alternative weighting models were also developed. The EI scores were calculated for a large number of processed and unprocessed foods in the Australian food system and used to assess 9,341 individual Australian adult daily diets obtained from the Australian Health Survey. A diet quality score, assessing the level of compliance with the food based Australian Dietary Guidelines, was also applied. For the current average diet, energy-dense and nutrient-poor discretionary foods made the largest contribution to the total EI score. Moderating the intake of these foods should be a priority in strategies seeking to promote healthy sustainable diets. Using quadrant analysis, a dietary pattern with a lower EI score was identified that achieved the Australian Dietary Guidelines, however the reduction in EI score was modest, at around 15%, and insufficient to achieve planetary boundary targets. Notably, gains in climate impact reduction were achieved at the expense of increases in water-scarcity footprint, highlighting the trade-offs between different objectives. To achieve healthy diets within planetary boundaries in Australia, the greater share of impact reduction may need to occur in the food production system.