Dear Editor: We thank Aleksandrowicz et al. (1) for calling attention to the recent review of studies assessing environmental impacts of dietary patterns by Hallstrom (2) and to the 2014 report to the Intergovernmental Panel on Climate Change that examined strategies for mitigation of climate change across sectors, including the agriculture, forestry, and other land use sectors (3). These, along with other recent publications (4, 5), add to this rapidly expanding, multifaceted area of research. We agree that changes in eating patterns could have substantial, but uncertain, potential to mitigate the environmental impacts of food systems (3, 5, 6). The review by Hallstrom et al. (2) of available, but limited, literature on environmental impacts of current and theoretical dietary scenarios reports that reducing consumption of animal-based products (in particular, ruminant meat) could decrease greenhouse gas (GHG) emissions considerably (2). It was noted also that the choice of methodology to assess dietary impacts can affect the scientific quality and outcome of studies. As noted in our review (6) and by others (2, 4, 7, 8), methodologic differences including the choice of functional unit in life cycle assessments (LCA), the gold standard for evaluating environmental impacts across multiple sectors (8), can lead to different and conflicting conclusions. The study by Vieux et al. (9), for example, reported that high nutritional quality diets that include higher fruit and vegetable consumption were not associated with low GHG emissions, as anticipated. These results were explained in part by whether GHG emissions were reported on the basis of caloric intake (CO2 equivalents/100 kcal) or food weight (CO2 equivalents/100 g). The specific LCA method used also can lead to different and potentially conflicting conclusions. Whereas the studies reviewed by Hallstrom et al. (2) use attributional LCA methodology to assess impacts of dietary scenarios on GHG emissions, a recent study in Australia used an environmentally extended input-output LCA method to estimate GHG emissions for different food sectors (7). Australian dietary guidelines, as in other developed countries, recommend increased consumption of fruit, vegetables, legumes, and dairy along with reduced consumption of energy-dense foods and drinks that are high in saturated fat, added sugar, salt, or alcohol (referred to as noncore foods) to achieve recommended intakes of dietary essential nutrients. The climate impact of dietary patterns based on Australian Dietary Guidelines was estimated to be about 25% lower than that of the average Australian diet. The impact of dietary scenarios modeled with only a 10% reduction in red meat and eliminating noncore food items was similarly estimated to be 25% lower. Reducing consumption of red meat as a sole dietary strategy to mitigate GHG emissions posed nutritional challenges for obtaining key nutrients, including highly bioavailable iron and zinc. In this study, increased consumption of core foods and lower consumption of noncore foods could have the cobenefits of 25% lower GHG emissions from Australian dietary patterns and enhanced population health. Given the variability in LCA methodologies among researchers, agreement on the appropriate LCA methodology and functional unit is critically needed to inform future guidance on healthy and sustainable diets. Consumer lifestyle preferences and circumstances also play a major role in shaping environmental impacts associated not only with dietary patterns, but also with food waste, transportation, shelter, and household energy use (3, 10). Barriers to substantive changes in dietary habits include both implications to human health and cultural and societal resistance to behavior change (3). Indeed, concerted efforts to improve public health through recommended changes in dietary patterns to achieve nutrient adequacy have fallen short (3, 11). Similar anomalies in consumer behavior have been observed in relation to household energy use (3). The report to the Intergovernmental Panel on Climate Change notes that impacts of GHG emissions, energy consumption, and land use varies considerably across regions and countries and that the potential for successful mitigation strategies will be most effective when country-specific approaches for sustainable development are in line with national priorities (3). Others similarly report that both positive and negative environmental impacts and opportunities and barriers to GHG emissions mitigation are region specific; thus, mitigation options should be examined on a case-by-case basis (5). Further, as noted by Aleksandrowicz et al. (1), mitigation opportunities in developed countries differ from those in low- and middle-income countries. Food systems are diverse not only within individual countries but also within the broader economic and sociopolitical context globally (3, 5). Decision-makers need suitable tools to analyze the intended effects and unintended consequences of potential mitigation strategies while also considering potential trade-offs. A recent committee report from the Institute of Medicine in the United States proposed an analytic framework for researchers, decision-makers, and other stakeholders to examine the possible impacts of future dietary guidance on population health and the environmental, social, and economic aspects of food systems (4). The framework, which has applicability across the globe, is designed to facilitate understanding of the environmental, health, social, and economic effects of the food system and how they are linked; foster enhancement of data collection systems and methodologies; inform decision-making in food and agricultural practices and policies; and minimize unintended health, environmental, social and economic consequences. However, measurement tools to fully assess changes in the totality of the food system are lacking. Bridging current population-level gaps between current and recommended dietary patterns can lead to improved health and potentially lower the environmental impacts of agriculture and food systems. However, researchers in agriculture, public health, nutrition, food safety, sociology, economics, complex systems, and the food industry will first need to address methodologic inconsistencies and further build the scientific research base to close gaps in understanding impacts of dietary patterns across environmental, social, and economic dimensions of food systems.

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