4.1. Discussion
The results from this study are congruent with findings from previous studies conducted in both Europe and the U.S, while also adding new information to the current body of literature regarding the relationship of dietary patterns and impact of indicators of sustainability.
Without having accounted for statistical uncertainty (see Limitations discussion below), our results indicate that the three omnivore diets studied have the greatest environmental impact and are related to the highest GHG emissions, land use and water use. The two vegetarian diets have the lowest impact on the environmental indicators studied, with the Vegan dietary pattern scoring the lowest for all three indicators. These findings correspond with previous studies conducted in the U.S. and internationally regarding the impact of diets on the environment [ 52 53 ]. However, these findings are important, given that they are using 2020 U.S. dietary guidelines as well as recent NHANES data, while previous studies are primarily using 2010 and 2015 U.S. dietary guidelines [ 20 24 ].
When examining the different food groups that contribute the most to GHG emissions, land use and water use, we note some distinct patterns. In the case of the three omnivore diets, the food group that contributes the greatest percentage to each of the environmental indicators in our study is red meat. Dairy milk is also of concern for all three indicators, and Low Omega-3 fish is a high contributor to GHG emissions. For the two dietary patterns that do not include animal protein, the dairy food group and the nuts/seeds food group are the food groups that score the highest for GHG emissions. For water use, the nuts/seeds group are the largest contributors for the Vegan pattern, while dairy milk is the largest contributor in the Vegetarian diet. Lastly, for land use, for the Vegetarian diet, dairy milk, dairy cheese, and whole grains are the largest contributors while for the Vegan diet, whole grains and refined grains are the largest contributors. However, the food groups that are the largest contributors to the three environmental indicators in the Vegetarian and Vegan diets contribute much less to GHG emissions, land use and water use than the food groups that are the highest contributors to the omnivore dietary patterns.
2 eq, it has a relatively high warming potential and is considered more detrimental to the environment than CO 2 eq. Previous studies have found similar results with red meat/beef being the highest contributor to several critical environmental indicators [ 54 ]. As energy is lost at each trophic level, the production of meat is less efficient and consequently produces more GHG emissions per unit of energy compared to plants [ 55 ]. Red meat from ruminants has high levels of methane emissions caused by the decomposition of their manure under certain conditions, and by enteric emissions. Although methane, which has a low half-life in the environment compared to COeq, it has a relatively high warming potential and is considered more detrimental to the environment than COeq.
2 eq per day per capita, to 2.45 kg CO 2 eq per day per capita, and average beef consumption declined 40% per capita, which contributed to more than 50% of the observed GHG savings in the diet over the study period [ Our findings suggest that the most impactful diet-related change that Americans can make towards a more sustainable diet is a shift towards a Vegetarian or Vegan diet and reducing the consumption of red meat and potentially dairy products [ 56 ]. These key takeaways are in line with previous studies that found that shifting from an average meat containing diet to a vegetarian diet would require anywhere from 18–50% lower GHG emissions, or 23–31% less GHG emissions in the case of a vegan diet [ 57 58 ]. The EAT Lancet Commission report, which may be considered the most exhaustive review of studies related to this topic, recommends a diet that reduces meat consumption by 50% for individuals living in the U.S. [ 7 ]. Although such a shift in dietary behaviors may not seem feasible for many Americans, a recent study found that from 2003–2018, the mean GHG emissions associated with the U.S. diet reduced by approximately 35%, from 4.02 kg COeq per day per capita, to 2.45 kg COeq per day per capita, and average beef consumption declined 40% per capita, which contributed to more than 50% of the observed GHG savings in the diet over the study period [ 59 ]. Men aged 20–34 had the greatest decrease in rate of reduction in GHGs associated with diet changes, while Black women had the lowest GHG emissions associated with their diet [ 59 ]. However, even with this substantial decrease in GHG emissions due to dietary change, Americans are still exceeding established GHG limits to meet global targets and additional efforts to decrease diet related GHG emissions are warranted.
60, Interestingly, the diet which is recommended by the U.S. Dietary Guidelines for all Americans to follow (i.e., the Healthy U.S.) does not recommend a substantial decrease in red meat. As a result, the Healthy U.S. dietary pattern has a greater impact on GHG emissions and land use than the Current dietary pattern. These results are congruent with several other studies examining the potential negative environmental impacts from switching from the current to a USDA recommended omnivore diet [ 25 61 ]. Given the importance of consuming diets that are sustainable, this recommendation seems to contrast with what Americans should be consuming.
62, Americans and American politicians struggle to see the connection between individual-level behaviors and policies contributing to climate change [ 30 ]. Perhaps this study provides the evidence needed to clearly establish the association of individual-level diet and climate change in the U.S. context. A diet which includes less red meat and more plants is not only beneficial for sustainability, but it can also have a major impact on the health of individuals, although each person’s nutritional status must be taken into account [ 9 63 ]. A recent cohort study found that dietary patterns that were associated with less cardiovascular risk were also associated with less GHG emissions, lower fertilizer, cropland and water needs [ 64 ]. Given the impact of overconsumption of red meat on both the environment and the health of Americans, this underscores the need to encourage Americans to consume less red meat.
66, According to the Socio-Ecological Model [ 29 ], potential strategies to change behaviors can focus on the individual level, the household level, the community level or the policy level. Given the scale of influence of the Dietary Guidelines for Americans on food systems, incorporating sustainability into the development of the next set of Dietary Guidelines has the potential to have great benefit in terms of long-term food security [ 60 ]. Other countries have been able to do this successfully, such as Canada, Sweden, Denmark, Spain and Switzerland, and can serve as example to the U.S. [ 65 67 ]. Other policy strategies that may help reduce the overconsumption of red meat in the U.S. include economic policies (i.e., taxing red meat and decreasing cost of high omega-3 fish and fruits and vegetables) and agricultural policies and regulations (i.e., shifting subsidies from factory farms to smaller farms, voluntary and non-voluntary mitigation strategies for livestock farming) [ 68 ].
Past studies suggest that individual dietary behavior change can best be accomplished through policy change, however actions by governments must be paired by societal support to be effective [ 69 ]. A recent review of environmental sustainability in national food-based dietary guidelines found that only a handful of these national guidelines that include statements about environmental sustainability provide specific advice about to implement these guidelines [ 70 ]. Strategies focused on the individual and household level may include creative health promotion via social media campaigns that aim to increase willingness to shift towards plant-based eating by raising awareness of the detrimental health and planetary effects of red meat, and showing that alternatives can be both healthy, tasty and cost effective [ 71 72 ]. These campaigns could also be incorporated into the educational component of household-targeted U.S. food assistance programs such as Women, Infants and Children (WIC) and the Supplemental Assistance Nutrition Program (SNAP), paired with alternative offerings to standard food packages (i.e., alternative milk and meat) without special dietary restrictions required. Strategies focused on the community level include social marketing campaigns or promoting these climate-friendly dietary shifts in schools, universities, government-subsidized programs, and via meal programs in larger corporations. Public health nutrition researchers and practitioners will be key instruments in accomplishing this societal change needed in order to achieve greater policy change [ 66 72 ].