We have shown that dietary GHG emissions associated with self-selected diets in the UK are strongly associated with the amount of animal-based products in the diet. After adjustment for sex and age, an average 2,000 kcal high meat diet had 2.5 times as many GHG emissions than an average 2,000 kcal vegan diet. This is the first study to demonstrate these differences in real self-selected diets of meat eaters and those who abstain from meat. There were also significant trends towards lower saturated fat, higher fibre and higher fruit and vegetable intake (but a higher intake of sugars) as the quantity of animal-based products in the diet decreases. Previous analyses of the same cohort have demonstrated lower BMI (Davey et al. 2003) and fewer ischaemic heart disease events (Crowe et al. 2013) in diet groups with lower intakes of animal products. Improved cardiovascular outcomes for vegetarian diets have been demonstrated in meta-analyses of cohort studies conducted in western populations (Huang et al. 2012; Key et al. 1999). Although observational studies are susceptible to residual confounding, non-systematic reviews of RCTs have demonstrated beneficial effects of plant-based diets on lipids (Ferdowsian and Barnard 2009) and weight status (Berkow and Barnard 2006). This suggests that advice to reduce the amount of meat and animal-based products in the diet would be consistent with the definition of a ‘healthy, sustainable diet’, although reductions of meat at the population level may pose nutritional challenges for key nutrients including iron and zinc which should be monitored (Millward and Garnett 2010).

A recent representative survey in the UK using four day weighted food diaries, the National Diet and Nutrition Survey 2008/10 (Bates et al. 2012), found that the average amount of meat consumed in adults aged 19–64 (including non-consumers) was 110 g/day, which suggests that the majority of adults in the UK would be categorised as ‘high meat consumers’ in our analysis. Reducing the amount of animal-based products in the diet represents an achievable way for an individual to reduce their carbon footprint. Assuming that the average daily energy intake in the UK is 2,000 kcal, then moving from a high meat diet to a low meat diet would reduce an individual’s carbon footprint by 920kgCO 2 e every year, moving from a high meat diet to a vegetarian diet would reduce the carbon footprint by 1,230kgCO 2 e/year, and moving from a high meat diet to a vegan diet would reduce the carbon footprint by 1,560kgCO 2 e/year. For context, an individual travelling on an economy return flight from London to New York has an addition to their carbon footprint of 960kgCO 2 e (Carbon Footprint. Carbon footprint calculator. www.carbonfootprint.com/calculator.aspx Accessed July 2013). A family running a 10 year old small family car for 6,000miles has a carbon footprint of 2,440kgCO 2 e (Carbon Footprint. Carbon footprint calculator. www.carbonfootprint.com/calculator.aspx Accessed July 2013), roughly equivalent to the annual carbon saving of two high meat eating adults moving to a vegetarian diet.

Two previous studies have estimated the difference in dietary GHG emissions of self-selected dietary groups (Vieux et al. 2013; Masset et al. 2014). Both of these studies were based on the representative Individual and National Survey on Food consumption in France. Vieux et al. (2013) showed that those who consumed a healthy diet, defined by low energy density, high nutrient density and low consumption of saturated fat, sugar and sodium, had higher dietary GHG emissions than those who consumed an unhealthy diet. Consumption of ruminant meat and pork, poultry and eggs was similar in both healthy and unhealthy diet groups. Masset et al. (2014) showed that diets with lower than average GHG emissions tended to be less healthy, defined using a nutrient density index. These low GHG diets consisted of approximately 20 % less meat, fish and eggs than the average diet. They also identified a subset of the population who consumed a healthy and low GHG emissions diet which did not cost more than an average diet and which was characterised by a higher content of plant-based products. In contrast to the study reported here, the authors did not directly compare diet groups defined by levels of meat consumption. Other studies have estimated the difference in dietary GHG emissions between diet groups, but have used modelled reduced meat dietary scenarios for comparison. The modelling has been conducted using three methods: diets have been constructed from scratch with reference to nutritionists (Baroni et al. 2007); observed meat-eating diets have been modified with selected ‘replacement foods’ (Berners-Lee et al. 2012; Saxe et al. 2013;); or optimisation programmes have constructed a low-meat diet that meets several pre-determined sustainability and nutritional criteria (Macdiarmid et al. 2012; Vieux et al. 2012; Wilson et al. 2013). Most of these modelling studies suggested that reducing animal-based products would reduce dietary GHG emissions (Baroni et al. 2007; Berners-Lee et al. 2012; Macdiarmid et al. 2012; Saxe et al. 2013), but one scenario considered by Vieux et al. (2012) suggested that reduced meat diets may increase dietary GHG emissions. In this scenario, meat was isocalorically replaced in the diet by fruit and vegetables - since the GHG emissions per 100 kcal of food in their database were generally higher for fruit and vegetables than for meat products, this resulted in an increase in GHG emissions. Saxe et al. (2013) found that a vegetarian diet in Denmark would reduce dietary GHG emissions compared to the average Danish diet by 27 %, compared to our estimate of 35 % reduction between meat eaters and vegetarians. The difference may be due to the criterion in the Saxe et al. analysis that the vegetarian diet should contain equal protein levels as the average Danish diet – in the EPIC-Oxford sample protein levels are significantly lower in vegetarians than in meat-eaters. By comparing the actual diets of self-selected diet groups, our results are not limited by restrictive criteria that may not be representative of true dietary behaviour. However, our data are based on cross-sectional comparisons between dietary groups, and as such they do not tell us how people would replace meat in the diet (although, the diet groups do at least represent diets that are actually consumed in the population). This is an important limitation that should be addressed by future longitudinal studies with repeated dietary measures.

The GHG emissions data used in this paper were developed specifically for this analysis. Although the nutrient intakes estimated by the FFQ have been validated against food diaries and some biomarkers (Bingham et al. 1994; Bingham et al. 1995), the GHG emissions have not. The GHG estimates for food items used in this paper are subject to uncertainty that is not captured in the confidence intervals shown in this paper – for example, LCAs of animal-based foods have shown considerable variation in GHG estimates, due to differences in methods used and in forms of agricultural production (Nijdam et al. 2012; de Vries and de Boer 2010; Roy et al. 2009). Estimates of total food-related GHG emissions using measurements of food consumption (such as the FFQ) are underestimates because they do not take account of food wastage. Throughout the analyses presented here we have assumed that GHG emission related to food wastage is reasonably similar across all food groups, but this may not be the case. Estimates of food wastage in the UK suggest that wastage of fruit and vegetables is higher than for meat products (Quested et al. 2013), which could reduce the difference in GHG emissions between the dietary groups. Consumption estimates derived from FFQs are also prone to under-reporting (Becker and Welten 2001; Scagliusi et al. 2008). Similarly, we have not adjusted for differences in raw and cooked weight of foods in these analyses. In most instances, such an adjustment results in smaller cooked weights, as it incorporates discarding inedible material (e.g. bones, skin, cores etc.), which would inflate the size of the dietary GHG estimates reported here.

The diets observed in the EPIC-Oxford cohort may not represent current consumption patterns in the UK. Firstly, they are taken from the baseline data collection period which was conducted in the 1990s. Secondly, a large proportion of the meat-eaters in the EPIC-Oxford cohort consists of family or friends of vegetarians and vegans, who are likely to have diets which differ from those of the general population in the UK. This difference can be assessed by comparing the amount of meat consumed in meat-eaters recruited through the snowballing technique with those recruited directly by participating GPs. This shows that 24 % of meat-eaters recruited via the snowballing technique were high meat consumers, compared to 41 % of meat-eaters recruited directly. Restricting the data by removing participants directly recruited makes little difference to the results (results not shown). Thirdly, the cohort tended to consume a healthier diet than the current UK population. For example, average consumption of fruit and vegetables in the EPIC Oxford cohort is over 6 portions per day, even in the meat eaters. The most recent National Diet and Nutrition Survey (Bates et al. 2012) estimated that average consumption in the UK was 4.1 portions per day.

Further work is needed to establish a defined ‘healthy, sustainable diet’, which should include the recommendation to reduce the consumption of animal-based foods. This is already a recommendation put forward by the Health Council of the Netherlands. Guidelines for a healthy diet: the ecological perspective. Health Council of the Netherlands: The Hague and The (2011) Although this study has only considered the impact of food on GHG emissions, reducing animal-based food consumption has also been proposed as a mechanism for achieving global food security given future trends in yield and agricultural land use (Foley et al. 2011; Godfray et al. 2010; Ray et al. 2013) and could also play a role in reducing water stress and biodiversity loss (Steinfeld et al. 2006). Although the epidemiological evidence largely supports the health-promoting effects of a vegetarian diet, this evidence is primarily drawn from observational studies and residual confounding cannot be ruled out. Further research is required to determine the longitudinal health effects of reducing animal-based products within the diet, preferably in randomised trials. This work should consider micronutrient intakes in vulnerable groups (Macdiarmid 2013; Millward and Garnett 2010) as well as risk factors for cardiovascular disease and cancer.

The current trend in the UK is towards increased meat consumption. The percentage of vegetarians and vegans in UK adults has decreased from an estimated 5 % in 2000/01 to 2 % in 2008/10, although these estimates are based on relatively small samples (Bates et al. 2012; Henderson et al. 2002). Over the same period FAOSTAT estimates that total meat supply in the UK increased from 78.5 kg/person/year to 84.2 kg/person/year (Food and Agriculture Organization. FAOSTAT. http://faostat3.fao.org/home/index.html#HOME Accessed July 2013), including an increase in consumption of beef – in 1961 (when FAOSTAT records began) total meat supply was only 69.2 kg/person/year. It is necessary to find strategies and interventions that can turn this trajectory around and support a population that is increasingly unfamiliar with a low meat diet.