Discussion

Research concerning the deleterious climatic impact of the food system generally relies on typified diets or diets based on average food consumption patterns, thereby not capturing the diversity observed across a representative sample of the population⁽ Reference Heller, Keoleian and Willett ^{26 )}. In many studies, dietary emissions are based on consumption-level data from published data (food consumed)⁽ Reference Macdiarmid, Kyle and Horgan ^{21 ,} Reference Saxe, Larsen and Mogensen ^{29 –} Reference Vieux, Soler and Touazi ^{31 ,} Reference Aston, Smith and Powles ^{46 –} Reference Wilson, Nghiem and Ni Mhurchu ^{48 )} or GHGE from agricultural commodities⁽ Reference Risku-Norja, Kurppa and Helenius ^{49 –} Reference Popp, Lotze-Campen and Bodirsky ^{52 )}. It has been argued that food consumption surveys provide a more realistic distribution of dietary intakes across societal demographics⁽ Reference Heller, Keoleian and Willett ^{26 )}. However, when possible, energy misreporters should be accounted for in analyses to allow for a more realistic interpretation of results⁽ Reference Vieux, Darmon and Touazi ^{30 )}. This is frequently overlooked in many studies and may result in less accurate and lower reported emission levels. The considerations outlined highlight the notable challenges in making direct and meaningful comparisons between studies as no agreed methodology exists to determine GHGE associated with food consumption. It also highlights the need for harmonisation of national population analysis of dietary GHGE outputs, with the need to be consistent in data type (commodity v. consumption), accounting for energy misreporting, as well as using conversion factors with the same system boundaries.

Dietary emissions were estimated in the present study to be 6·5 kg CO₂eq/d for a representative sample of the Irish population. Every attempt has been made to calculate as accurately as possible the GHGE associated with food consumption by using full LCA while also accounting for misreporters. Tukker et al.⁽ Reference Tukker, Goldbohm and de Koning ^{28 )} used food balance sheets from the FAO to estimate 7·1 kg CO₂eq/d as the average GHGE associated with food consumption in the EU. Although the method used by Tukker et al. overestimates per capita consumption, it is an accurate procedure for calculating food-related environmental impacts such as dietary emissions⁽ Reference Heller, Keoleian and Willett ^{26 )}. Therefore, it would be reasonable to conclude that at the very least emissions related to food consumption in Ireland are comparable to those across the EU.

Comparability between international and national studies can be difficult due to the varying types of data source used and/or differences such as system boundaries in the emission factors adopted. Researchers from the UK⁽ Reference Berners-Lee, Hoolohan and Cammack ^{19 ,} Reference Green, Milner and Dangour ^{22 ,} Reference Hoolohan, Berners-Lee and McKinstry-West ^{34 )} , France⁽ Reference Vieux, Darmon and Touazi ^{30 )} and the Netherlands⁽ Reference Temme, Toxopeus and Kramer ^{35 )} have obtained dietary data from similar national nutritional surveys as NANS and provide useful comparisons. What is more, Ireland, France, the Netherlands and the UK are comparable in that they share diets that are characterised by low ratio of vegetal to animal energy, high intake of animal fats, and relatively low intakes of cereals and vegetables⁽ Reference Tukker, Goldbohm and de Koning ^{28 )}. Various studies have approximated UK dietary emissions as 5·7⁽ Reference Green, Milner and Dangour ^{22 )}, 7·3⁽ Reference Berners-Lee, Hoolohan and Cammack ^{19 )} and 8·8 kg CO₂eq/d⁽ Reference Hoolohan, Berners-Lee and McKinstry-West ^{34 )}. Some of the differences in UK estimates can be attributed to the differences in system boundaries, the particular wave of the UK national consumption survey used, emission factors adopted, and whether the study includes children as well as adults. French and Dutch adult dietary emissions have been reported as 4·2 kg CO₂eq/d⁽ Reference Vieux, Darmon and Touazi ^{30 )} and 4·3 kg CO₂eq/d⁽ Reference Temme, Toxopeus and Kramer ^{35 )}, respectively. The omission of post-retail emissions and emissions associated with waste may partially explain the higher emission profile of Ireland in comparison to France⁽ Reference Vieux, Darmon and Touazi ^{30 )}. Furthermore, meat consumption for adults in the French dietary survey used to derive food consumption was 35 % lower than that of the NANS, which would have greatly influenced overall GHGE^{( 53 )}. Dutch dietary emissions were also lower than those observed in Ireland; although Dutch emission factors considered GHGE associated with food waste, energy misreporters were not accounted for in the analysis⁽ Reference Temme, Toxopeus and Kramer ^{35 )}. Indeed, the authors suggested that dietary emissions were underestimated by a similar order of magnitude (15 %) to the proportion of energy under-reporters observed the study⁽ Reference Temme, Toxopeus and Kramer ^{35 )}. Additionally, Dutch meat consumption was 44 % lower than observed in Ireland⁽ Reference van Rossum, Fransen and Verkaik-Kloosterman ^{54 )}. However, the emission factors applied to meat were lower for both the French and Dutch studies, which may further contribute to their lower overall dietary emissions compared with those of Ireland.

The majority of emission factors used in the present study were those used by Green et al.; in their study, UK dietary emissions for adults were estimated at 5·7 kg CO₂eq/d⁽ Reference Green, Milner and Dangour ^{22 )}. However, the UK estimates did not account for energy misreporting; hence their findings were broadly similar to those observed herein of 6·0 kg CO₂eq/d when the total population including energy misreporters was considered. Meat consumption was 10 % higher in Ireland than observed by Green et al. and may be one of the many factors which may explain why their reported GHGE were slightly lower. A smaller number of emission factors were obtained from Heller and Keoleian and they estimated daily average GHGE in the USA at 5·0 kg CO₂eq/d using commodity data rather than habitual data⁽ Reference Heller and Keoleian ^{41 )}. A sensitivity analysis revealed that Irish dietary emissions were in line with US estimates when using as many of their emission factors as possible (see online ‘Supplementary material 2’, Table S1).

Differences in GHGE across sociodemographic and socio-economic groups were mostly influenced by the quantity of food consumed. With the exception of gender, the emission intensity of diets (GHGE/MJ) was not significantly different across the sociodemographic profiles. Previous research on adult Irish men has shown that they use dietary choices to express masculinity, which can influence a preference for certain foods such as meat⁽ Reference Newcombe, McCarthy and Cronin ^{55 )}. Women, on the other hand, often restrict food intake, therefore not eating as much and not consuming meat in the same quantities as men⁽ Reference Cronin, McCarthy and Newcombe ^{56 )}. Hence, male dietary preferences may explain the difference observed in both the emission intensity of diets and the variation in dietary emissions between genders. Significant differences in dietary emissions were also observed according to age. Those aged 18–35 years consumed higher quantities of unhealthy foods such as processed meats and alcohol, which leads to overall higher dietary emissions when compared with other age groups. Indeed, other studies have also noted the unhealthy eating patterns of young adults⁽ Reference El Ansari, Sebena and Stock ^{57 ,} Reference El Ansari, Stock and Mikolajczyk ^{58 )}. Therefore, those aged 18–35 years failed to adhere to both the environmental and nutritional aspects of a sustainable diet^{( 23 )}. Foods of animal sources contributed most towards GHGE, with red meat responsible for the greatest proportion of Irish dietary emissions. Furthermore, red meat had the highest ratio of GHGE per MJ of energy provided. Higher intakes of animal-based foods were noted in high emitter tertiles. However, the tertiles were not indicative of the complexities and underlying structure of dietary patterns as only a single variable (total dietary emissions) was used for quantile classification.

Foods of animal origin provide many beneficial micronutrients and their high content of essential amino acids and micronutrients should not be overlooked when formulating dietary recommendations that consider both nutrition and GHGE⁽ Reference Smith, Sones and Grace ^{59 )}. Clearly, tensions exist in balancing the environmental and nutritional facets of food consumption. It is suggested that to achieve sustainability and public health objectives a transition to a less meat-based diet is required⁽ Reference Friel, Dangour and Garnett ^{24 )}. In an effort to address this issue many non-government organisations traditionally favoured climate change messages which ask for modest reductions of meat and encourage consumption of ruminant meat derived from grass-fed systems⁽ Reference Laestadius, Neff and Barry ^{60 )}. Recently non-government organisations such as the World Resource Institute and Chatham House have been more uncompromising and both have called for a reduction in consumption of animal-based foods^{( 61 , 62 )}. Research from Tom et al.⁽ Reference Tom, Fischbeck and Hendrickson ^{63 )} suggests that per kilojoule, production of vegetables requires more water than meat. Attention should therefore be paid to the potential swapping of one environmental concern for another when developing recommendations to reduce dietary emissions⁽ Reference Tom, Fischbeck and Hendrickson ^{63 )}. However, the European food system is very different to that which exists in the USA. While Meier and Christen identified that blue water needs were higher for vegan and vegetarian diets in Germany⁽ Reference Meier and Christen ^{64 )}, Vanham and co-workers found that the overall water footprint of vegetarian diets was lower than that of other dietary patterns in separate studies of the EU⁽ Reference Vanham, Mekonnen and Hoekstra ^{65 )} and Austria⁽ Reference Vanham ^{66 )}. Consideration is consequently advised when assessing the sustainability of diets as the climatic impact is only one facet of wider environmental concerns.

While vegan and vegetarian diets can substantially reduce dietary emissions, partial replacement of meat could potentially induce higher climatic effects if the substituted food groups are higher in GHGE than those being displaced (i.e. if chicken was substituted for cheese)⁽ Reference Hallström, Carlsson-Kanyama and Börjesson ^{67 )}. Careful consideration is therefore required when formulating health recommendations based on environmental concerns. To provide more holistic assessments, it has been proposed that the sustainability of alternative diets is based upon nutrient and energy contents⁽ Reference Masset, Vieux and Verger ^{32 )}. Hypothetically meat can be replaced by plant-based substitutes but an appreciation of the nutritional equivalency of the replacement food is necessary^{( 68 )}. Replacing the energy and protein attained from average beef consumption could be compensated by the consumption of bean varieties, which leads to GHGE reductions (see online ‘Supplementary material 2’, Table S2). However, the size of the reduction in GHGE is dependent on the quality of meat being replaced. Moreover, replacing the energy induced by beef by instead consuming vegetables such as broccoli is difficult and requires large intakes which yield similarly high GHGE as beef (see online ‘Supplementary material 2’, Table S3). Thus, there are inherent difficulties in implementing dietary guidelines which adhere to all the principles of sustainable diets.

Typically, diets which are in accordance with health and nutrition guidelines for fruit and vegetables often conflate in reducing dietary GHGE⁽ Reference Macdiarmid, Kyle and Horgan ^{21 ,} Reference Green, Milner and Dangour ^{22 )}. Evidently, low dietary emissions were generated through the consumption of fruits and vegetables in the present study. High-sugar snacks were the second highest contributor to energy intake in the diet while also providing the least amount of GHGE per unit of energy provided. Indeed, many of the food groups which had the lowest emission factors are not conducive to good health if consumed excessively. Minimising dietary GHGE by only consuming foods low in GHGE therefore does not necessarily conform to maximising human health outcomes⁽ Reference Vieux, Soler and Touazi ^{31 )}. For instance, from a public heath perspective, increased and overconsumption of high-sugar foods may lead to negative health outcomes while concurrently resulting in lower GHGE⁽ Reference Yang, Zhang and Gregg ^{69 )}. In addition, meat was the highest source of dietary emissions but had a low energy profile. Recommendations to reduce red meat will result in reductions in GHGE but consideration must also be given to its health properties. In contrast, high-sugar snacks were low in GHGE but high in energy content; they also lack nutritional quality. Therefore, when devising low-carbon diets a balance is warranted as to both the nutritional and environmental impacts of certain food groups⁽ Reference Tilman and Clark ^{70 ,} Reference Briggs, Kehlbacher and Tiffin ^{71 )}. Green et al.⁽ Reference Green, Milner and Dangour ^{22 )} suggest that dietary emissions could be reduced by as much as 40 % through dietary changes. However, consumers are often unwilling or unsuccessful in altering dietary behaviour⁽ Reference Chance, Gorlin and Dhar ^{72 )}. Indeed, campaigns over numerous decades calling for increased intakes of fruits and vegetables have been largely unsuccessful despite active targeting by government and health organisations⁽ Reference Rekhy and McConchie ^{73 )}. The advocates of meat-free diets may therefore be met with resistance as consumers favour diets that contain some meat⁽ Reference Schösler, de Boer and Boersema ^{74 ,} Reference Graça, Calheiros and Oliveira ^{75 )}. ‘Less but better’ and ‘less and more varied’ meat consumption have been subsequently proposed as a dietary mitigation strategy which may appeal to consumers⁽ Reference de Boer, Schösler and Aiking ^{76 )}. Meat can be defined as ‘better’ if it achieves a spectrum of outcomes for climate change, the environment, animal welfare, human health, livelihoods, social justice and social values⁽ Reference Sutton and Dibb ^{77 )}. However, consumers often find ways of disengaging from many of the issues concerning meat consumption by ‘explaining away’ their behaviour through blame avoidance⁽ Reference Graça, Calheiros and Oliveira ^{75 ,} Reference Rothgerber ^{78 )}.

There are many alternative non-voluntary measures which could be enforced to lower dietary emissions. Decreasing dietary emissions can be achieved by using economic (e.g. Pigouvian tax), social (e.g. educational campaigns) and legal instruments. Many arguments have been made in favour of a meat consumption tax, rather than a meat production tax⁽ Reference Edjabou and Smed ^{79 –} Reference Wirsenius, Hedenus and Mohlin ^{81 )}. It has even been suggested by Chatham House that without government intervention consumers are unlikely to reduce their consumption of animal-based produce^{( 62 )}. However, consumers generally respond unfavourably to such taxation measures and there is likely to be some public resistance to a meat tax⁽ Reference Vanhonacker, Van Loo and Gellynck ^{82 )}. Conversely, a Finnish Mandatory Vegetarian Day is an example of a legal instrument to stimulate reduced meat consumption. A weekly forced food restriction day is implemented in the Helsinki School District whereby meat and fish have been eliminated from the school menu. The effectiveness of the intervention is unclear however and may have unintended consequences such as increased food waste arising from a hedonic dislike of vegetarian food⁽ Reference Lombardini and Lankoski ^{83 )}.

Sustainability can be an important motivator of behaviour and may be adopted in future campaigns to change dietary patterns and promote health⁽ Reference Clonan, Wilson and Swift ^{84 )}. Hence, policy makers, nutritionists and health professionals need to increase the public’s awareness not only of health but also emphasise the environmental impact of dietary choices⁽ Reference de Boer, de Witt and Aiking ^{85 )}. This may however prove challenging, considering the limited success that health organisations have had in changing consumers’ behaviour towards fruit and vegetable consumption⁽ Reference Rekhy and McConchie ^{73 )}. Although some subsets of individuals are motivated to eat less meat as it appeals to their psychological desires to protect the environment and promote good health, this motivation is not shared by all⁽ Reference Schösler, de Boer and Boersema ^{86 )}. Nevertheless, de Boer et al.⁽ Reference de Boer, de Witt and Aiking ^{85 )} found that consumers’ willingness to eat less meat increased with its perceived effectiveness in reducing dietary emissions. This highlights the importance of social instruments such as education and consumer awareness campaigns. Improving understanding is difficult when one considers consumers lack of awareness of GHGE generated through diet⁽ Reference de Boer, de Witt and Aiking ^{85 )}. Motivating people to change embedded and habitual food behaviour is compounded by a reluctance within government, industry and even non-government organisations to inform consumers of the climatic impact of diet⁽ Reference Laestadius, Neff and Barry ^{87 )}. Moreover, findings from a recent Eurobarometer study have shown that for consumers in Ireland climate change is not deemed as serious an issue as in other EU member states^{( 88 )}. Asking individuals to trade perceived enjoyment and tradition for sustainability is challenging when the benefits of sustainability are poorly understood. Although 60 % of Irish adults perceive sustainability as being important in the food choices they make, only 34 % fully understand the concept^{( 89 )}.

The climatic burden that accumulates throughout the food system could be potentially lowered not only by altering dietary patterns but also through changing other consumer behaviours. Towards the end of the food chain, consumers throw away food, let it spoil or engage in behaviours which contribute to unnecessary food waste. In Europe, this translates to between 95 and 115 kg of food waste per capita annually^{( 90 )}. Food loss, on the other hand, occurs earlier in the food system. Lowering food waste and food loss represents a potentially effective method to reduce dietary GHGE⁽ Reference Venkat ^{42 )}. Approximately one-third of the food produced globally for human consumption is lost or wasted^{( 90 )}, with as much as 40 % of food lost and wasted in the industrialised world occurring at retail and consumer levels^{( 91 )}. If food waste can be reduced the climatic impact of dietary patterns could be considerably lowered. Furthermore, mitigation measures which address food waste, rather than consumption, may receive less resistance from consumers than other actions⁽ Reference Vanhonacker, Van Loo and Gellynck ^{82 )}.

Consumption of animal-based foods has been linked to both positive and negative health outcomes and there are many contrasting arguments both for and against the consumption of red meat⁽ Reference McAfee, McSorley and Cuskelly ^{92 )}. While research has associated red meat consumption with negative health impacts⁽ Reference Boada, Henríquez-Hernández and Luzardo ^{93 )}, other studies suggest it can have many beneficial health outcomes⁽ Reference Binnie, Barlow and Johnson ^{94 ,} Reference De Smet and Vossen ^{95 )}. Nevertheless, it is important to note that many of the micronutrients derived from meat can also be acquired from consumption of plant-based foods⁽ Reference Temme, van der Voet and Thissen ^{96 ,} Reference McEvoy, Temple and Woodside ^{97 )}. Yet consumers’ reluctance to alter their behaviour with regard to meat consumption may hinder the potential environmental benefits of a more plant-based diet. Therefore, placing the onus on the supply chain to reduce GHGE is of particular importance. Mitigation strategies that occur before the farm gate could theoretically lower emissions associated with consumption of animal products. Reductions in the carbon footprint of red meat at farm level can be as high as 30 % if all producers in any given system, region and climate adopt the efficiency practices implemented by those with the lowest GHGE per unit of production⁽ Reference Gerber, Steinfeld and Henderson ^{17 ,} Reference Hyland, Styles and Jones ^{98 )}. Moreover, other on-farm mitigation measures could lower emissions further⁽ Reference Schader, Muller and Scialabba ^{99 )}. Ireland has implemented a sustainable food programme which enables farmers and producers to set and achieve sustainability targets by providing a framework to deliver long-term improvements in areas such as emissions mitigation with the aim of reducing GHGE associated with food production^{( 100 )}. However, some research suggests that these production mitigation options alone are unlikely to be sufficient; changes in diets are also required to meet EU emission targets⁽ Reference Bryngelsson, Wirsenius and Hedenus ^{101 )}.

Emission factors should ideally account for where food was produced, processed, distributed, etc. Thus a full understanding of the source and supply chain for each food consumed in NANS would be necessary to give an absolute and accurate estimate of dietary emissions. Nevertheless, the methods used in the present study provide a good indicator of GHGE associated with food consumption in Ireland. Indeed, the primary method to appraise dietary emissions in this research area is through sourcing emission factors from the literature rather than ones which are country specific. However, this may be limiting when assigning emission factors to foods with a high carbon footprint that may not be fully applicable in an Irish context. Globally, animal-based foods have considerable variations in emission factor estimates due to differences in methods applied and variation in agricultural production systems⁽ Reference Röös, Sundberg and Hansson ^{11 )}. This was highlighted in the dairy emission factors applied from Finnegan et al.⁽ Reference Finnegan, Goggins and Clifford ^{43 )}, which were lower than those used in other studies. Indeed, a sensitivity analysis revealed that a 20 % decrease in the emission factors used for animal-based products would lead to a 10 % overall decrease in the dietary emissions elicited in the present study (see online ‘Supplementary material 2’, Table S1). It would not be unreasonable to assume other emission factors could also be lower than the ones found in the literature when one considers the highly competitive Irish agricultural sector and low food miles associated with Ireland’s relatively small land area. Taking account of such factors in the future could result in lowering daily Irish dietary emissions.