In order to explore the environmental impact of different dietary regimens, three dietary groups were selected: omnivorous, ovo-lacto-vegetarian, and vegan. The analysis of food patterns highlighted the definition of well-matched diet groups based on participants’ self-reported eating habits.

Conversely to previous data of environment impact obtained on hypothetical diets and meals, the present results are based on recorded intakes and have the clear advantage of being realistic. The three dietary regimens were equivalent to one another for energy content, but differed from the custom diets designed in previous studies7, 28. As an example, the environmental impacts of different dietary patterns (omnivorous, vegetarian, and vegan) were assessed in Italy by designing a weekly well-balanced plan, with a daily average energy intake ranging between 2100 and 2300 kcal7. In another study dealing with the environmental impact of omnivorous, vegetarian and vegan diets, three weekly menus were designed, and the average energy intake resulted to be a little higher than 2000 kcal/d28. Recorded energy intakes were indeed higher than those hypothesised before, especially for O, and showed a remarkable variance within our study groups, but all fell within the reference values for Italian healthy adults31, 32 with a normal BMI. It should be noted that the mean energy intake of our population was similar to a designed dietary plan of 2400 kcal in which the environmental impact of omnivorous, ovo-lacto-vegetarian and vegan diets were compared33. Despite these similarities on energy intake, modelling approaches referring to an average diet may produce unrealistic food combination and quantities, since eating patterns vastly differ among individuals34. Indeed, our three diet groups differed in their nutrient profiles. Specifically, carbohydrate intake was always matching the 45–60% of daily calories recommended for the Italian population31, 32, with the O and V groups close to the lower and the upper values, respectively. Total fat intake fell within the reference range of 20–35% of daily calories31, 35 only in the V group, while O and VG volunteers had fat intakes slightly over the recommended values. Protein intake was within the recommended 10–20% of daily calories36 for all the diet groups. When compared to dietary models, which are standardised to match the energy contribution for protein, lipid and carbohydrate with recommended values7, 28, our assessment revealed: 1) lower intakes of carbohydrates in the O and VG groups, 2) lower intakes of proteins in the VG and V groups, and 3) higher intakes of lipids in all the diet groups, especially for O and VG.

These differences between real and hypothetical diets underline the need to observe and analyse recorded dietary intakes in order to properly assess the environmental impact of dietary approaches. Only a few studies have assessed the environmental impact of actual dietary intake calculated on the basis of CF, WF, and EF simultaneously. Both Donati et al.30 and Germani et al.29 evaluated the carbon, water, and land footprints of the actual food intake of the Italian adult population, but without taking into account any differences in the dietary regimen. Data on the environmental impact of the Italian household consumption are consistent with our results for all the three environmental indicators29. Germani et al. also compared the actual Italian food consumption to a MD model, highlighting the lower value of the three indices in MD compared to the actual Italian diet29. Similar results were found in other studies, in which a shift of the Italian average food consumption towards a MD pattern resulted in a reduction of the food environmental impact on natural resources, especially on greenhouse gas emissions37 and water usage38. Indeed, in accordance with numerous studies, the MD emerges as a dietary regimen with beneficial environmental and health effects14, 29, 39, 40. Along with the nutritional profiles, the notable adherence to the MD revealed a general healthy eating pattern of our participants in all the three dietary regimens. These results are consistent with those previously reported by Benedetti et al.41 in which Italians appeared to have a moderately high level of adherence to the MD.

In addition to the MD, other predominantly plant-based diets appear environmentally better than meat-based ones21, 42, 43. Our results are consistent with those reports, since, in the present study, VG and V diets represent a clear environmental advantage with respect to the O one for all the three environmental evaluated indicators. This aspect, which is related to the biggest environmental impact generated by a greater consumption of animal products, had already been hypothesized7, 28.

However, the big differences highlighted when observing virtual scenarios are not so evident when taking into account real-life contexts. In particular, the V approach was not associated with significantly lower environmental footprints when compared to VG one. A likely explanation might be that, while unprocessed plant-based foodstuffs usually replace animal-based products in hypothetical vegetarian and vegan diets18, the real plant-based diets are instead characterised by industrially highly-processed plant-based meat and dairy substitutes (e.g. seitan burger and soy yoghurt). Some people opt for highly processed, high-fat products instead of nutritious plant-based foodstuffs44. Moreover, the lower energy density of plant-based foodstuffs results in a higher food intake for V with respect to VG (around 12.5% in terms of food weight), possibly explaining the lack of environmental benefits of a vegan diet in comparison with an ovo-lacto-vegetarian choice.

Comparison of the environmental impact of different populations remains very difficult, as data found in the literature vary widely among studies. Differences in geographical areas, data sources, and dietary assessment strategies could represent explanatory factors for this variability45. In the present study, as an example, CF equalled 3.96 ± 0.98, 2.60 ± 0.62, and 2.34 ± 0.50 kg CO 2 -eq per person/d for the O, VG, and V group, respectively, as assessed with a 7-d food record. Without considering the specific diet of the participants, the same parameter was 3.44 kg CO 2 -eq per person/d in the Italian INRAN-SCAI cohort (2313 subjects, aged 18–65 years) using a self-recorded 3-d dietary record29. Differences among European populations were detected in various studies resulting in: 3.24 kg CO 2 -eq per person/d for the UK-NDNS cohort (1491 subjects, aged 19–94 years), assessed through a 4-d food record46; 3.55 and 4.69 kg CO 2 -eq per person/d for women and men, respectively, of the INCA2 French cohort (2624 adults) recording a 7-d food record47; 3.87 kg CO 2 -eq per person/d for the Nederland EPIC cohort (40011 adults), measured through a food frequency questionnaire48; and 3.76 and 5.04 kg CO 2 -eq per person/d, if evaluated with a food frequency questionnaire or with a 7-d weighed dietary record, respectively, in 166 Sweden volunteers49.

Beyond the impact of dietary choices, variations in the average environmental impact of a particular diet or a population have been attributed to differences on individual choices50, gender51 or household structure52. In the present study, the high inter-individual variability registered within each dietary group is worth attention. In particular, it is interesting to note the presence of some VG and V subjects with individual values of environmental impact higher than some of the O study group. The high impact of these specific subjects was mainly attributed to a high consumption of proteins and fats, but energy intake also contributed. High meat consumption and high-fat vegetarian diets were hypothesized to require more land resources than other food choices53. Inter-individual variability was also conditioned by energy intake of each subject, which is associated with adequate nutritional needs and influenced by energy density of the diet. When impact data were adjusted by energy intake, as also reported by Soret et al.44, the analysis brought to light an extremely high environmental impact of some subjects, in particular fruitarians.

A focus on a specific population, even though participants belong to four geographically distant areas in Italy, and the fact that seasonality, farming and cattle rearing typology, and food production methods were not taken into account for the environmental impact evaluation should be considered the main weaknesses of this study. In addition, the environmental database used presents some intrinsic limitations. Although it is regularly updated and based entirely on public data and information from scientific literature, it cannot be considered complete. As a consequence, data related to food subgroups and not to the single food item were used. Correspondence among specific foodstuffs and the subgroup to which these foods were assigned, as well as the environmental impact values for each food subgroup, are provided at Supplemental Tables S1 and S2, respectively. The WF of fish and seafood were not available and the environmental data on coffee and tea were not easily attributable because linked to dry powder and not to ready-to-use products. Nevertheless, this is the first study to evaluate simultaneously the carbon, water, and land impact of real recorded and weighed food intakes in groups, although not large, of O, VG, and V.

To reach an environmentally sustainable solution, animal-based foodstuffs should be partially replaced with fruits, vegetables, legumes, and cereals, according to nutritional guidelines. At the same time, observations regarding inter-individual variability are of critical importance as the generic definition of every dietary pattern may dramatically conceal the impact of individual choices on environmental footprints. This emphasizes the need for thinking not only in terms of dietary group but also of individual dietary habits, irrespective of dietary choice. These aspects should be considered before launching equivocal messages to the general population through the media, and/or underpinning certain dietary choices.

Dietary recommendations in terms of environmental impacts deserve further studies, considering, for instance, the choice of locally grown and seasonal products as well as agricultural and processing techniques. Health status should be also included in this multifactorial scenario dealing with food choice, energy intake, and environmental impact. This holistic approach, merging health preservation and disease prevention with environmental sustainability in the dietary habits framework should be pursued. Educating people to make little changes in their dietary behaviours could be a key action towards this common goal.