What is a sustainable diet?

Have you watched David Attenborough’s film ‘A life on our planet’ yet? Did the first half scare you? It certainly worried me. The impact of a growing global population on a planet undergoing an environmental sustainability crisis… but thankfully as the second half implies there is something that we can all do.

Extensive evidence indicates that an effective solution to protect the environment (whilst also being great for our health too), is to move towards a more plant-based diet (one where meals are based around grains, fruit, vegetables, legumes, nuts and seeds and if included, animal foods are eaten in much smaller amounts)(1). Why? Let’s take a closer look.

Figure 1: Our current food system

Source: British Dietetic Association – One Blue Dot

Figure 1 illustrates our existing food system, of which each stage has a significant impact on our environment. Shockingly, it’s currently responsible for:

20-30% of Greenhouse Gas emissions (GHGe) globally.

Deforestation, biodiversity loss, soil and water pollution around the world.

70-80% of all global human water use.

85% of fisheries being fully exploited and/or overfished.

10 tonnes of avoidable waste – around 25% of food purchased is spoiled or wasted every year (meaning an additional waste of land and water and ‘unnecessary’ GHG production)(2).

GHGe are responsible for global warming. In 2017, the global temperature was recorded as approximately 1°C above pre-industrial levels and increasing at a rate of around 0.2°C per decade(3). In order to try to prevent the catastrophic consequences of extreme weather conditions, the UN Intergovernmental Panel on Climate Change (IPCC) proposed a safe limit for increase of no more than 1.5°C by 2050. To achieve this, we need to reduce GHGe by 70-95%(4).

As you can clearly see from figure 2 below, when it comes to our current consumption patterns and food system, livestock farming is by far the biggest contributor of GHGe. Almost all of these are from ‘enteric fermentation’ (a natural part of the digestive process in animals such as cows and sheep, where microbes in their stomach decompose and ferment food, producing methane as a by-product), manure, the production of animal feed and the carbon dioxide emissions that occur from clearing land. As pigs and chickens are ‘non-ruminants’, they contribute less GHGe than cattle(5).

Figure 2: % GHGe from foods in the UK diet

Source: British Dietetic Association - One Blue Dot

The study illustrated in Figure 3 demonstrates the amount of GHGe linked with different types of common diets. It further demonstrates that as individuals reduce their meat consumption, the rate of GHGe reduce significantly.

Figure 3: GHGe associated with different types of diets

Source: Scarborough, et al. (2014) via FRCN(6)

This is not to say that a sustainable diet has to be vegan or vegetarian but it does indicate that even small but achievable steps to adapt your eating habits to contain less meat and dairy can have a beneficial effect on our environment.

This wouldn’t just be due to a reduction in GHGe, as livestock farming is linked with many other impact factors as well…

Source: FCRN. (2016)(5)

The large majority of soy produced globally is used for animal feed (7) (see figure 5), yet its production has been a key cause of deforestation and biodiversity loss, particularly within the Amazon. This is extremely significant as the Amazon rainforest forms a large part of the carbon cycle (have a quick google if you are unsure what this is), absorbing ¼ of all carbon each year, so a decline in its ability to do this puts our environment in trouble(1).

Figure 5: What Soy is being used for

Source: FCRN. (2020)(7)

An important point to make clear, is that assessing the sustainability of food is very complex due to the massive number of different factors involved at each stage of our food system. Consequently, focusing on only one impact measure could result in others important considerations being ignored and affect the overall interpretation. Figure 6 clearly draws the data for GHGe, land and water use when producing a variety of protein foods, together in a visual representation. Overall, plant proteins generate less GHGe, use less land and less water than livestock farming. However, there is one exception when it comes to water use; nut agriculture, particularly almonds, is water intense despite the lowest GHGe per 100g. This is important when nuts are grown in regions where water is already scarce and receives little rainfall (global warming may also cause dry lands to become dryer and wetlands to get wetter). It also highlights that impact measures do not always correlate with each other. There are also still many issues that this graph does not display.

Figure 6: GHGe, Land and Water use per 100g protein

Source: British Dietetic Association - One Blue Dot

I think it’s now clear why we need to reduce our consumption of meat and dairy, and that we should replace these with more environmentally friendly plant foods. However, when it comes to the latter, is there anything that we should be mindful of? The answer is yes (if able).

In general, lower impact produce are those that are field grown (for example not grown in greenhouses), transported shorter distances by road or sea (not air) and those that are more robust (i.e. require less rapid transport modes such as air) and less perishable, thus less prone to being spoiled or wasted(5) – see figure 7 below.

Figure 7: Hierarchy of GHGe from fruits and vegetables

Source: FCRN. (2016)(5).

Different regions and countries have better growing conditions for certain foods, meaning that, even after transport, the total GHGe of imported food can be lower than home-grown food, so “food miles” alone are not always a good indicator of sustainability(5). Figure 8 illustrates how lettuce grown out-of-season in the UK in Winter and thus indoors, in a fossil-fuel heated and lit greenhouse, has a higher GHG footprint compared to lettuce grown in fields in Spain, where the weather is naturally warmer during this time and then imported the fairly short distance by road or sea to the UK. Nevertheless, UK lettuce grown in season, over the summer and thus in fields, as opposed to heated greenhouses, produces the lowest amount of GHGe. It can therefore be extremely useful to try and pick foods in-season, where able.

Figure 8: The effect of season and location on GHGe from lettuce production.

Source: Edwards-Jones, G., et al. (2008) via FRCN(5)

Freezing food requires energy and packaging. While some research shows higher GHGe for frozen, canned and packaged vegetables versus fresh, if less food is wasted from frozen produce, then the difference in GHGe per consumed food may be less significant(5). Furthermore, if you eat frozen produce “out of season” that has been produced locally, this could offset the transport GHGe of eating fresh produce that has been transported long distances by air.

As a result of all the evidence on sustainability and the environment, The British Dietetic Association (BDA) recently developed recommendations for achieving a sustainable diet(2)

1. Reduce your red meat consumption, especially beef and lamb (to less than 70g per day OR 500g per week (cooked weight) - 70g is equivalent to a piece of steak about the size of a pack of cards, 3 average-sized rashers of bacon or slices of ham or a sausage) and avoid processed meat. Avoid compensating reductions in red meat by increasing white meat consumption, instead prioritise plant food sources of protein due to their much lower environmental burden and rounded nutritional profile.

2. If you consume fish, focus on sustainably sourced fresh water and farmed fish (look for the MSC logo for sustainable wild fish or ASC logo for sustainable farmed fish). The Marine Conservation Society (MSC) also do an app that some may find useful when deciding which fish to buy.

3. Moderate dairy consumption, as dairy cattle, like beef cattle are environmentally burdensome. If plant-based alternatives are used, calcium and, where available, iodine fortified variants should be chosen.

4. Increase your fruit and vegetable intake. Try opting for seasonally and locally produced fruit and vegetables which are not grown in greenhouses. If out of season fruit and vegetables which are grown abroad are consumed, canned or frozen variants will be more sustainable. Any produce that is airfreighted, pre-packed and / or pre-prepared or grown in greenhouses is best reduced/avoided.

5. Be mindful of portions. Over consumption requires excess food to be grown and produced.

6. Limit food waste, due to its significant environmental impact, secondary to both resource use and gases released during decomposition. As figure 10 highlights, it is also extremely expensive for households. Try buying tinned/frozen produce and remember ‘best before’ dates are not the same as ‘use by’.

Figure 10

Source: British Dietetic Association - One Blue Dot

N.B. The scope of this discussion does not include important social aspects of sustainable diets such as inequities in access to food and income-related issues of household food insecurity. It is also acknowledged that there may be some benefits to animal farming, which are not discussed here i.e. under the right conditions grazed pastures can store increased levels of carbon, some livestock consume agriculture by-products, such as parts of plants inedible to humans, by-products such as wool, for example(1).


1. PEN The Global Resource for Nutrition Practice (2009) Sustainable Food Systems: Plant-based Diets and the Environment Background {online} viewed 21st October 2020 <>

2. The Association of UK Dietitians (2020) One Blue Dot - The BDA's Environmentally Sustainable Diet Project {online} viewed 21st October 2020 <>

3. Allen, M.R., O.P. Dube, W. Solecki, F. Aragón-Durand, W. Cramer, S. Humphreys, M. Kainuma, J. Kala, N. Mahowald, Y. Mulugetta, R. Perez, M. Wairiu, and K. Zickfeld (2018) Framing and Context. In: Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [Masson-Delmotte, V., P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J.B.R. Matthews, Y. Chen, X. Zhou, M.I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, and T. Waterfield (eds.)]. In Press. {online} viewed 21st October 2020

4. Garnett, T., & Finch, J. (2016). How can we reduce food-related greenhouse gas emissions (Foodsource: chapter 4). Food Climate Research Network, University of Oxford. {online} viewed 21st October 2020

5. Garnett, T., Smith, P., Nicholson, W., & Finch, J. (2016). Food systems and greenhouse gas emissions (Foodsource: chapter 3). Food Climate Research Network, University of Oxford. {online} viewed 21st October 2020

6. Garnett, T., Scarborough, P., Finch, J. (2016). What is a healthy sustainable eating pattern? (Foodsource: chapter 9). Food Climate Research Network, University of Oxford. {online} viewed 21st October 2020

7. Fraanje, W. & Garnett, T. (2020). Soy: food, feed, and land use change. (Foodsource: Building Blocks). Food Climate Research Network, University of Oxford. {online} viewed 21st October 2020