What does a low carbon diet look like?

For most of us, once we’ve reduced our carbon emissions from travel and in the home, the next place to look is at our diet. The carbon emissions from the average European diet are around 1.5 – 3 tonnes CO2e/year (depending on how much of the food chain is included), as part of a total carbon footprint of around 9 tonnes CO2e/year (data from here and here). We can look at carbon emissions from our diets in two basic ways; we can take a ‘typical’ diet and calculate the overall total based on the quantities of things eaten, or we can look at data for a particular foodstuff with a view to eating more or less of it.

There have been a plethora of studies on the environmental impact of diet, and since different assumptions can lead to different conclusions it can be difficult to know which reports to believe. It’s then more difficult to know from a secondary source the extent to which data has been cherry picked to suit a particular agenda (or whether there was any primary data source in the first place). So as when reading about all things, I suggest checking the references.

Originally a statistical tool used to assess the effectiveness of pharmaceuticals, meta analysis and systematic review are standardised ways of collating and analysing data across multiple studies in order to draw a single set of conclusions. Quality criteria for studies are set, followed by a large data gathering exercise, and then some re-analysis to make results comparable. It’s not foolproof, but it’s the best approach available. The systematic review of the environmental impact of diet that I’m drawing on here is by Hallstrom and colleagues in 2014. It’s available here. It included studies on vegetarian and vegan diets, and then some intermediate diets (e.g. beef/lamb replaced by pork/chicken, and low meat diets). Results were all compared to a normal ‘reference’ diet, and are illustrated in the figure below.

diet improvements
% reduction in GHG emissions from broad dietary changes compared to a ‘reference diet’. From here

As you can see, almost all the diets studied had lower impacts than the ‘reference’ diet, often by 20-50%. Vegan diets had the lowest impact, followed by vegetarian. Diets replacing ruminant meat (beef, lamb) with monogastric meat (pork, chicken) also had lower impacts. Even having a ‘healthy’ diet had a lower impact than reference diets. As you can see, the precise decrease in impact varied from one study to another as well as between broad dietary groups, which is not surprising given the differing assumptions. This variability does demonstrate the importance of not being obsessed with precision in studies of this type; the range of choices within each dietary type that we might make if we follow it ourselves might render a precise answer unhelpful.

Keep it simple – reduce consumption of beef and lamb

Differing results between studies should not distract us from the important facts. Ruminant agriculture is environmentally disastrous; a fundamental feature of the digestive systems of cattle and sheep (that have evolved to break down cellulose) is that they produce significant methane, which is a more potent greenhouse gas than CO2. It is possible to reduce methane production from ruminants (by feeding them differently, or using different farming techniques), but it is disingenuous to dwell on this in comparison to the really simple action of not eating ruminants in the first place. As it stands, ruminant agriculture is responsible for around 15% of anthropogenic GHG emissions, and as populations shift to more ‘western’ diets, the problem will worsen (the rate of increase in the number of farmed ruminants on the planet is about 25 million/year). There is a straightforward and short discussion of the problems of ruminants in relation to climate change here (it is much more readable than many scientific papers).

I live in Wales, and hill farmers will often use the ‘there isn’t much else we could do with the land’ argument in favour of upland sheep and cattle, but the maths is really simple; whilst there are lots of environmentally positive things that could be done in the uplands (e.g. managing for wider ecosystem services) which we can calculate the impacts of, even doing absolutely nothing with the land would be far better from a climate change perspective than grazing ruminants on it, because it would reduce methane emissions.

But I read a study that said cattle were ok really! Please can I believe that one instead?

There is a school of thought that says that grass feed cattle are actually good for the climate because the grasslands they graze on sequester carbon, and the manure they produce is good for the soil. However, the argument does not stand up to detailed scrutiny; this would only offset around 20-60% of the emissions from grazing systems (depending on assumptions) and any beneficial effect would slow down and stop entirely as the soil reached carbon equilibrium. The ‘grass fed cattle are good’ argument is discussed at great length in ‘grazed and confused’, available here (8 page summary), and here (in all it’s 127 page glorious detail).

The discussion so far has been limited to diets in affluent countries where the overall level of nourishment is high, and we have highly effective food supply chains where we can buy what we want. However there is an ‘ecological leftovers’ school of thought that states that if the animal is reared on land that is not suitable for any other form of food production and consumes no food that could otherwise be fed to humans, then one might consider it to be an entirely appropriate way of providing dietary protein where it might not otherwise be available. There may well be countries/climates/societies in which this is true, but the UK is not one of them! The issue of whether ruminants can be said to be efficient in this way is discussed in a fascinating and somewhat philosophical paper here.

Protein, pleasure and functional units

Life cycle assessment studies always refer to what is known as a functional unit; the service delivered. This enables comparisons to be made between scenarios and is specified in such a way so as to compare like with like. In studies of whole diets, the functional unit generally relates to a year of a nutritionally balanced diet. But for individual foodstuffs it’s more complicated; we can’t compare a kg of chick peas with a kg of beef. Should we be comparing it per unit of protein, or according to its calorie content, per ‘portion’, or on some other measure? Food has a hedonistic value; don’t always eat or drink because we need to, it’s far more complicated than that. Most people would agree that a £10 bottle of wine is nicer than a £5 bottle of wine, and that we don’t eat chocolate for its nutritional value. At this point, if you’re counting carbon you could probably work out the carbon impacts of the various forms of pleasure you partake in. Or you could just properly savour the occasional bacon sandwich and not worry about it. Pleasure is important!

Our choice of protein source is a key determinant of the carbon emissions from our diet, and if we start from the perspective that we are sufficiently well nourished and we are eating meat/cheese/eggs largely for pleasure rather than as protein, then we would compare GHG emissions for a ‘portion’ of each of these foods. Assumptions on portion size are likely to be rather arbitrary – obviously one can choose to be greedy or abstemious when eating a slab of meat. But it also allows us to think about the issue in a slightly different way; processing and/or cooking methods can allow the same amount of meat to go further. This could be via a beef burger having a relatively low meat content (in contrast to the 100% ‘premium quality’ labelling that is frequently used to market meat), or via bulking out a bolognese or a stew with other ingredients.

Expressing results per unit of protein is made even more complicated by the fact that not all protein sources are nutritionally similar; vegetable proteins are harder to digest than animal protein for example. Disclaimers done, let’s look at foodstuffs using protein as a functional unit. The most readable paper on this is a compilation/comparison of LCA studies published here.

kg protein
GHG emissions from various feedstuffs, expressed as kg CO2 equivalents per kg protein. Each dot represents a data point from an individual LCA study. The bands represent the range of results obtained, which vary according to the production system under study and the assumptions made. Taken from here.

Not surprisingly, beef and lamb are particularly high in GHG emissions. Extensively farmed beef (grass fed) generally has higher emissions, because the system is less ‘efficient’; the animals eat more and take longer to grow to slaughter weight than if they are intensively reared. The relatively low impact of beef from dairy cows illustrates difficulties in attributing environmental impacts in a system that produces more than one product. This is a common problem in LCA, and is discussed at length in relation to dairy/beef here. Whilst most studies showed seafood to have relatively low GHG emissions, the range of results are skewed by a study in which the LCA of bottom trawled fish was calculated (lobster was found to be particularly GHG intensive).

The striking thing about this graph is that because beef and lamb are so bad, it makes the differences between other sources of protein look relatively minor; at a simple level, the ranges of results from the different studies overlap by quite a lot for the various potential protein sources. We could get bogged down in the detail of different production systems and assumptions here, but another way of looking at it is to look at the total GHG emissions, compared to the emissions from other aspects of our lifestyle. Dietary guidelines suggest about 0.8g of protein per kg of body weight per day. That’s around 56g of protein per day for an average man. If consuming anything other than beef, lamb, or seafood, the protein component of this man’s diet is responsible for up to around 2kg CO2e/day. This might be 5-10% of his total lifestyle impact. The other very simple way of looking at this is to say that a certain amount of protein is essential, and that beef/lamb are somewhere between 5 and 25 times worse than other protein sources. So whilst we could argue the toss about whether to eat an egg, some chickpeas or some tofu, it probably doesn’t matter very much from a GHG perspective, as long as we’re not eating beef or lamb.


What about everything else we eat?

I’m deliberately not writing about fruit and veg, or starchy foods, or fats. Whilst there are a plethora of studies on the GHG emissions of these foodstuffs, and we could talk about production systems and food miles, changes in these areas are relatively insignficant unless you are already vegetarian or vegan, as shown in the pie chart below.

average uk diet
Contribution of major food types to the GHG emissions for a typical diet in the UK. Drawn using data from here.

If you’re interested in data on other foods, there are simple summary tables in the WWF ‘how low can we go’ report on GHG emissions from UK food available here; page 37 for individual foodstuffs, page 45 for the contributions to a typical UK diet (this is the source I have used for the pie chart above). The Food Climate Research Network (FCRN) also have excellent resources on their website.

Or we could just stop eating so much?

25% of the UK population are obese (body mass index, BMI >30), and 62% of us are either overweight or obese (BMI greater than 25). Leaving aside the health impacts of this, it’s pretty obvious that eating less food will reduce your carbon impact, especially if the reason for the weight problem is excessive fat consumption (which is often going to come from meat or cheese). It’s probably also true that people with weight problems eat a higher proportion of highly processed food, which will also add to the carbon impact. The first graph in this post showed the range of GHG emission reductions from eating a ‘healthy’ diet (generally at least 10%) compared to a typical diet. Regardless of whether or not you’re overweight, all of us could usefully eat smaller portions of carbon intensive foods.

Final thoughts

The best thing you can do is to go vegan; whilst the assumptions underlying LCAs are always worth scrutinising, the quantity and quality of data on the climate impacts of livestock production make this difficult to dispute, whether you consider it at the level of an individual food source or as part of a broad diet. But veganism is not popular with everyone, and as you’ll have seen from the discussion above, there are plenty of middle ground options that make a significant difference to your carbon footprint. The single biggest thing you can do is to avoid beef and lamb. If you eat pork or chicken (preferably chicken) instead, that’s ok. It’s not as good as being vegetarian or vegan, but it will drastically reduce the climate impact of your diet. More processed vegetarian/vegan foods (tofu, quorn etc) also have much lower impacts than beef/lamb, so switching to these for a few meals a week is also a good option. And it is certainly the case that low carbon dietary choices are also going to be better for your health.


Postscript: This blog post may come across as rather militant. I don’t claim any moral high ground on this subject, it’s just something I’m interested in. I am not vegan, and I don’t think I ever will be. I grew up in a family with a very traditional ‘meat and two veg’ diet; I ate meat every evening, and cheese most lunchtimes. Since becoming more interested in the environment in my 20’s, my diet has changed a lot. These days I limit my meat intake to once or twice a week, almost always pork sausages from the local butcher (fried breakfast is one of life’s greatest pleasures). I don’t drink much milk; enough for about 2 cups of tea a day. I eat cheese about once a week. I love chickpeas, and I also eat tofu and quorn. I eat an egg most days, and a couple of fish fillets a week. I probably eat beef or lamb once every couple of months. I am not suggesting that this is an ideal diet.

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