A plant-based diet has a lower impact: myth or not?

As a science-based method, LCA is an excellent tool to bust the myths that surround sustainability. In this monthly series, we look at some common sustainability ideas to see if they are myth or true. In today’s episode: reducing impact by going without meat.

The relationship between food and environmental impact is a popular topic of discussion and debate. Traditional wisdom states that eating a plant-based diet is better for the environment than consuming meat. The “meatless Monday” movement, for example, claims that one good reason to go meatless one day per week is to reduce our carbon footprint and conserve resources such as water and fossil fuels.

However, in December of last year, meat lovers everywhere celebrated when a new study was published stating that “vegetarian and healthy diets could be more harmful to the environment”. As a vegetarian myself, I am very interested the research on this subject. So, myth or not? What type of diet is better for the environment?

Choosing the right functional unit

Anyone who has conducted an LCA study understands the importance of choosing a functional unit: the basis for your comparison. In many cases, you can’t simply compare product A and product B, because they have different performance characteristics.

For example, a disposable water bottle can only be used once, while a reusable water bottle can be used many times. If the purpose of the LCA is to compare packaging systems, you can’t compare the impact of one bottle to the impact of the other bottle. A much better approach would be to compare drinking a set amount of water (say 100 liters) out of each.

What can go wrong if you pick the wrong functional unit?

Choosing the right functional unit is also very important when comparing vegetarian diets to diets that include meat. The study mentioned above, by Carnegie Mellon, found that “eating lettuce is more than three times worse in greenhouse gas emissions than eating bacon.” When I first saw this title, I immediately wondered about the functional unit – and it turns out that the functional unit the researchers chose in this study was calories.

In my opinion, that’s misleading. Lettuce has very few calories and bacon is very high-calorie meat. To get the same number of calories from lettuce as you get from a few strips of bacon, you need to eat a lot of lettuce. Three strips of bacon have about 130 calories. For comparison, an entire head of lettuce has only 50 calories. Do you know anyone who would choose to eat three heads of lettuce as a substitute for three strips of bacon? Nutritionally, these are very different – they serve very different functions.

What functional unit should we use to compare diets?

If we compare meat products and vegetable products on a per kilogram basis, we see very different results. While this may also not be a perfect functional unit, since bacon and lettuce are not comparable nutritionally, it shows how important it is to select a functional unit that makes sense.

This is what the comparison looks like per kilogram. As you see, pork has more impact in all three categories, especially carbon and energy. The data is from the Agrifootprint (economic allocation) library, one of several LCI libraries dedicated to agricultural products.

Product Carbon (kg CO2e) Energy (MJ) Water (m3)
Pork 3.33 12.5 0.0221
Carrot 0.0913 0.459 0.00306
Spinach 0.246 0.793 0.017

An even more appropriate functional unit might compare the impacts of eating a balanced diet that includes meat with the impacts of a balanced vegetarian diet over a period of time, rather than per calorie or per kilogram of one specific product. Even measuring an entire meal, rather than one product, would paint a more complete picture.

A 2010 study by ESU-services found in a number of examples that vegetarian meals had about 1/3 the impact of meat-based recipes. Again, there are many factors and individual choices that can be made, so it would be likely difficult to give a definitive answer.

Low-impact plant-based diet: myth or not?

Status: Confirmed

While this new study brings up an interesting point about the efficiency of delivering calories, blogs and other news outlets took the conclusions much too far by stating that bacon is better for the environment than lettuce.

The important take-away is not that we should swap all meat for vegetables, but that we should consider the alternatives given the purpose of each. Of course, there are many other important factors at play in modeling these products, such as different allocation methods, but overall I think this myth is mostly confirmed.

Uncover more sustainability myths

This is the tenth part of our ‘Sustainability Mythbusters’ series. See other episodes:

Paula Bernstein

Paula Bernstein worked for PRé from 2013 to February 2018. Her areas of expertise included environmental product performance, LCA databases, and supply chain sustainability measurement. Paula collaborated structurally in LCA and sustainability metrics implementation projects for many industries, such as apparel, food, and building & construction. She also worked closely with the PRé software team to implement databases in PRé’s software package SimaPro.

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