Gennaio 13, 2016

38 heads of lettuce per day or 47 slices of bacon?

38 heads of lettuce per day or 47 slices of bacon?

Controversial or polemical topics have a better chance to go viral on social media. Unfortunately, articles that deal with these topics are often also the ones that tend to omit the necessary fine prints to appropriately interpret them, or the ones that use overgeneralization. Scientific articles that go viral are no different.  


The latest example of this trend is an article published by a group of researchers from the Carnegie Mellon University (Environment Systems and Decisions, Tom et al., online first). The study compares the energy requirements, water footprint, and greenhouse gas emissions of diet regimens recommended in recent advice from the US Department of Agriculture (USDA). The authors found that following the recommended USDA diet with reduced meat intake and increased dairy, seafood and vegetables would result in an increase of the related energy demand by 38%, of the water consumption by 10% and of greenhouse gas emissions by 6%.


The universal mantra that echoed on media outlets around the globe, and that was instantly shared thousands of times on social media, translated the results to lettuce produces more greenhouse gas emissions than bacon does. A battle of clicks and vehement comments burst between vegetarians, vegans and meat eaters. Incidentally, the study does not evaluate vegetarian diets or meat-diets. Moreover, the authors never actually compared lettuce to bacon, nor mentioned them before a press release of their University.


What’s left to say, then, about lettuce and bacon?


All the results in the study are calculated based on the caloric content of food. Using the calorie as a common unit, as the study does, we may well say that, YES, lettuce is actually environmentally worse than bacon. However, one head of lettuce has circa 53 calories per 350 grams (0.78 pounds) of product and a slice of bacon of 8 grams (0.02 pounds) has 43 calories. In order to fulfil with lettuce the recommended daily 2000 calories intake, one would have to consume38 heads of lettuce. Who would eat such an amount of lettuce? And also, who would eat 47 slices of bacon for the same purpose? We can conclude that the calorie is not an appropriate unit of comparison. Using the fiber content, protein content or the servings to compare the environmental burdens of bacon and lettuce or any other produce, would have resulted in more valuable and realistic outcomes.


Another methodological aspect of the study is not viral either. The authors assess alternative dietary regimens using a meta-analysis of existing life cycle assessment (LCA) studies. LCA is a well-established technique that allows quantifying the impacts of any product throughout their entire life cycle. Meta-analysis of LCA results is not an unusual practice. Recently, the well-known Intergovernmental Panel on Climate Change (IPCC) also used a meta-analysis of LCA studies to calculate the carbon dioxide intensities of alternative energy sources. The results of IPCC became a standard scientific reference. However, comparing alternatives using the results of different LCA studies without taking care of the assumptions that generated them is risky. Not all LCA studies are created equal. Great differences exist in terms of quality, scope, geographical extent, reference year of the data, and uncertainty of results.

These details warn us against the risk of overgeneralization. Scientific communication on hot topics that go against the received wisdom loses, at times, key details along the way. These details are fundamental to objectively judge the quality of a study. Reading the sources and contacting the domain experts is often key to avoid claims that are not supported by science. A healthy diet and a reduced consumption, especially of red meats like beef and lamb, are still key to a healthier environment and a healthier living. This has been demonstrated by many influential scientific studies. This is a fact. Free interpretations of results, or lettuce versus bacon debates, are just based on unsupported opinions.

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About Stefano Cucurachi

Stefano Cucurachi

Stefano Cucurachi, Ph.D., is a postdoctoral scholar at the Bren School of Environmental Science and Management of the University of California, Santa Barbara (UCSB). Stefano joined UCSB after working as a researcher at the University of Leiden, The Netherlands, and at the ETH-Zurich, Switzerland. Stefano’s expertise is on quantitative sustainability assessment, with a focus on life cycle assessment (LCA). Stefano’s work currently focuses on the way that quantitative sustainability assessment can be improved to better help decision-making for sustainability. He has contributed to the sustainability sciences with a number of publications in the area of quantitative methods and uncertainty analysis.

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