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Further Attention to the Environmental Implications of Dietary Choices

January 5, 2016
Michelle Tom

This post is written by Michelle Tom, lead author of Energy use, blue water footprint, and greenhouse gas emissions for current food consumption patterns and dietary recommendations in the US, and provides further commentary on her paper. Michelle is a research associate at Carnegie Mellon University who recently earned a PhD in Civil and Environmental Engineering at CMU. Her research investigates the relationship between human health and natural resource use and greenhouse gas emissions within the transportation and food supply sectors of the U.S. 

This post is written by Michelle Tom, lead author of Energy use, blue water footprint, and greenhouse gas emissions for current food consumption patterns and dietary recommendations in the US, and provides further commentary on her paper. Michelle is a research associate at Carnegie Mellon University who recently earned a PhD in Civil and Environmental Engineering at CMU. Her research investigates the relationship between human health and natural resource use and greenhouse gas emissions within the transportation and food supply sectors of the U.S. 


I’m writing this piece as a follow-up to the excellent blog that appeared on the FCRN website that summarizes our paper “Energy use, blue water footprint, and greenhouse gas emissions for current food consumption patterns and dietary recommendations in the US,” recently published in Environment, Systems, and Decisions. This follow-up will address some interesting questions that we’ve received regarding our study and hopefully further clarify our results and their implications.

Two of the main food groups that drive our results are fruits and vegetables.  In our analysis we find that fruit has the greatest cumulative energy use and blue water footprint per Calorie, while vegetables represent the third highest energy use and blue water footprint per Calorie. The environmental effects of fruits and vegetables are also compounded by USDA recommendations for similar increases in consumption of both fruits and vegetables.  Thus, it can be inferred that an iso-Caloric substitution of all fruits with vegetables would yield better environmental outcomes.  This then begs the question:  to what extent would cumulative energy use, blue water, and emissions be different if vegetables were preferred over fruit?

A preliminary analysis reveals that adherence to an iso-Caloric USDA diet in which increases of fruits are substituted with vegetables, would be accompanied by an increase in energy use, blue water footprint, and greenhouse gas emissions, relative to the current US diet; however, this increase would be less than what we would see if fruit and vegetable consumption increased in roughly comparable amounts, as advised by the USDA.  Specifically, an all-vegetable increase would lead to increases of approximately 30% for energy use, 6% for blue water footprint, and 10% for GHGs.  In contrast, in an iso-Caloric USDA diet, including fruits, energy use increases 43%, blue water increases 16%, and emissions increase 11%.

In all of our models we assume standard food mixes of typically consumed fruits and vegetables in the United States.  But for each dietary scenario, we’ve adjusted the overall Caloric intake of fruits and vegetables without altering the percentages of individual foods (broccoli, carrots, apples, etc.) that comprise the food groups.  For example, broccoli and carrots make up roughly 1% and 2%, respectively, of overall vegetable intake in the US.  Under dietary Scenario 2 (iso-Caloric shift to the USDA recommended diet), the USDA recommends that Americans incorporate 84% more vegetables into their diets.  Thus, for our analysis we increase the Caloric intake of broccoli and carrots by 84%, keeping the proportions (1% and 2%) the same.  The percentage allocation of Calories to individual foods is based on the USDA Loss-Adjusted Food Availability data series.

A question that is always important in scientific inquiry is the generalizability of results beyond the sample population.  In this case, this may refer to the applicability of our findings to other advanced industrialized countries such as those in Western Europe.  The impact intensity data of individual foods evaluated in our research are drawn from numerous environmental life-cycle assessment (LCA) studies.  Due to lack of US-based data, many of the LCAs, particularly for energy use, were conducted in industrialized European nations.  Nonetheless, we believe that our findings are specific to the United States since consumption patterns and recommended diets differ among countries.  In Germany, for example, Meier and Christen (2013) find that an iso-Caloric shift from the current German diet to the German Nutrition Society’s (GNS) official food-based dietary recommendations would be accompanied by reductions in energy use, blue water use, emissions, and land use.  These results differ significantly from our findings due, in part, to GNS recommendations for larger reductions in meat, poultry, and eggs, smaller increases in vegetables and dairy, and a reduction in fruit consumption. 

While our results do indicate that shifting to the USDA recommended diet would have negative environmental ramifications, this is not to say that having a healthy diet is inherently unsustainable.  There is variance within food groups in terms of environmental resource use and emissions.  In fact, some fruits and vegetables, such as grapes, carrots, and lima beans, have relatively low resource use and emissions.  Thus, a diet that incorporates more low-impact fruits and vegetables and fewer high-impact fruits and vegetables would be more sustainable.  The environmental implications associated with changing the makeup of meat and dairy consumption are more complex, as changes that may be less detrimental to the environment in some respects are more detrimental in others.  For example, while reduced emissions and blue water use savings could be achieved by replacing red meat with poultry or seafood, this would increase energy use.  Whether dietary guidelines can be developed to promote both healthy and sustainable living is an important and interesting question for future research.

Reference:

Meier T, Christen O (2013) Environmental impacts of dietary recommendations and dietary styles: Germany as an example. Environ Sci Technol 47(2):877–888

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Comments

John Kazer's picture
Submitted by John Kazer (not verified) on

Thank you for the blog, Michelle.

Please can you clarify the basis of the science behind using energy as a primary metric for fruit and vegetable consumption amounts?  For example, is there any consideration of the required amounts of other nutrients and would a (near) doubling of vegetable intake over-compensate these requirements?

I find it odd that grain intake, a key source of energy and other nutrients, do not really seem to increase under the USDA guidelines you've applied.

michstom's picture
Submitted by michstom (not verified) on

Hi John,

This project developed from my dissertation topic on obesity and the environment.  Since Caloric intake is largely responsible for body weight, it naturally followed that food Calories be the metric by which we measure food consumption. 

With respect to nutrients, the USDA guidelines were established with attention to the nutritional inputs necessary to maintain a healthy diet.   So by virtue of our utilization of these guidelines, we do account for required amounts of nutrients to a limited extent.

Grain intake does not increase very much under the USDA guidelines because according to the USDA, Americans already consume large amounts of grains (over 20%).  In fact, US adults consume more grains, by Caloric intake, than any other food group.

I hope this response answers your questions.

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