Call it Bacongate. Or, think of it as the short-lived, but too good to be true, victory for bacon lovers.
A study out of Carnegie Mellon University garnered a lot attention this week when one of its authors made a counterintuitive claim: Eating lettuce was worse for the environment, in terms of greenhouse gas emissions, than eating bacon.
That surprising claim was featured prominently on the press release for the study, which also included a statement from one of the study's authors, Paul Fischbeck: "Eating lettuce is over three times worse in greenhouse gas emissions than eating bacon," he said. "Lots of common vegetables require more resources per calorie than you would think."
Before chewing too long over that statement, it helps to look at the findings of the study itself, which is published in the journal Environmental Systems and Decisions. The researchers looked at how different dietary choices could impact the environment. They found that if Americans consumed about 320 fewer calories per day, greenhouse gas emissions would fall by 9 percent.
They also found that if Americans switched their diets to be more in line with US Department of Agriculture (USDA) dietary recommendations — which include eating fewer solid fats and added sugars, and eating more fruits, vegetables, low-fat or fat-free dairy products, and seafood — but kept consuming about the same number of calories, greenhouse gas emissions would actually rise by 11 percent. If Americans switch to a USDA-recommended diet and also reduce their calorie count, greenhouse gas emissions still rise, but by only 6 percent.
Michelle Tom, a Ph.D. candidate at Carnegie Mellon University and a coauthor on the paper, explained their findings.
"Essentially, according to the USDA, Americans consume far too much processed sugars, fats, and oils and too few veggies, fruits, and dairy products," she said. "Unfortunately, the vegetables, fruits, and dairy foods have higher emissions per calorie than sugars, fats, and oils."
It's that dynamic — the switch away from sugars, fats, and oils — she said, that is driving their finding that greenhouse gas emissions rise when following the USDA-recommended diet.
Brent Kim of the Johns Hopkins Bloomberg School of Public said the problem with the bacon-lettuce comparison is that it was done by calorie, and the foods differ greatly in how calorie-dense they are.
Consider a three-ounce serving of pork. It would be unlikely for someone to attempt to eat enough lettuce to replace the calories they would have gotten from that pork. In fact, according to Kim, to equal the amount of calories from three ounces of pork, a person would need to eat more than two heads of lettuce, or 24 cups.
Thus, comparing lettuce and bacon in terms of greenhouse-emissions per calorie is like comparing apples and oranges. People also eat foods like those for different reasons, he said — meat for protein, and vegetables for fiber. That's why it doesn't make sense to compare the foods head-to-head in the first place.
He called it a "ridiculously unrealistic comparison."
Tom confirmed that by their analysis, they found that "lettuce is roughly three times more emissions-intensive per calorie" than pork is. Another way to think about it is by serving size. And, after a back-of-the-envelope calculation, she said that by serving, lettuce was indeed "far less emissions-intensive" than pork.
She points out that vegetables like spinach, squash, carrots, and broccoli are all low on the emissions spectrum.
Fischbeck clarified that their analysis was about pork in general, not just bacon.
"Whether bacon would be higher or lower than pork is an interesting question that has been discussed recently because of the press release," he said, "but there is no agreement!"
A broader point, Kim said, is that that meat from ruminants — cows, goats, and sheep — are the most emissions-intensive food. He said that reducing meat and dairy consumption, and eating a more plant-based diet, is better for the climate.
Vegetables, as a category, are less greenhouse gas-intensive than meat, as a category, even when measured on a per-calorie basis, according to the Carnegie Mellon study.
Martin Heller, a research specialist at the University of Michigan's Center for Sustainable Systems, studies the environmental impact of food. He echoed Kim's analysis.
"The studies in this area have been pretty consistent in demonstrating that animal-based foods have a larger-impact on the environment than plant-based foods," he said. Even a move towards chicken and away from beef, coupled with a reduction in meat consumption, can help greenhouse gas emissions, he said.
David Tilman, an ecologist and professor at the University of Minnesota, studies the impacts of agriculture on the environment. He said that taken as a whole, agriculture — which includes elements like land-clearing, ruminants like cows, and the use of fertilizers and fuel for tractors — contributes approximately 30 percent of greenhouse gas emissions globally. He's also the author of a 2014 study in the journal Nature that explored the health, and environmental impacts, of different diets. The diet that had the lowest greenhouse gas emissions, as compared to a "typical" diet, he said, was vegetarian.
One well-known dietary fact, he said, is that some unhealthy foods, like sugars, fats, and oils, actually have a low environment impact.
Tilman said he thought the Carnegie Mellon study was "a nice analysis" and "carefully done." He referred to the bacon-lettuce comparison as "sort of silly."
From a greenhouse gas emissions perspective, beef and lamb are the worst emitters, he said; vegetables like legumes, peas, and beans are good.
"We really have to be wise about this," he said. "We're trying to help the health of people, and help them live better lives, with a healthier diet, as well as trying to help the world. And there are many, many diets that do that. And what they all have in common is having much less consumption, especially of red meats."
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