Vegetarians may be no greener than bacon-lovers, according to new research. In fact, lettuce eaters may be three times worse.

Looking at the way food is consumed in the United States, a student-professor team from Carnegie Mellon University (CMU) in Pittsburgh concluded that following the US Department of Agriculture (USDA) recommended nutritional guidelines to eat more fruits, vegetables, dairy, and seafood is actually more harmful to the environment than a diet of typically “less healthy” foods.

“Eating lettuce is over three times worse in greenhouse gas emissions than eating bacon,” said Paul Fischbeck, professor of social and decisions sciences and engineering and public policy, in a statement. “Lots of common vegetables require more resources per calorie than you would think. Eggplant, celery and cucumbers look particularly bad when compared to pork or chicken.”

Dr. Fischbeck and his co-authors, whose study appeared in the journal Environment Systems and Decisions, found that the USDA recommendations tended to have higher greenhouse gas (GHG) emissions per calorie than alternatives. Following the supply chain from growing to processing to transporting food, and onto store shelves and tables, as well as household storage, the researchers measured the strain on environmental resources in the form of energy use, water use, and GHG emissions.

Eating fewer calories and reducing weight was found to have a positive effect on the environment by shoring up energy use, water use, and emissions from the food supply chain by some 9 percent, according to the study.

By comparison, eating the USDA recommended “healthier” foods – a variety of fruits, vegetables, dairy, and seafood – increased the environmental impact across all three categories the study examined: Energy use went up by 38 percent, water use by 10 percent and GHG emissions by 6 percent, according to the study.

The initial findings of the study were “surprising”, according to senior research fellow Anthony Froggatt at Chatham House, a think-tank unaffiliated with the research that is looking at the connection between meat consumption and greenhouse gas emissions.

Mr. Froggatt said in an interview with the Independent that it is “true lettuce can be incredibly water intensive and energy intensive to produce”, but comparative exercises like the one performed by CMU can yield vastly different results depending on how the foods are raised or grown.

“We usually look at proteins rather than calories, and as a general rule it is still the case that reducing meat consumption in favor of plant-based proteins can reduce emissions,” he said.

Still, with global population ballooning, “we have a pressing need to eat better and farm better,” Tamar Hospel, a food-policy columnist for The Washington Post, wrote last August.

Lettuce, Ms. Hospel writes, “has almost nothing going for it. It occupies precious crop acreage, requires fossil fuels to be shipped, refrigerated, around the world, and adds nothing but crunch to the plate,” citing its high water content and relatively low nutritional value.

Michelle Tom, a co-author of the study, acknowledges the relationship between diet and environment is “complex.”

“What is good for us health-wise isn’t always what’s best for the environment,” she said in a statement. “That’s important for public officials to know and for them to be cognizant of these trade-offs as they develop or continue to develop dietary guidelines in the future.”

The Carnegie Mellon findings may contradict the soon-to-be-released USDA dietary guidelines which are expected to recommend more veggies and less meat consumption as part of a diet that’s “more health promoting and is associated with lesser environmental impact than is the current average US diet,” an advisory panel draft said, according to the Associated Press.