Is that salad really good for the environment? (Photo by Renee Comet for the Washington Post)

The idea that being vegetarian is better for the environment has, over the last forty years, become a piece of conventional wisdom.

Its popular rise began in 1971 with the publication of the surprise best-seller Diet for a Small Planet and then spread far and wide: earlier this year it made its way into a key government report for recommendations for the American diet.

As that report from the Dietary Guidelines Advisory Committee put it: “Consistent evidence indicates that, in general, a dietary pattern that is higher in plant-based foods, such as vegetables, fruits, whole grains, legumes, nuts, and seeds, and lower in animal-based foods ...is associated with lesser environmental impact than is the current average U.S. diet.”

This notion isn’t, however, something that scientists have agreed on, and some new research undermines the longstanding idea.

A paper from Carnegie Mellon University researchers published this week finds that the diets recommended by the Dietary Guidelines for Americans, which include more fruits and vegetables and less meat, exacts a greater environmental toll than the typical American diet. Shifting to the diets recommended by Dietary Guidelines for American  would increase energy use by 38 percent, water use by ten percent and greenhouse gas emissions by six percent, according to the paper.

“We were very surprised by our results,” said Paul Fischbeck, professor of engineering and public policy at Carnegie Mellon University. “It’s not what we set out to do - in fact, we expected the exact opposite.”

The findings on the government-recommended diet, which the researchers described as “perhaps counterintuitive,”  stem from the fact that the “healthy” diet includes larger amounts of fruits, vegetables, dairy products and fish, which have relatively large environmental impacts when compared to some foods in our current diet such as foods with added sugars.

“You can’t just assume that a vegetarian diet will reduce your carbon footprint, which is what people think,”  Fischbeck said.

[Here's what Congress is doing about concerns regarding the accuracy of the Dietary Guidelines]

The Carnegie Mellon paper was funded by the Colcom Foundation and the Steinbrenner Institute for Environmental Education and Research at Carnegie Mellon University.

While the research builds on previous work that likewise undermines the conventional wisdom, the debate over the environmental virtues of vegetarianism are unlikely to subside any time soon.

For one thing, the vegetarians have a point: scientists on both sides have concurred that eating beef - though not other meats - has daunting environmental impacts.

Because of the amount of grain and land used to produce a pound of beef, as well as the volume of methane the animals produce, the nation’s intake of beef has significant environmental ramifications, particularly in terms of greenhouse gas emissions. Indeed, the environmental impacts from beef production dwarf those of other animal foods such as dairy products, pork and poultry.

“The key conclusion - that beef production demands about one order of magnitude more resources than alternative livestock categories - is robust,” according to a paper last year in the Proceedings of the National Academy of Sciences.

Perhaps not surprisingly then, six other studies, all cited by the federal committee providing expert advice to Dietary Guidelines for Americans, indicated that diets including less meat are better for the environment. To take but one example, Cornell University researchers reported in the American Journal of Clinical Nutrition in 2003 that “meat-based food system requires more energy, land, and water resources than the lactoovovegetarian diet.”

On the other hand, other papers echo the findings from Carnegie Mellon, suggesting that diets with less meat are no guarantee of environmental benefits. For example, a 2013 paper published by French researchers in the American Journal of Clinical Nutrition found that some diets “containing large amounts of plant-based foods” had the highest levels of greenhouse gas emissions.

“Our results therefore seem to contradict the widely accepted view that diets that are good for health are also good for the planet,” they reported.

So how do the scientists reach such different conclusions?

The reasons, it turns out, are illuminating.

To oversimplify somewhat, research of this type consists of adding up the amounts of greenhouse gas emissions and other environmental costs estimated to come from the production, transportation and marketing of individual foods included in a diet. For example, in the Carnegie Mellon research, the scientists collected estimates of the water use, energy use, and the greenhouse gas emissions for scores of individual foods, and then added up the environmental impacts incurred by various diets.

One of the reasons that the studies vary is that the scientists made different assumptions about the foods involved in each diet studied - and this turns out to be critical. The environmental impacts of individual foods vary tremendously (see chart below), and consequently, the results of these papers shift dramatically depending on the particular vegetarian or meat-eating menu.

Some of the environmental impacts of individual foods are quite strikingly opposite what you might guess: On a per calorie basis, producing lettuce creates nearly as much greenhouse gas emissions as does beef, according to the CMU research; in fact, lettuce generates roughly three times what pork does. Fresh fish, too, is associated with surprisingly high levels of greenhouse gas emissions.

As a result of the varying impacts among foods, small changes in the diet to be analyzed can have large impacts on the results: for example, cutting out pork and eating more lettuce, for example, might be good for you but it will take a higher toll from the environment.

Another important reason for the conflicting findings is that some of the research only considers the environmental costs of what people actually eat. But a significant percentage of any given food is wasted, and in order to get a complete tally, the environmental costs of producing the wasted food also ought to be included. Including food waste tends to raise the environmental costs of fruits and vegetables because more of them tend to be wasted: According to Fischbeck and colleague Michelle Tom,  while about 40 percent or more of fruit goes to waste, only about 33 percent of meat does.

The data on the environmental effects of the food supply, moreover, can be extremely complicated to measure, vary from place to place, and are a subject of significant scientific uncertainty. But for all the debate over the issue, and the complexity of data, a couple of nuggets seem clear.

First, while it might be nice to think that what’s good for you is also good for the planet, it’s not necessarily the case. To take just one example that may please some people: Ingredients associated with junk foods, such as added sugars and saturated fats, have lower environmental impacts, according to the CMU estimates.

Second, the choices you make for dinner do have environmental consequences. They’re just not as simple as you might think.