Vegetarians, put down your pitchforks. Meat-eaters, less of the smug grin. Yes, this article is about some of the environmental issues associated with vegetable-rich diets within the U.S., but it is not an attack on the ethics-driven dietary choices of people and it shouldn’t be used as such.

A new study has looked at what would happen if the entire U.S. population followed exactly the dietary guidelines from the Department of Agriculture (USDA). The team modeled three different scenarios: a reduction in calories consumed but no changes in diet; a shift to a more vegetable-heavy diet but no caloric reduction; and a mix of the two, which is the one recommended by the USDA.

The study, which is published in Environment Systems and Decisions, aims to highlight how complicated it is to balance environmental issues and human needs.

The researchers investigated three parameters for the scenarios: energy use for food production, blue water footprint (the amount of freshwater necessary to produce a product), and greenhouse gas (GHG) emissions. In scenario 1, quantities of these variables were reduced by about nine percent, which is unsurprising since the only change was the consumption of fewer calories.

The diets in scenario 2 and 3 have more fish, vegetables, and fruit compared to the average U.S. diet. The higher intake of these healthy foods is balanced by a reduction in meat, solid fats, and added sugars. While this diet is good news for our bodies, as evidenced by the many benefits thought to be offered by the famous Mediterranean diet, the study suggests it might not be so good for the environment.

All three parameters actually increased under scenarios 2 and 3. For scenario 3, energy use went up by 38 percent, water use by 10 percent, and greenhouse gas emissions by six percent. Scenario 2 was found to be even worse. But why is that? Per Calorie, production of vegetables requires more energy and water than meat, but generates only one-quarter of GHG emissions compared to beef.

Also, the study investigated a vegetarian diet, not a vegan one. "Dairy, by far, has the greatest impact on increased GHG emissions because it has the third highest emissions intensity value, which is then compounded by USDA recommendations for substantial increases in dairy," wrote the authors in the paper.

“There’s a complex relationship between diet and the environment,” Michelle Tom, a Ph.D. student at Carnegie Mellon University and lead author on the paper, said in a statement. “What is good for us health-wise isn’t always what’s best for the environment. That’s important for public officials to know and for them to be cognizant of these tradeoffs as they develop or continue to develop dietary guidelines in the future.”

Another food group recommended by the USDA guidelines is fish. Fish has a tiny blue water footprint, as most of the fish that reach our tables come from the ocean, but this advantage is offset by high energy demands. Fisheries often need to move far from the shore, requiring significant amounts of fuel for transport and creating GHG emissions along the way.

This study is not universal, though. It’s specific to the U.S., so dietary requirements, infrastructure, and current eating habits need to be taken into account when making general conclusions about vegetable-rich diets. A similar dietary guideline in Europe would actually reduce energy use, blue water footprint, and GHG emissions, largely due to the fact that food is often produced more locally.

The researchers also looked at how the parameters change for the three scenarios based on food waste. Between 34 and 42 percent of food produced is wasted every year. The paper shows that if we could significantly reduce the amount of wasted food, our impact on the environment would be drastically reduced no matter our diet. Last week, for example, France passed a very strict law to fight food waste, an important step toward a global culture of sustainable food production.