Update: This story received criticism from our readers. Since it was originally published, we’ve updated the article with some new information and context (see below).

Advocates of vegetarianism – including everybody's favourite Governator – regularly point out how how harmful human consumption of meat is to the environment, but is opting for a fully vegetable-based, meat-free diet a viable way to cut down on energy use and greenhouse gas emissions?

Nope – according to a new study by scientists in the US – or, at least, it's not that simple. Researchers at Carnegie Mellon University (CMU) say that adopting the US Department of Agriculture's (USDA) current recommendations that people incorporate more fruits, vegetables, dairy and seafood in their diet would actually be worse for the environment than what Americans currently eat.

"Eating lettuce is over three times worse in greenhouse gas emissions than eating bacon," said Paul Fischbeck, one of the researchers. "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."

If these findings seem surprising in light of what we know about the impact of meat on the environment, you're probably not alone. You're also not wrong – meat production does take a high toll on the environment.

But what we need to bear in mind is that the energy content of meat is also high, especially when compared to the energy content of many vegetables, which is why going on a salad diet is great for your waistline. Consuming less energy content means less you in the long run.

But what if you don't want to lose weight? What if you just want to replace the same amount of energy you get from meat with energy from vegetables? Well, then, to put it very simply, you need to eat a lot of vegetables. And when you contrast meat and vegetables on their impact per calorie as opposed to by weight, veggies suddenly don't look quite so environmentally friendly.

In the study, which was funded by CMU's Steinbrenner Institute for Environmental Education and Research and the sustainability-focused Colcom Foundation, the researchers examined three different scenarios in terms of their energy and water use, and the greenhouse gas emissions that stem from the growing, processing, and transporting of food involved.

In the first scenario, the impact of food production on the environment could be lessened if people simply ate less of what they already do. Shifting from the current average US diet – which is particularly high in calories – to a reduced calorie intake designed to achieve 'normal' body weights for the population, rather than perpetuating the two thirds that are currently obese, would result in a 9 percent decrease in energy use, water footprint, and emissions.

However, in a second scenario, keeping calorie levels the same but adjusting the foods eaten to incorporate USDA recommendations that people eat more vegetables, fruits, dairy, and seafood would see energy use increase by 43 percent, with the water footprint increasing by 16 percent and emissions by 11 percent.

And the third scenario – reducing calories while also shifting to the recommended foods – takes a greater toll on the environment too, with increases in energy use, water footprint, and gas emissions of 38 percent, 10 percent, and 6 percent respectively.

The researchers acknowledge that their findings may be somewhat surprising in light of the zeitgeist over meat's impact. "These perhaps counterintuitive results are primarily due to USDA recommendations for greater caloric intake of fruits, vegetables, dairy, and fish/seafood, which have relatively high resource use and emissions per calorie," they write in Environment Systems & Decisions.

But controversial as the findings may sound, comparing the respective impact of different foods based on their calorie content isn't new or radical.

"If you stop eating beef, you can't replace a kilogram of it, which has 2,280 calories, with a kilogram of broccoli, at 340 calories. You have to replace it with 6.7 kilograms of broccoli," Tamar Haspel wrote last year for the Washington Post. "Calories are the great equaliser, and it makes sense to use them as the basis of the calculation."

Inevitably, producing far greater amounts of foods like broccoli to compensate for the calories lost from meat and other high-energy food sources involves larger amounts of energy, water and emissions than any simple kilo-for-kilo comparison of environmental footprint allows. Take a look at the graphic here to see how the impact of foods is reordered when they're ranked by calorie content as opposed to by weight.

"There's a complex relationship between diet and the environment," said Michelle Tom, one of the team. "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 cognisant of these tradeoffs as they develop or continue to develop dietary guidelines in the future."

Update: The researchers did not find that vegetarians or vegetarianism are harmful to the environment, or that producing vegetables is more harmful to the environment than producing meat.

What they found, in light of the data they examined, is that producing some vegetables and other foods results in high use of natural resources – and that eating more of those foods (as recommended for health by the USDA) in two particular scenarios results in higher energy use, blue water footprint and greenhouse gas emissions.

One limitation of only looking at the per-calorie level of resources and emissions in foods, however, is that it doesn't necessarily reflect what people actually eat. As others have pointed out, some foods that require a lot of natural resources to produce – such as lettuce, for example – would likely never constitute the basis of a diet, vegetarian or otherwise, since they're so low in calories.

Other research suggests that eating less meat is a good thing for the environment. One previous study found that following a lacto-ovo vegetarian diet (no meat, fish, or poultry) would result in a 33 percent decrease in greenhouse gas emissions, and vegan diets go even further, with a 53 percent decrease in emissions.

But in terms of the Carnegie Mellon University study, what the researchers are saying, to borrow Hilary Hanson's phrase at The Huffington Post, is that "not every plant product is more environmentally friendly than every meat product." (Original emphasis.)

This point is also well expressed (along with much other commentary) in this Washington Post article by Tamar Haspel.

"You can't lump all vegetables together and say they're good. You can't lump all meat together and say it's bad," Paul Fischbeck, one of the researchers, explained to Hanson, in reference to food's impact on the environment. "My bottom line is that there are no simple answers to complex problems. Diet and the environmental impact of agriculture … is not a simple problem."

The importance of research like this is how it helps people to think about the extent to which our individual diets affect the environment and the resources around us. This is especially relevant right now, with the planet under more stress than ever, due to the size of countries' populations, environmental damage resulting from climate change, and competition for limited resources. It also highlights how environmental benefits and health benefits can sometimes clash.

But, standing back, it's clear from the competing (and sometimes controversial) findings of studies like this that scientists have yet to find the best way of measuring the impact of one particular type of diet compared to another. Hopefully, with more research, this is something that can be figured out, and we can produce enough sustainable and healthy food to feed people everywhere.