When they looked at only carbon dioxide emissions associated directly with energy consumption, they came up with the vegetarian diet far less damaging to the planet than the others.
However, the researchers admit that their findings can't be considered exact. Take fish, for instance.
"The seafood portion of American diets is heavily skewed toward what is called charismatic predator fish," Eshel said, which are harder to catch. "Sword, shark and tuna and so on require long-distance ocean journeys, and those efforts are not efficient. They require a lot of labor and a lot of fossil fuel."
Switching from red meat to fish won't necessarily help the planet, unless you can develop a taste for anchovies, which prefer to remain closer to shore and travel in large schools that can be easily caught and harvested.
Considering the fact that so much of our energy goes into such areas as transportation, is it realistic to think of lowering greenhouse gases by changing the typical American diet?
The researchers insist it is. The United States, Eshel says, accounts for about 28 percent of the world's total greenhouse gas emissions.
"The U.S. has five sectors of the economy that are large emitters," he said. "Those are transportation, industrial, commercial, residential and agriculture."
Energy used in agriculture has grown substantially in recent years, he says, and now stands at around 18 percent or 19 percent of the nation's energy use.
The researchers say their findings show that at least 6 percent of that use comes from the production of foods that are not energy efficient, like cattle and the food to feed them. Considering that the "mean American diet is responsible for an additional ton-and-a-half of greenhouse gas emissions a year from each of us," the savings could be substantial.
"It's a huge deal," Eshel said.
So do these two vegetarians want us to give up our hamburgers and tuna and eat cauliflower?
"I hate to sound preachy," Eshel said, but it wouldn't hurt to give a little.
"I say eat whatever works for you, but just keep in mind that the less animal-based food you eat, and the more you replace those calories with plant-based food, the better off you are, in terms of your health as well as your contributions to the health of the planet."
Bring on the anchovies!