If Americans adopted the recommendations the USDA's "Dietary Guidelines for Americans, 2010," diet-related greenhouse gas emissions would increase 12 percent, according to scholars, and if Americans reduced their daily caloric intake to the recommended level of about 2,000 calories while shifting to a healthier diet, greenhouse gas emissions would decrease by only 1 percent.

What must happen is that Americans must switch to no animal products, say Martin Heller and Gregory Keoleian of the University of Michigan's Center for Sustainable Systems, who looked at the greenhouse gas emissions associated with the production of about 100 foods, as well as the potential effects of shifting Americans to a diet recommended by the U.S. Department of Agriculture. 

"The take-home message is that health and environmental agendas are not aligned in the current dietary recommendations," Heller said.

In its 2010 dietary guidelines, USDA recommends that Americans eat more fruits, vegetables, whole grains, fat-free and low-fat dairy products, and seafood. They should consume less salt, saturated fat, trans fat, cholesterol, added sugar and refined grains but they don't explicitly state that Americans should eat less meat. An appendix to the report lists the recommended average daily intake amounts of various foods, including meat, which is significantly less than current consumption levels. Heller and Keoleian used the USDA's Loss Adjusted Food Availability dataset as a proxy for per capita food consumption in the United States.

While a drop in meat consumption would help cut diet-related greenhouse gas emissions, increased use of dairy products—and to a lesser extent seafood, fruits and vegetables—would have the opposite effect, increasing diet-related emissions, they believe. In the United States in 2010, food production was responsible for about 8 percent of the nation's greenhouse gas emissions they cite, though those numbers are based on a 'it takes a gallon of gas to make a pound of beef' metric that has been widely discredited. However, it is true that, in general, animal-based foods are responsible for more greenhouse gas emissions per pound than plant-based foods.

The production of both beef cattle and dairy cows is tied to especially high levels of greenhouse gas emissions by vegetarian activists but accurate numbers are difficult to calculate. One thing that is true is that cows convert plant-based feed into muscle or milk, which takes lots of feed. Growing that feed often involves the use of fertilizers and other substances manufactured through energy-intensive processes. And then there's the fuel used by farm equipment.

In addition, cows burp methane, and while that effect is exaggerated, even a short-lived gas like methane has for be factored in.

Greenhouse gas emissions associated with producing the U.S. diet are dominated by the meats category, according to Heller and Keoleian. While beef accounts for only 4 percent by weight of the food available, it contributes 36 percent of the associated greenhouse gases, they believe. So they are lobbying for a switch to diets that don't contain animal products and say that would lead to the biggest reductions in this country's diet-related greenhouse emissions.

Critics of vegetarian emission claims about meat production argue that it would create a world where only the rich would eat meat, which would lead to drastic disparities in health.

Heller said he's not arguing that all Americans should go vegan, and he believes that animals need to be part of a sustainable agricultural system. However, reduced consumption would have both health and environmental benefits.

In their paper, Heller and Keoleian also looked at wasted food and how it contributes to U.S. greenhouse gas emissions. They concluded that annual emissions tied to uneaten food are equivalent to adding 33 million passenger vehicles to the nation's roads.