If everyone in the world followed the USDA-recommended diet, there wouldn't be enough agricultural land to grow all the food, a new study has found.

The researchers from the University of Guelph and the University of Waterloo, both in Ontario, said an additional gigahectare of fertile land — roughly the size of Canada — would be required to feed everyone, highlighting the fact that dietary guidelines should be based on more than just nutrition.

The study, funded by a Canadian government grant through the Natural Sciences and Engineering Research Council, sought to address the issue of sustainability in the global diet.

The researchers drew from current yield data (production, imports and exports by nation) for various crops from the Food and Agriculture Organization of the United Nations to estimate the amount of land required to grow what the USDA considers a healthy diet low in calories and saturated fats.

"Our analysis shows that there is not enough land for the world to adhere to the USDA guidelines under current agricultural practices," they wrote in the study, which was published earlier this month in the peer-reviewed journal PLOS ONE. "This is despite the fact that the USDA guideline diet is already less land-intensive than the current U.S. diet."

'Much larger than we thought'

Madhur Anand, the study's senior author and a professor of global ecological change and sustainability at the University of Guelph, said the numbers "were a little surprising."

"It's just sometimes you know that there might be an issue but you just don't know the extent of it. It was just much larger than we thought," she said.

"It's important to know that this may have global implications if we were to have everybody follow this diet … and there would be land limitations to implementing it," said Anand, who used the USDA guideline because it was the most readily available one when the researchers began their study six years ago.

Based on their findings, the researchers recommended that national dietary guidelines:

Should be developed using not just health but also global land use and equity as criteria.

Be co-ordinated internationally.

Navin Ramankutty, a professor of global food security and sustainability at the University of British Columbia, agrees that dietary guidelines should consider more than just health.

Madhur Anand, the study's senior author, said she got the idea for the current paper after she completed another paper in 2010 that looked at global forest cover. (University of Guelph)

"A health guideline only focuses on what kind of macro nutrient people get. They're just going to say, 'OK, you need to consume so much protein.' It's not going to say where that protein source should come from," said Ramankutty, who was not involved in the study. "And for sustainability, it makes a huge difference where the actual protein comes from, and I think to point that out is very useful."

But Ramankutty said he can't imagine countries co-ordinating on dietary guidelines: "I don't think that makes sense."

The researchers also analyzed the current yield data by country and continent and found:

Australia, Brazil and the United States could spare the most land, while India, Mozambique and Saudi Arabia required the most land to meet the USDA guidelines.

North America, South America and Oceania could spare significant amounts of land if they adhered to the less land-intensive diet, while Africa, the European Union and Asia would require more agricultural land.

Ramankutty said those results "were not entirely surprising."

"So the result that the Western Hemisphere is going to spare land by switching to the guideline makes [sense] completely in line with intuition," he said. "The same thing for the Eastern Hemisphere, where a lot of people are not getting enough protein these days. If they switched to the guideline, clearly they will use more land. They're also going to be less malnourished."

'A wake-up call'

Co-author Evan Fraser said the findings were "a wake-up call and a path forward."

"Feeding the world over the next generation is one of the biggest global challenges that we face," said Fraser, who holds the Canada Research Chair in global food security. "And this is not an easy problem to solve. It's right up there with climate change and international trade issues and all these big, thorny issues of the 21st century."

He said the study "helps articulate and gives us some sense of the scale of the problem."

The message here is that "eating healthy and sustainably is both necessary, is a big challenge, but is also doable given certain changes that need to be made."

And those changes include shifting to diets that are higher in fruits and vegetables, shifting to plant-based proteins, reducing waste and investing in science to increase crop yields, he said.

"If we don't do those things, we're going to need a heck of a lot more land," Fraser said.