"We wanted to know, really, how many crops feed the world, and what's happening with them," says Colin Khoury, a visiting researcher at the International Center for Tropical Agriculture, or CIAT, in Cali, Colombia.

Khoury and his collaborators went through 50 years of data collected by the United Nations' Food and Agriculture Organization. And they uncovered two big trends.

Photo: International Center for Tropical Agriculture

The first: "Hey, actually, there's places where diets are diversifying, where they're adding crops," says Khoury.

In parts of Asia, such as China, rice is a declining portion of the average person's diet as they add in other foods that are now more available. In the U.S., meanwhile, people are eating more imported foods, like mangoes and coconut water.

But here's the second discovery: Those bigger menus of food also are getting more and more similar to each other, from Nanjing to Nairobi. Everybody is relying more and more heavily on a few dozen global mega-foods.

Many of those foods are part of what you'd call a standard Western diet, including wheat, potatoes and dairy. But other mega-crops come from the tropics, such as palm oil. "It's grown on a large scale in Malaysia and Indonesia, but it's become a global commodity in diets essentially everywhere," says Khoury.

Smaller crops, meanwhile, are getting pushed aside. Sorghum and millet, for instance, are grown quite widely around the world but they're losing out to corn and soybeans. Other small crops that you only find in certain areas could disappear altogether.

"In the Andes, aside from the potato, most of the rest of the traditional roots and tubers, crops like oca and maca, are declining in the amount of production, and the amount of consumption," says Khoury.

A lot of things are driving this trend, he says. There's a rise in international trade, but also "people moving to cities, having more access to supermarkets, to fast food, having less time to cook, not having gardens.

The trend toward greater dependence on fewer crops continues, Khoury says. And so do the risks. It's dangerous to depend on just a few crops because any one of them could be hit by some disaster, such as disease.

But governments and international organizations can still help to safeguard diversity in our food sources. They can act to preserve the many genetic varieties of mega-crops that still exist, and also preserve and encourage cultivation of minor crops, he says.

Rachel Laudan, a historian at the University of Texas and author of Cuisine and Empire: Cooking in World History, says that it's important to remember that what drives the rise and fall of crops is demand for them, after they've been transformed into something mouth-watering. "They're only interesting if they've been processed in some way," she says.

Think of all the things people do with wheat, she says. "You can get bread. You can get flatbread. You can get cakes. You can get pie. You can get beer!"

That's what turned wheat into a mega-crop. Root crops, by contrast, were difficult to store, transport or process into interesting foods.

We could still turn some of these minor crops into attractive foods if there's enough interest in them, she says. In fact, interest may be reviving.

Consumers in the U.S. and Europe are snapping up little-known tropical fruits and so-called ancient grains, like farro and quinoa. Perhaps some of those minor crops can get their own spot on the global menu.

Copyright 2014 NPR.

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In the New Globalized Diet, Wheat, Soy and Palm Oil Rule