Interesting Map -- Green is the Vegetable Intensive Diet.

Source: The Washington Post

If you grow corn, you can produce corn for human food, corn for animal feed, or corn for alcohol fuel. Presumably:

• If people eat the corn grown of the farm, all the calories go to people. 
• If animals eat the corn, how much of the corn goes to nourish people depends on which animals. If you feed the corn to a pet, none gets to people; if you feed it to chickens, it that corn produces a considerable amount of protein for people's nourishment; if you feed it to cows, less protein gets to people per bushel of corn.
• If the corn is used to produce alcohol to be added to gasoline, it may help to get corn, beef, eggs, milk and chicken to market, but it doesn't do much for nutrition.

In poor countries, people eat a lot of plant food and little meat; the meat that they do eat is likely to be from animals that produce relatively more protein for humans per pound of feed. Thus they get a lot more calories out of the crops that they grow than we do.

Of course, food that gets to the food system does not all get to be eaten. There are a lot of losses from the farm to the table. (And not all the food that is eaten is used efficiently by sick people.)

If the world population keeps growing (and it is predicted to hit 11 billion by 22000, it may be appropriate to switch away from meat and get more of our protein from vegetables. That is probably a good idea for health anyway. Homo sapiens evolved as omnivores, not carnivores. The heavy meat diet that many Americans eat takes a tole on their health.