If you're reading this from outside the US, please excuse the national reference. This claim refers to how the US ranks in Oxfam's compilation of how easy it is to get a balanced, nutritious diet.
A nice synopsis of the report is found on NPR. Max Lawson, of Oxfam, is quoted on NPR as saying "Food is very, very cheap in the U.S. compared to most countries," he explains. "But the fact is you end up with people malnourished in one of the richest countries because they don't have access to fresh vegetables at a cheap enough price to make a balanced diet."
Readers of Eat Drink Vote will not be surprised by this. As stated on the book's back cover "While encouraging readers to vote with their forks for healthier diets, this book insists that it's also necessary to vote with votes to make healthier diet choices more available, accessible, and affordable for everyone."
The full report may be downloaded here.