American adults are fatter than ever — 26.7 percent are obese, according to a new federal report — and no state meets the U.S. goal of keeping that rate below 15 percent. But the nine states that top the fat chart, with more than 30 percent of their populations obese, are all in the South: Alabama, Arkansas, Kentucky, Louisiana, Mississippi, Missouri, Oklahoma, Tennessee, and West Virginia — what Dr. William Dietz of the Centers for Disease Control and Prevention (CDC) calls "the heart disease and stroke belt." Why are Southerners significantly more rotund? Here are four theories: (Watch an ABC report about lazy Louisiana)

1. The air is thinner up north
The leanest state? Colorado. And that makes sense, says Katherine Hobson in The Wall Street Journal. A 2010 study in Obesity magazine suggested that people at higher altitudes eat less and burn more fat in daily living. That puts the low-lying South at a disadvantage. The CDC's Dietz agrees that "altitude may be one partial explanation" for the South's girth.

2. Southerners love their cars
The thinnest areas of the U.S. are also those with the greatest reliance on public transportation, says the AP's Mike Stobbe. A "recreational" infrastructure that offers lots of public trails, like Colorado's, also helps. The no-brainer keys to staying thin are eating less and exercising, and you don't get as much exercise if you go everywhere in your car.

3. It's a culture thing
Given the clear-cut "geographical differences," says dietician Timi Gustafson in the Seattle Post-Intelligencer, "culture and lifestyle may play a role" in the South's obesity rate, too. Part of the cultural difference is "health education, or rather the lack thereof." And the "so-called 'grocery gap'" in poor rural areas makes it harder to get fresh produce and other nutritious foods.

4. A tradition of unhealthy (irresistible) food
The fact that America's obesity Top-10 list mostly consists of Southern states, says the Anderson, SC, Independent Mail in an editorial, surely reflects "(and simultaneously places blame on) our delicious traditional foods." Unfortunately, nothing "can compare with a Southern cook's 'cat's head' biscuits and fried green tomatoes." Why is "everything that tastes so good... so bad for us"?