Once considered an affliction of the lazy and indulgent, obesity now affects about one-third of Americans. The epidemic has swept up the wealthy, middle class and the poor; city dwellers, suburbanites and those in rural areas; and people of all races and ethnicities.

The causes, researchers say, are numerous. These include a diet of calorie-dense but nutrient-deficient food found in grocery and convenience stores, public planning strategies that favor motorists over walkers and cyclists, and simply bad habits.

And while the causes are many, the costs are enormous. Obesity’s associated costs add $93 billion to the nation’s medical bill annually. Each year, 112,000 people die from obesity-related causes, and the condition is responsible for an increased risk of chronic diseases like Type 2 diabetes, cancer and heart disease.

To better understand the local and state implications of the obesity epidemic, we ranked the nation’s heaviest cities. In doing so, we discovered states with multiple offenders, metropolitan areas with expanding waistlines and a high representation of Southern cities. Worse yet, after claiming the title of the most sedentary city, Memphis, Tenn., has also ranked first as the country’s most obese.

Behind the Numbers
To determine which cities were the most obese, we looked at 2006 data on body mass index, or BMI, collected by the Centers for Disease Control’s Behavioral Risk Factor Surveillance System, which conducts phone interviews with residents of metropolitan areas about health issues, including obesity, diabetes and exercise.

In this case, participants report their height and weight, which survey analysts use to calculate a BMI. Those with a BMI between 18.5 and 24.9 are considered at a healthy weight, those with a BMI between 25 and 29.9 are considered overweight, and those with a BMI of 30 or higher are considered obese. About 32% of the nation is obese, according to the Centers for Disease Control; Memphis ranked above the national average at 34%.

Though data is collected for roughly 145 metropolitan statistical areas, we looked only at the country’s 50 most populated cities and ranked the top 20. Because of an insufficient number of survey responses, data from some cities, including Sacramento, Calif., Columbus, Ohio, and Buffalo, N.Y., was not included. Had we included every area on the list, the smaller cities of Huntington, W.V., and Ashland, Ohio, on the West Virginia, Kentucky and Ohio state borders would have far outpaced every city on the list with obesity rates of 45%. Of the 50 cities we did rank, Boston entered last, with only 19%.

Noticeable Trends
Many of the cities on the list have high poverty rates and high frequencies of fast-food consumption.

In the city of Memphis, which does not include the outlying areas surveyed by the CDC, 24% of residents live below the poverty line. According to the U.S. Census Bureau, the national average is 13%. The same trend was noticeable in the cities of Milwaukee (No. 17), Detroit (No. 5), and San Antonio, Texas, (No. 3) where 26%, 33%, and 18% of residents, respectively, live beneath the poverty line.

While fast-food consumption is a minor factor influencing obesity rates, purchasing patterns often reflect larger health issues and habits in certain communities. The average American had purchased fast food 16 days of the month between January and September of this year, according to Quick-Track® research conducted by the consumer tracking group Sandelman & Associates. Thirteen cities on our list, including Memphis, Austin, Texas, and Indianapolis, met the national average or higher. Residents of San Antonio eat fast-food 20 days of the month, and had the highest frequency of the cities on our list.

The Causes
Despite public health warnings about maintaining a frequent exercise regimen, limiting fast-food consumption and avoiding weight gain, there is no single cause of obesity, a fact that often frustrates experts, legislators-and obese people.

Other factors contributing to our ballooning waistlines, says Marian Levy, director of the master’s of public health program at the University of Memphis, include enormous food portions, declining exercise rates and cheaper, unhealthy food. When asked about Memphis, however, Levy emphasizes a local culture built around Southern hospitality.

“We express our caring about people through food,” she says, describing generous helpings of fried fish, chicken and okra often shared with neighbors and friends. “We have to realize that if we truly care about people, we want them to be healthful.”

In Memphis, as in other cities on our list, reversing the obesity crisis can seem like trying to plug a thousand holes in a sinking ship. Public health campaigns are a start. Healthy Memphis Common Table, a nonprofit organization trying to promote better fitness and nutrition choices, provides residents with a list of exercise facilities and walking paths in addition to health tips and testimonials about the benefits of weight loss.

Another tactic, notes Levy, is vending machine legislation that will require schools pre-K through eighth grade to replace unhealthy foods and beverages in vending machines, on school store shelves, at fundraisers and a la carte cafeteria items with more nutritious alternatives. She hopes the legislation, which is being implemented for the current school year, will improve the diets of Memphis-area school children, 71% of whom receive a free lunch from school cafeterias.

Still, “there’s not going to be a silver bullet,” Levy says. “There has to be a simultaneous change at the environmental level, in schools, communities and families.”

The Solutions
It’s that community-wide change in lifestyle that experts say will result in fewer cases of obesity.

“You see cities taking this on in a range of different ways,” says Leon Andrews, the project director of the Institute for Youth, Education and Families at the National League of Cities. Andrews is currently overseeing a one-year project in which six cities, including our third most obese, San Antonio, receive assistance in combating childhood obesity and promoting community wellness.

Andrews identified five ways cities could specifically address childhood obesity, as well as larger community health issues. These included improving public space and utilizing parks and recreation areas to encourage physical activity, as well as pursuing healthy food alternatives through community gardens and farmer’s markets.

“More cities are becoming aware of [obesity] and looking to play a role in improving the situation,” Andrews says. He also pointed out that city leaders often preferred to follow a successful example as opposed to chart a new course: “They definitely want to be the second, but may not want to be the first,” he says. Regardless, it’s clear that rising rates of childhood obesity-17% of children and adolescents ages 12 to 19 are overweight-has prompted cities like Birmingham, Ala., San Diego and Richmond, Va., all on our list, to become more proactive in terms of obesity prevention.

Others, such as Walter Willett, a professor of epidemiology and nutrition at the Harvard School of Public Health, believe our salvation lies mainly in ridding the grocery store of food he calls “not fit for human consumption.” Among the items he would like to see purged, he says, are the “shelves of sugar water, the breakfast cereal section, dominated by refined starch and sugar, and white bread and rolls.”

According to Willett, a healthier diet, in combination with increased levels of physical activity and environments that promote exercise, would drastically improve the country’s obesity problem. “If we do this right,” he says, “we’ll improve our quality of life in many different ways.”

Rebecca Ruiz - ForbesLife