When Craig Nash was forced to purchase an extra airplane seat, that was it.

“It just hurt so bad,” says the Chicago resident, whose largesse prompted airline employees to ask him to buy an extra seat. “It was just so embarrassing. It got to where (I thought) I’ve just got to do something.”

Nash, weighing in at 420 pounds, decided to shed the weight, but found he couldn’t do it himself. He fell back on old habits, and when he auditioned for a weight loss reality show, he was twice rejected.

But when he heard about the 50 Million Pound Challenge on the radio, he found his saving grace.

“Next thing you know, I sold my car, and said ‘I’m going to force myself to do more,’” says Nash, 47, who has lost 200 pounds because of the national program sponsored by State Farm Insurance, and led by Dr. Ian Smith, a Chicagoan who hosts VH1’s “Celebrity Fit Club.”

Smith developed the free weight-loss and support program three years ago in response to the obesity epidemic that affects more than 60 percent of Americans. The program has helped more than one million Americans lose more than 4 million pounds, giving them tools to track their weight loss and plan healthy diets. Unlike some diet programs, it costs nothing.

This week, Smith, Nash and other Chicagoans have been filming commercials promoting the program at various locations, including Water Tower Place, Millennium Park and Union Station.

Smith, who lives in the Loop, says Chicago is in the middle of the pack when it comes to tackling obesity. A network of parks and bike paths helps, he says, but the city needs a more aggressive city-wide approach.

“I think Chicago has the tools to be a healthy city. What Chicago is missing is a galvanized approach to work together,” says Smith, 39.

He says Chicago summers – when people celebrate the end of long, dreary winters – contribute to unhealthy lifestyles. Celebrating summer leads to eating unhealthy food and drinking, which contribute to obesity, he says.

Another problem, he says, is the lack of access to healthy food in some communities. So-called food deserts, especially on the South and West Sides, deprive people of access to healthy food. A study released yesterday by the Mari Gallagher Research and Consulting Group said a new grocery store in Roseland could add 15 years to the lives of residents.

“Definitely food access is one part of the problem,” says Adam Becker, executive director of the Consortium to Lower Obesity in Chicago Children, which in 2004 found that one-quarter of Chicago kindergartners were overweight or obese.

“If you think about what’s the most affordable food, it’s the one with the most preservatives…as opposed to fruits and vegetables, which are more expensive,” he says.

“We can’t make economics a barrier to access,” Smith says. “Weight loss and health is a priority. You have to make yourself and your health a priority.”

Try telling that to Marilyn Winters, who lost 117 pounds through the 50 Million Pound Challenge. At one point she weighed 267 pounds; now she’s training for the Chicago Marathon. Or to Nash, who gave up red meat and fried foods to become a vegetarian, and bikes 200 miles a week.

“It changed my life,” Nash says. “My wife has a new husband for our 25^th anniversary, coming up this August.”