Downsize Fitness has only one requirement - membership is reserved for those overweight only.

In a culture that values thin, the fat debate is on fire.

From plus-size models strutting the catwalks, to curvaceous superstars like Adele belting it out at the Oscars (and winning one for "Skyfall") and Lena Dunham proudly bearing it all in her hit HBO show, "Girls," plus-size is going mainstream.

Two-thirds of Americans are overweight or obese, and several clothing stores, like Fashion to Figure, are marketing cute clothes for bigger girls.

"I think in today's world, when there's so many positive role models out there, Octavia Spencer, Adele, Melissa McCarthy from 'Bridesmaids' and lots of people like that, I think, are changing the way things are viewed," said Fashion to Figure CEO Michael Kaplan. "And those biases hopefully will become removed over time."

Vogue, a magazine famous for featuring super-skinny figures, will soon feature Kate Upton, a swimsuit model known for her curves, on their cover.

So is there still real shame in being overweight? For the gym-goers at Downsize Fitness in Chicago, the answer is, "Yes."

At Downsize Fitness, membership is reserved for the overweight only -- those who are at least 50 pounds overweight -- because they feel "fat shamed."

"Before I came here, I did try to lose weight," said one gym-goer named Chris Almaguer. "But with me being my size, I would literally stay up all night, maybe around 4, 5 a.m., go outside just to walk so no one would see me."

"There is a need for a place that people, heavy people need to go an concentrate on losing weight," said David Lanz.

They are all self-described outcasts from the mainstream gym scene, where they often felt ostracized.

"There's not a culture of acceptance in America for overweight people," said Francis Wisniewski, co-founder of Downsize Fitness. "You can still make fun of it in movies. You can still discriminate against based on size. So it's still that last thing in America that you are not protected on."

Downsize Fitness is now part of the national argument: Are overweight people still treated differently?

Just this week, CVS Pharmacy told its employees they had to submit information on their weight, body fat and glucose levels or pay a "surcharge," an extra $600 per year, for the company's health insurance plan.

Another hot button question: Should overweight airline passengers have to buy an extra seat?

Blogger Kenlie Tiggerman said she was humiliated by a Southwest Airlines gate agent in 2011.

"The gate agent came up to me and he asked me how much I weighed, what size clothes I wore," she told "Nightline" in a 2012 interview. "He said that I was too fat to fly, that I would need an additional seat."

Sharon Robertson has also felt the pain of harsh stares and ridicule. She joined Downsize Fitness last year, when her starting weight was 376 pounds and she said she was still reeling from the sting of a bad experiences with a personal trainer at a mainstream gym.

"I don't know if she was afraid of training a fat person or what it was," Robertson said. "So all I was able to do was sit in a corner and work on a treadmill. It definitely hurt."

In six months at Downsize Fitness, Robertson dropped 20 pounds. Her goal weight is 225.

"The workouts are stressful, they are trying," Wisniewski said. "We ask people to come five times a week. We call people if they don't show up."

Basic membership at the gym is $50 a month. A premium membership, which includes a trainer, a nutritionist and unlimited group classes, costs $250 a month.

Some critics say that Downsize Fitness segregates obese people, but Wisniewski believes it is for a purpose.

"Gyms are built for fit people to stay fit," he said. "I don't think they are built for fat people to get fit. In a way, we are segregating, but we are segregating for a reason."

Lewis Cline also joined Downsize Fitness six months ago, when his starting weight was 310 pounds.

He said he tried to lose weight at one of those "other" gyms and was a member for 12 years, paying $75 a month. But after only going for one year, he never went back.

"It wasn't something where I was comfortable going into the gym," Cline said. "You are on your own. There is no one there to help you. There is no one to explain what you should be doing."

Since joining Downsize Fitness, he has dropped 50 pounds and his goal weight is 180.

Exercise physiologist Jennifer Ventrelle agreed that it makes a difference to be comfortable when deciding to engage in healthy behavior.

"It's not about, 'Oh, it's OK that you're big. You can stay the way that you are. It's OK that you are not physically active because you don't feel comfortable in the gym,'" she said. "You're going to be working just as hard. It's just in a different environment. You're being more accommodating to someone so that they can engage in healthier behaviors."

Chris Almaguer is Downsize Fitness' biggest success story. The first day he entered the gym, he weighed nearly 500 pounds. Just one year later, he has lost a whopping 202 pounds. His goal weight is to get to 190.

"It was more like this, I was just trying to hold myself up and my back was just hurting too much," he said.

He is now one of the gym's most active members.