She had always struggled with her weight, but in January 2006, Kai Hibbard was in real trouble: At just 26 years old, her 5-foot-6 frame carried 265 pounds.

Her best friend staged a mini-intervention. “She said, ‘Hey, I love you, but you’re super-fat right now,’ ” Hibbard recalls. The pal encouraged Hibbard to try out for the smash NBC reality show “The Biggest Loser.”

“So I made a videotape,” Hibbard says, “and the next thing I know, I’m on a reality TV show.”

Hibbard had never seen “The Biggest Loser.” She had no idea what she was in for.

“The whole f- -king show,” she says today, “is a fat-shaming disaster that I’m embarrassed to have participated in.”

Since its premiere in 2004, “The Biggest Loser” — which pits obese contestants against one another in a race to lose the most weight — has been one of the most popular reality shows of all time.

The 16th-season finale will air live on Jan. 29. Average weekly viewership is 7 million people, and about 200,000 people audition per season.

The show rakes in about $100 million annually in ad sales, with ancillary products such as cookbooks, DVDs, protein powder, clothing, video games and branded weight-loss camps bringing in tens of millions of dollars more per year.

In a country where two-thirds of the population is overweight or obese, “The Biggest Loser” has multifaceted appeal: It’s aspirational and grotesque, punitive and redemptive — skinny or fat, it’s got something for you. It’s not uncommon to see contestants worked out to the point of vomiting or collapsing from exhaustion. Contestants, collegially and poignantly, refer to one another as “losers.”

“You just think you’re so lucky to be there,” Hibbard says, “that you don’t think to question or complain about anything.”

Contestants are made to sign contracts giving away rights to their own story lines and forbidding them to speak badly about the show.

Once selected, Hibbard was flown to LA. When she got to her hotel, she was greeted by a production assistant, who checked her in and took away her key card. When not filming, she was to stay in her room at all times.

“The hotel will report to them if you leave your room,” Hibbard says. “They assume you’re going to talk to other contestants.”

Another competitor, who spoke to The Post on the condition of anonymity, says that when she first checked in, a production assistant also took her cellphone and laptop for 24 hours. She suspects her computer was bugged.

“The camera light on my MacBook would sometimes come on when I hadn’t checked in,” she says. “It was like Big Brother was always watching you.” The sequestration lasts five days.

After an initial winnowing process, 14 of 50 finalists are taken to “the ranch,” where they live, work out and suffer in seclusion. (The remaining 36 are sent home to lose weight on their own, and return later in the season.)

Those who remain, Hibbard says, are not allowed to call home. “You might give away show secrets,” she says. After six weeks, contestants get to make a five-minute call, monitored by production.

“I know that one of the contestants’ children became very ill and was in the ICU,” Hibbard says. “He was allowed to talk to his family — but he didn’t want to leave, because the show would have been done with him.”

Once at the ranch, contestants are given a medical exam, then start working out immediately, for dangerous lengths of time — from five to eight hours straight.

“There was no easing into it,” Hibbard says. “That doesn’t make for good TV. My feet were bleeding through my shoes for the first three weeks.”

“My first workout was four hours long,” says the other contestant. She came on the show a few years ago at more than 300 pounds. On her first day, she was put through this regimen:

Rowing

Body-weight work

Kettle bells

Cool-down on treadmill

Interval training

Stairmaster

Outside work with tires

At one point, she collapsed. “I thought I was going to die,” she says. “I couldn’t take any more.”

Her trainer yelled, “Get up!,” then made a comment about a sick and overweight relative.

“I got up,” she says. “You’re just in shock. Your body’s in shock. All the contestants would say to each other, ‘What the f- -k just happened?’ ”

The trainers, she says, took satisfaction in bringing their charges to physical and mental collapse. “They’d get a sick pleasure out of it,” she says. “They’d say, ‘It’s because you’re fat. Look at all the fat you have on you.’ And that was our fault, so this was our punishment.”

Hibbard had the same experience. “They would say things to contestants like, ‘You’re going die before your children grow up.’ ‘You’re going to die, just like your mother.’ ‘We’ve picked out your fat-person coffin’ — that was in a text message. One production assistant told a contestant to take up smoking because it would cut her appetite in half.”

Meanwhile, their calories were severely restricted. The recommended daily intake for a person of average height and weight is 1,200 to 1,600 calories per day. The contestants were ingesting far less than 1,000 per day.

Hibbard says the bulk of food on her season was provided by sponsors and had little to no nutritional value.

“Your grocery list is approved by your trainer,” she says. “My season had a lot of Franken-foods: I Can’t Believe It’s Not Butter spray, Kraft fat-free cheese, Rockstar Energy Drinks, Jell-O.”

At one point, Hibbard says, production did bloodwork on all the contestants, and the show’s doctor prescribed electrolyte drinks. “And the trainer said, ‘Don’t drink that — it’ll put weight on you. You’ll lose your last chance to save your life.’ ”

Such extreme, daily workouts and calorie restriction result in steep weight losses — up to 30 pounds lost in one week.

“Safe weight loss is one to two pounds per week, and most people find that hard,” says Lynn Darby, a professor of exercise science at Bowling Green State University. “If you reduce your calories to less than 800 to 1,000 a day, your metabolism will shut down. Add five to eight hours of exercise a day — that’s like running a marathon, in poor shape, five days a week. I’m surprised that no one’s ­really been injured on the show.”

In fact, contestants have been seriously injured, but it’s not often shown. The first-ever “Biggest Loser,” Ryan Benson, went from 330 pounds to 208 — but after the show, he said, he was so malnourished that he was urinating blood. “That’s a sign of kidney damage, if not failure,” Darby says. Benson later gained back all the weight and was disowned by the show.

In 2009, two contestants were hospitalized — one via airlift. And 2014’s Biggest Loser, Rachel Frederickson, became the first winner to generate concern that she had lost too much weight, dropping 155 pounds in months. She appeared on the cover of People with the headline “Too Thin, Too Fast?” Frederickson (5 feet 4, 105 pounds) admitted to working out four times a day, and within one month of the finale had gained back 20 pounds.

“Just calorie restriction in and of itself has to be supervised,” Darby says. “I mean, people die. Then add that exercise load on top of it. The joints of someone who has never exercised absorbing the force of 300 pounds of jumping or bouncing? It’s just not safe.”

Hibbard says she and other contestants sustained major physical damage.

“One contestant had a torn calf muscle and bursitis in her knees,” Hibbard says. “The doctor told her, ‘You need to rest.’ She said, ‘Production told me I can’t rest.’ At one point after that, production ordered her to run, and she said, ‘I can’t.’ She was seriously injured. But they edited her to make her look lazy and bitchy and combative.”

Hibbard’s own health declined dramatically. “My hair was falling out,” she says. “My period stopped. I was only sleeping three hours a night.” Hibbard says that to this day, her period is irregular, her hair still falls out, and her knees “sound like Saran Wrap” every time she goes up and down stairs. “My thyroid, which I never had problems with, is now crap,” she says.

“One of the other ‘losers’ and I started taking showers together, because we couldn’t lift our arms over our heads,” says the other contestant. “We’d duck down so we could shampoo each other.”

The trainers, she says, were unmoved. “They’d say stuff like, ‘Pain is just weakness leaving the body.’ ”

This contestant says she and most of her castmates came away with bad knees. “There was one guy whose back was so bad, he could only exercise in the swimming pool. By the end of the show, I was running on 400 calories and eight- to nine-hour workouts per day. Someone asked me where I was born, and I couldn’t remember. My short-term memory still sucks.”

So why do so many contestants stick with the show?

“You’re brainwashed to believe that you’re super-lucky to be there,” Hibbard says. One doctor told a contestant she was exhibiting signs of Stockholm syndrome, and Hibbard herself fell prey to it.

“I was thinking, ‘Dear God, don’t let anybody down. You will appear ungrateful if you don’t lose more weight before the season finale.’ ”

The other contestant had a similar response. Despite “the harassment and the bullying, I wanted to please them,” she says. She lost seven pounds in one week and apologized. “I’d lost 12 pounds the week before,” she says.

For Hibbard, the low point came when she and her fellow “losers” were brought to a racetrack, where they were housed in individual horse stalls. When a bell went off, they had to run neck-and-neck like animals, picking up sacks filled with their lost weight on the way.

“I walked,” she says. It was her minor form of protest. “They edited it to look like I was lazy,” she says, “but I wasn’t participating because it was humiliating.”

When Hibbard got home, her best friend and boyfriend took her straight to the doctor. “She said I had such severe shin splints that she didn’t know how I was still walking,” Hibbard says.

The show’s most famous trainer, Jillian Michaels, quit “The Biggest Loser” for the third time in June 2014, with People magazine reporting she was “deeply concerned” about the show’s “poor care of the contestants.”

In a statement to The Post, NBC said only: “Our contestants are closely monitored and medically supervised. The consistent ‘Biggest Loser’ health transformations of over 300 contestants through 16 seasons of the program speak for themselves.”

Expert Darby doesn’t buy it. “With most weight-loss programs, people gain at least half of the weight back,” she says. “And the people who are most successful in our studies are the ones who make small changes over the long term — so I can’t imagine that anyone on ‘The Biggest Loser’ has weight loss that’s sustainable.”

Hibbard, who lost 121 pounds to end up at 144, put weight back on, but won’t say how much. Yet she feels a responsibility as someone once held up as false inspiration.

“If I’m going to walk around collecting accolades, I also have a responsibility [to tell the truth],” she says. “There’s a moral and ethical question here when you take people who are morbidly obese and work them out to the point where they vomit, all because it makes for good TV.”

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