Rachel Frederickson's weight loss brings out trolls

A day after Rachel Frederickson won the latest US season of The Biggest Loser, after shedding nearly 60 per cent of her body weight, attention wasn't focused on her $US250,000 ($281,357) win but rather the criticism surrounding her loss.

Experts cautioned that regardless of her current weight, the criticism being levied on social media about her losing too much isn't helpful.

A more constructive message is needed, they say, centring on overall healthy living and body image.

Ms Frederickson, 24, dropped from 118 kilograms to 47 under the show's rigorous exercise and diet regimen, and time spent on her own before the finale.

Biggest Loser Australia trainer Shannan Ponton has also weighed in on the controversy, saying he was shocked by Ms Frederickson's dramatic weight loss.

"That's a hell of a lot of weight to lose," he said.

Before and after: The Biggest Loser winner Rachel Frederickson.

Some took to Twitter to call for a minimum body mass index (BMI) threshold, while one even quipped the victory confetti might knock Ms Frederickson over.

Ms Frederickson was a three-time state champion swimmer at Stillwater Area High School in Minnesota, then turned to sweets for solace after a failed romance with a foreign exchange student she followed to his native Germany.

Ms Frederickson's newly thin frame lit up Twitter on Wednesday, with many viewers pointing to the surprised expressions on the faces of trainers Jillian Michaels and Bob Harper during the show's Tuesday night finale.

Many tweeted that Ms Frederickson looked anorexic and unhealthy, while others congratulated her for dropping 70 kilograms.

Jillian Lampert, senior director of the Emily Program, an eating disorder treatment program based in St Paul, said Ms Frederickson's body mass index was below normal.

But she said the criticism directed against Ms Frederickson isn't helpful.

"As a society we often criticise people for being at higher weights - that's part of why we have the TV show The Biggest Loser - and then we feel free to criticise lower weight," Ms Lampert said.

Mr Ponton says while it's easy to rush to conclusions, Ms Frederickson's slim figure does not necessarily mean she's unhealthy.

Stopping short of agreeing with the Twitter backlash, Mr Ponton says instances of contestants losing too much weight are the exception, not the rule.

"Literally hundreds of people have been through the doors of The Biggest Loser both in the US and in Australia," he said.

"And that's the first one who's been accused of taking it too far."

Australian contestants undergo strict physical and psychological testing throughout filming to ensure they lose weight healthily, Mr Ponton added.

In the US, participants are also subject to psychological screening before they sign up for the popular reality TV show.

But asked whether such tests scrutinise a contestant's chances of developing an eating disorder, he said possible contestants usually already had one.

"I think most of our contestants have an eating disorder - an overeating disorder," he said.

Ms Lampert said a more constructive message to send young people would centre on well-rounded health and the importance of eating well, moving well and sleeping well.

Joanne Ikeda, a dietitian and retired faculty member at the University of California at Berkeley's Department of Nutritional Sciences, added that the focus needed to be on embracing a diversity of body size.

"We should be happy we don't all look like Barbie and Ken," she said. AP with AAP

Rachel Frederickson.

Rachel Frederickson.