The author of Positive Bodies: Loving the Skin You're in believes the latest instalment of the popular reality show, in its eighth season in Australia, focuses solely on boosting television ratings rather than the self-esteem of the contestants.

"It's not what size you are that's important, but how healthy your organs and brain are," she said following the first elimination episode of The Biggest Loser: The Next Generation.

The new season follows the journey of parents and their children grappling with obesity. The aim this season, according to the opening credits, is "to tackle and break the cycle of generational obesity".

The contestants have been living the "Camp Biggest Loser" lifestyle for less than a month and every episode so far has been peppered with twists and turns, tears and tantrums.

''Many of the contestants have experienced teasing and bullying for their size and this is not good for their self-esteem,'' Dr Lewis said. ''But equally, being yelled at by instructors that are supposed to be helping you achieve your health goals can be detrimental too, and this can be seen in the tearfulness of some of the contestants. [It] indicates they're being pushed too hard and isn't very good for their self-esteem or body image," she said.

In week one it was revealed that before the cast began their Biggest Loser ''journey'' everyone was secretly filmed in their homes and having their eating habits monitored and critiqued by the trainers.

Last week they all took part in a 21-kilometre half-marathon after the heaviest contestant in the worldwide history of the show arrived.

Twenty-seven-year-old Kevin walked into the house and tipped the scales at more than 250 kilograms.

In the episode before the half-marathon, he struggled to complete a one-minute warm-up on a treadmill. He also struggled to control his temper and outbursts.

"It is completely unreasonable to expect someone who is morbidly obese and does no or very little physical activity to all of a sudden run a half-marathon,'' Dr Lewis said.

"What he probably needs is encouragement for his efforts, no matter how small. It is unreasonable to expect someone to go from being morbidly obese and not doing any physical activity, to exercising at the intensity expected of Kevin or to put someone like Kevin through unreasonable pain and suffering."

Dr Lewis said it was important that viewers be "media literate" when watching shows such as The Biggest Loser. She suggested parents talk to their children and highlight how certain elements of the show related and did not relate to reality.

The weight the contestants lose each week is one example of how unrealistic the series is. During the most recent weigh-in most contestants all lost more than five kilograms in a seven-day period.

"All the research evidence shows that slow, steady weight loss of about half a kilo per week is the best for healthy living and long-term weight maintenance. Losing kilos in one week is not going to be maintainable in the long run, and is actually quite dangerous,'' Dr Lewis said.

■ The Biggest Loser: The Next Generation airs on Southern Cross Ten every Sunday, Monday and Tuesday at 7.30pm.