Personal trainer fattens up to help his clients

Paul James, a personal trainer and former model, before his weight-gain attempt (L) and halfway through his challenge to add 40kg (R). Photos / Supplied

Australian personal trainer Paul James is going to extreme lengths in a bid to understand how his overweight clients feel.

The 32-year-old former model made it his New Year's resolution to add an additional 40kg to his 80kg frame.

"I wanted to walk a mile in the shoes of my clients so I could better empathise with them" he explained.

Since kicking his regular gym sessions to the curb in January, James has succeeded in gaining 35kg - and his shape has changed dramatically.

A rounded tummy sits in place of the six pack he once sported and he has noticed his thighs rubbing together and a persistently sore back.

Melbourne-based James says it hasn't been difficult to gain the weight, despite being in good shape before he started.

"I'm just making the incorrect choices ... it's not uncommon to what many people are doing every day."

And those incorrect choices are certainly having an effect on his health.

The Herald Sun this week reported that James' liver enzyme readings had spiked to double the healthy amount and his blood pressure had also climbed.

But James insists he's being careful.

"I've got a health professional who monitors me on a weekly basis.

"I'm going about it in a responsible manner and if my doctor told me to stop, I would."

Once he hits his goal weight of 120kg, James plans to maintain it for three months, before embarking on a programme to lose it all again by year's end.

He hopes living with the extra weight for a time will give him a better understanding of the "day-to-day realities of life" for people carrying extra kilos.

But he admits slimming down may not be quite the walk in the park he'd envisaged.

"A month ago I would have said it would be easy, but I'm enjoying the ease of not having to think about what I eat and when I eat all the time," he said.

"I'm also lethargic a lot of the time and a little bit unmotivated."

Nevertheless, James - who is filming his journey for a documentary - says he plans to get back on the treadmill as soon as he's able.

"I'm trying to show people that, no matter what condition you're in, you can bring it back with a healthy diet and exercise."