Five new leaders are set to overhaul their health when the long-running RTÉ reality show returns tonight.
And fitness expert Karl insisted he won't be pulling any punches when it comes to helping the volunteers, as well as those across the country following them, to reach their goals.
"Sometimes the bigger picture is, 'How can I get this person to do the very best in terms of their health and wellness over the course of eight weeks?'" he argues.
"And sometimes it's not being nice - sometimes it's challenging people a little bit.
"Sometimes you have to get a little tougher than I would normally be, purely because people have eight weeks, and you want them to walk away happy with what they did, so sometimes you have to be a little bit firmer. If people aren't happy, so be it."
"You'll always get a bit of criticism - that's perfectly cool. I'm big enough now, I can handle it."
Nice guy Karl was last year vilified by viewers after tackling leader Paul Devaney for appearing not to take the televised weight loss challenge seriously.
The dad-of-two from Galway went on to shed almost two stone before showing off his slimmed-down new look on the OT red carpet during the grand finale.
"People see a snippet," begins Karl of his harsh exchange with the 42 year-old construction worker.
"The whole Paul thing was very simple - each week you're trying to find the right way to unlock [leaders'] potential, and with him that was what was behind it.
"It was like, 'Listen, you're a bit of a joker, you really need to show us that you want to be here', because I knew that would get him fired up, which is exactly what happened.
"But I got, like I mean, online harassment about it, I really did.
"I got some serious grief over it, and that's fine because at the end of the day, his best interests were at heart.
"And had I not said it, I'm not sure he would have done as well as he did."
"When I started on the show, I was 24, I would have had a different reaction to [abusive messages]," he admits.
"Now I generally reply and say, 'Thanks a million for your comment, delighted to have it and happy to meet you for a coffee to discuss it properly in person -just let me know when suits you', and no one ever does.
"They're trying to get you riled up and have a row with you; I kind of kill it with kindness."
Mum-of-four Sarah O'Connor Ryan (38) from Tipperary is one of the five new leaders set to brave the Lycra and weighing scales each week in a bid to transform their health in 2022.
And personal trainer Karl is ready for the 'fat-shaming' criticism sure to be once more thrown at the programme as it enters its 15th season.
"It's not fat-shaming," defends the expert, who appears alongside psychologist Dr Eddie Murphy, dietitian Aoife Hearne and GP Dr Sumi Dunne on the series. "We're trying to educate people how to be healthier.
"One of the many aspects of health is weight, and you can dress it up any way you want, but it is one of the health indicators. By being overweight, you're putting yourself at a higher risk.
"We're trying to teach you how to manage that and to reduce your weight gain, and if you want to lose some weight, fantastic - jump on board.
"But it comes back to health and wellness all the time - it isn't purely weight loss.
"Last year's show was huge in terms of ratings because people really need help when it comes to their health," continues Karl.
"And with Covid, health has become deeper - there's psychological, there's mental, there's physical, there's emotional, there's weight, there's fitness. It's a wider conversation now, which is great.
"Particularly this year's show, you're going to see a lot more behind the scenes stuff that you never got to see in terms of the quantity of measurement that we do every single week - you're going to see exactly what we measure, which I'm really happy about.
"I think that's where the show becomes even more important for people, because however January's going to look, it's going to be a tough January. It's a great time to get moving so it's good to be back on."
Superfit Karl celebrates his 40th birthday later this month, but he's not ready to embrace his inner 'MAMIL' (middle-aged man in Lycra) just yet.
"I'm 40 in two weeks," shares the Meath native, who presents his own podcast, Real Health with Karl Henry. "I'm feeling good about it … I think. I feel fit and I feel healthy. I'm probably as fit as I've ever been at the minute. I didn't succumb to the 'Covid kilos'," he unsurprisingly confirms.
"I'm very normal. Over the years, I've put on weight, I've lost weight, and I suppose that's what helps me to relate to people in terms of what I do for a day job. The older I get the more health and wellness become important.
"I work in an industry full of shredded six-packers - I'm not one of those, I never have been one of those, I've no interest in being one of those. I just aim to be healthy."
Operation Transformation is on RTÉ One tonight at 9.30pm.