Louise Minchin, 50, completed the Norseman triathlon, held in Norway, at the weekend.
The presenter returned to the BBC Breakfast sofa yesterday, where they aired a video showing Louise take on the ultimate challenge.
The Norseman triathlon starts with a swim from the loading bay of a car ferry, then on to cycling 180 kilometres though the mountains, finished with a 42.2 kilometre run.
As the segment of Louise’s phenomenal achievement was aired on the BBC, one viewer asked how much the broadcaster paid to follow her.
They asked the host: “How much is she getting paid for doing that c**p?? It’s a public funded broadcaster we want to know.”
Louise, who raised money for mental health charity, Mind, by doing the triathlon, simply replied back: “Nothing.”
The viewer added: “So who’s paid for the flights and the accommodation and the film crew and the coverage …plus every other expense?”
Louise didn’t respond again, but fans backed the BBC Breakfast host as they praised her.
One person wrote: “Well I am happy that my money was spent on covering this amazing lady’s achievement. Such grit and determination, if only we could all have a sprinkle of it.”
Another supported: “Louise is doing it in her own time for @MindCharity don’t you know. Because that’s the type of thing good people do.”
Having returned to work alongside Dan Walker on BBC Breakfast yesterday, Louise shared a snap of herself with her co-star.
The presenter said in the caption she was “hobbling” but “happy” after completing the gruelling triathlon.
She said: “Hello, hello, good morning, sorry to be so late, the #norseman heat has ruined my manners #backonthesofa #hobblingbuthappy @nxtri.”
Louise had been updating her 161,000 followers on her journey over the past few days.
After finishing the race, Louise said she felt “broken” as she showed off her Norseman T-shirt.
She wrote: “I am a bit broken but delighted to be the proud owner of a white #Norseman t-shirt after a very long tough brilliant day in the stunning Norwegian countryside. A brilliant experience.
“Thanks for all your support for @mindcharity and thanks to everyone at @nxtri Next plan, rest.”
Louise followed it up with a picture of herself in tears as she emerged from the sea after the epic swim.
She said: “Morning, I am still recovering from that feeling of elation after jumping off a ferry to swim in a fjord with 250 other people, beneath snow touched mountains towards a bonfire on a beach. #Norseman it was for me. Thanks for your lovely messages.”
BBC Breakfast airs on BBC One from 6am.