But the presenter's own diet is not all carrot sticks and lettuce leaves. In fact, she says: "I'm partial to a really grubby backstreet curry, washed down with a load of shandy."
The Shropshire-born presenter adds: "Me and my partner (TV director Charles Martin) are always saying, 'Can you imagine if the cameras were here?'
"We had friends round for a roast dinner. I thought, 'I'm going to eat slowly, I'm not going to have all the added extras, no bread for me'. I suddenly found myself two glasses of wine down, having pudding with custard, the Italian chocolates, everything. I know what I am doing, but I still do it."
Richardson also recounts how, as a youngster, she and some classmates raided the kitchens of their strict Staffordshire boarding school at midnight to stock up on biscuits and crisps.
"We're all secret eaters; we all fall into the traps," says the 43-year-old, whose honesty and humour have made her a hit with viewers on shows such as Secret Eaters, The Sex Education Show and Supersize vs Superskinny.
She has also written a Body Blitz Diet book, promoted as "perfect for anyone who wants to bust their gut in as painless a way as possible".
"I've just been doing the voiceover for Secret Eaters, so I can see myself in the monitor to match the words to the pictures. In this episode, I was looking at myself thinking, 'I look awful'."
"I am like any other woman in that I'm very self-critical. I might be the diet guru, but am a very ordinary person as well. I think it means people can identify more strongly with me, because I'm not a skinny minnie.
"I'm not Gwyneth (Paltrow) and I'm never going to be Gwyneth."
Secret Eaters has a serious message behind its light-hearted approach. In the first episode of the new series, which starts this week, we meet Surrey office workers Faye and Laura, who can't understand why they are overweight.
After volunteering to be tracked by hidden cameras and private detectives, however, it emerges the pals have been loading their plates with huge portions and munching sugar-laden treats.
Confronted by evidence of their secret scoffing by Richardson - in an incident room piled high with the calorific food they have been eating - the blushing pair then embark on a healthy eating plan.
Despite the show being in its third series, Richardson insists participants still have not worked out how to hide their habits.
"They know what they are letting themselves in for and the same rules apply, they always know there are cameras going in their house," she says.
"But this year we have had to be bigger and cleverer about the cat and mouse games - getting other informants on the go, following them in cleverer ways, getting friends and family to play along, and rigging up offices and workplaces as well."
So does Richardson get fans of the show snooping on the contents of her supermarket trolley?
"Thankfully no, because they all think I'm (TV presenter) Dawn O'Porter," she says. "Whenever I do get stopped and anybody says, 'What are you doing with that?', I say, 'I don't know what you're talking about, I'm Dawn!' She can be very, very useful sometimes.
"People seem to think me, Dawn and Claudia Winkleman are exactly the same person."
Perhaps it's because, like O'Porter, Richardson has gamely thrown herself into the action on the shows she fronts. She ran naked into the sea to celebrate her body on Supersize vs Superskinny, and had an STI screening on The Sex Education Show.
It was for the same series that she had one of the most personal screen experiences ever, when, at the age of 37, she agreed to take a fertility test. The results turned out to be below average, and she admits the experience was "quite traumatic".
"I was very upset, and it really threw me. But I think the only reason I was happy for that to be broadcast was because I thought, 'If this in any way chimes with other women watching - and it will do - then we are all in that same boat, I'm sharing the difficulty of that with other people'."
Richardson, who lives in London with Martin, her partner of 18 years, is still interested in starting a family, but doesn't want to go through IVF and is not sure about egg donation.
"I think that ship has sailed for me," she says.
When asked what her father, a retired vicar, makes of some of her previous on-screen antics, Richardson says: "He learned many years ago to roll his eyes and go, 'That's just my daughter'."
"I think he is proud of me underneath it all. He said to me, 'As long as you are happy, that is what is important in life'. And he is right."