Mr Opik said the two were friends before the campaign
MP Lembit Opik, known for his colourful love life, has played down rumours he is dating a 21-year-old lingerie model.
The Lib Dem, who has been engaged to Cheeky Girl Gabriela Irimia and weather presenter Sian Lloyd, said he was "very good friends" with Katie Green.
He said she was a "lovely girl" but they had been working together on a campaign against eating disorders.
In a magazine interview, Ms Green praised him as "very charming" and said they had been for "plenty of dinners".
"I'm meant to be going up to his home in Wales soon for the weekend. I have always liked an older man and it's refreshing that he sees me as intelligent," she told Closer.
The interview prompted stories the two were dating but Mr Opik was more coy about the status of their relationship.
"I'm not one for semantic definition," the Montgomeryshire MP told the BBC.
"There's not two ways about it, we get on really well but actually we are trying to raise the profile of eating disorders," he said.
"We are spending a lot of time together but what we are trying to do is get people to realise that it's not too smart to lose weight when you can suffer organ damage or die.
"The fashion industry needs to wise up and stop making it look fashionable to be dangerously thin."
He said Ms Green - who hit the headlines when a modelling agency told her to slim down from a size 12 to a size 10 - had asked him to help with her "Say No to Size Zero" campaign because the two were friends.
But he said it was an issue close to his heart because he knew people who had had eating disorders and it was "terribly debilitating".
Mr Opik's love life has helped make him one of the most widely recognised MPs in the Commons and a staple of gossip magazines - having been engaged to Sian Lloyd, then Gabriela Irimia.
But his reputation may have harmed his efforts to become president of the Liberal Democrats - there were rumours of a Stop Lembit campaign and the contest was won by Baroness Scott.
He is not the only MP to get involved with efforts to curb the use of size zero models on the catwalk.
Then culture secretary Tessa Jowell said in 2006 "stick thin" models should be barred from London Fashion Week.
In 2007 she said they should undergo medical examinations before taking to the catwalk.