The radio and TV presenter and mum to four-year-old Ben revealed she had gone back to the vice in an interview with The Sun.
She said she ate well and, having just turned 40, had no health worries, but she revealed: "I've just started smoking again and I'm not proud. I've got to get it into check.
"Trouble is the most interesting people are the ones outside smoking. I always think, 'I want to be outside with the cool ones'. I also drink too much coffee."
Kate, who has been a single mum since she split up with Ben's dad Darren Emerson in 2011, presents the Boots Feel Good Forum radio show on Sundays.
She said she exercises by doing weights and resistance training.
"And I try to do the school run really quickly to get my heart rate up," she said. But she never diets.
"I refuse to wake up thinking, 'I'm not going to eat X, Y or Z today'," she said.
"You'll spend all day craving it, then you'll eat it and end up feeling a total failure.
"If things start to get a bit tight I just cut the c**p for a while.
"It took me a year to get back into shape after I had the baby."
And she revealed that she has been in training to be a counsellor after she becamse a single mum, saying: "I wanted to be the best person I could be for my son. As part of it you have to go into therapy, which was a really healthy thing for me to do after 20 years in telly. You learn a lot about yourself and not all of it very nice!"