The journalist and TV presenter told Closer magazine that the stress of moving house and her subsequent lack of free time meant that she was not following her usual healthy diet.
She said: "I'd gone up to a good size 14. I was really busy and I wasn't eating well or doing any exercise.
"I'm more organised about what I eat now. I'll always have a banana in my bag and I plan my meals properly."
The 50-year-old admitted that it was hard to maintain a fitness regime and keep the pounds off once she had finished training for the London marathon.
Lorraine Kelly's Nutrition Made Easy was published in 2009 and she has also produced a book on healthy eating for children, as well as an eating plan for babies and toddlers.