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The Chase presenter says his doctors told him to give up fatty food and alcohol, The Sun reports. Walsh's father died of heart disease when he was 59 years old.
The 60-year-old told the publication: "I was a time bomb. I produce too much cholesterol.
"It's a silent killer. My heart guy said, 'Look Brad, you need to get fit."
Walsh heeded the doctor's orders and cut out alcohol and carbs. He says the age his father died helped motivate him to get his health on track.
"I had it in the back of my mind that I just had to get past my dad's age.
"So turning 60 was a bit of a milestone ... I got on the scales and saw I'd hit 14st 9lbs (92kg), the heaviest I've ever been. I really was quite big."
The star has so far lost 4.5kg.
He was treated to a surprise birthday party on UK's ITV channel when he turned 60 in June.
This Morning presenters Holly Willoughby and Phillip Schofield reflected on Walsh's career with the quiz show host.
He was moved to tears by the celebration, the Mail Online reported.
Walsh told the hosts: "It seemed like only yesterday I had my 50th birthday, then all of a sudden, this thing's happening on the telly, I feel humbled and a bit emotional actually."