The presenter's mother, Margaret, was diagnosed with Alzheimer's disease in her late 50s and died aged just 61.
Katie, 45, said: "It was a tough time. I feel incredible admiration and respect when I think about what my father and my younger sister, who hadn't yet gone to university and still lived at home, endured when mum was ill.
"It was harrowing, something you wouldn't wish on anyone. I visited as often as I reasonably could but dad was adamant my mother would not have wanted me to drive three hours each way on the motorway just to feel unhappy and stressed."
Alzheimer's now affects 850,000 people in the UK and costs £26billion a year to treat.
It was harrowing, something you wouldn't wish on anyone
Dementia is caused by diseases, most commonly Alzheimer's, which result in the loss of brain cells.
Katie - who is among the favourites to win Strictly - refuses to be frightened about the prospect of early-onset Alzheimer's, which can have genetic links.
She said: "There's no point worrying but I do support dementia charities and keep an eye on new breakthroughs which are bringing hope.
"I do wish more investment was being pumped into research - the numbers of people suffering from it are only going to keep increasing."
Katie may be happily married to restaurateur John Vincent, but she has come clean about her crush on professional partner Anton du Beke. "I never thought for a moment I would be partnered with him," she said.
10 Things You Didn't Know About: Katie Derham
"He was always my favourite professional when I watched the show because he has real presence and bearing and looked like such a safe pair of hands - and I'd met him when I was in the audience.
"Anton has always been my guilty secret. I love my husband and John's absolutely thrilled I'm taking part. He's met Anton - and his girlfriend Hannah too - so there's no need to worry on that score."