Kelly Clarkson unveiled her svelte new figure on her new show Duets last week, and she's crediting her slimmer self to her boyfriend, Brandon Blackstock.

"No one wants to be untoned when you're dating someone," Clarkson tells Life & Style of her inspiration to slim down. "I've always fluctuated, but now I'm in a happy relationship."

Clarkson says that although Blackstock (stepson of country superstar Reba McEntire) keeps her "more active", she says the two will never work out together, admitting, "We did that once. I don't like huffin' and puffin' in front of my man and looking all red and gross and sweaty."