"I was chubby, always had big feet, the wrong shoes, bad hair," she said on last night's episode of Running Wild with Bear Grylls.
"When I grew up, I never heard positive reinforcement about body image from any female in my life," Winslet admitted, while traipsing about the mountains of Snowdonia, Wales with Grylls, 41. "I only heard negatives. That's very damaging because then you're programmed as a young woman to immediately scrutinize yourself and how you look."
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Winslet is determined to protect her three children from the negativity she faced. She told Grylls about one particular self-esteem-boosting exercise that she practices with her daughter Mia, 14.
"I stand in front of the mirror and say to Mia, 'We are so lucky we have a shape. We're so lucky we're curvy. We're so lucky that we've got good bums.' And she'll say, 'Mummy, I know, thank God,' " Winslet said. "It's paying off."
Interspersed with a significant amount of body talk was, well, mountain-scaling, goofy antics and even a hilarious recreation of Titanic's most iconic scene (of "never let me go" fame) with Grylls.
Turns out, Winslet is quite the adventurer.
"It's really fun making a fire with a tampon. It's really fun learning how to rock climb properly and tie knots properly and make tea with bits of boiled up bark," she said.
Watch Winslet's "Fireside Chat" with Grylls: