Jake Gyllenhaal was pushed to his limits by Bear Grylls on a gruelling survival expedition.
The Hollywood star, who has trained to get in shape to star in action movies, went to Iceland with the former soldier for his show Born Survivor and asked to be pushed - but admitted he was surprised by how tough he found it.
Bear revealed: "A couple of weeks ago I took Jake Gyllenhaal, reportedly the fittest man in Hollywood, on a Born Survivor to Iceland. He said he wanted to to be pushed and afterwards said he'd never been pushed so hard.
He went on: "We had crazy conditions - sleeping in snow caves, huge river crossings and these torrential storms. I was a little bit nervous as I promised Jake's management team I would keep him alive, but he was brilliant.
"There's something magical about the raw emotion of fear and nerves. I think that's what people like about the show."
Bear, 37, admits that having three sons - Jesse, Marmaduke and Huckleberry, who he describes as "little Mowglis" - has focused his attention on living.
He said: "You've got to get back in one piece to them and that's the only thing that matters.
"A couple of weeks ago one of our crew almost got killed in a rock fall. I think the more you survive, the more your experience and knowledge grows and the more wary you are next time.
"I always say at the start of filming, 'You can get it right a thousand times, but you only get it wrong once'."
:: Mud, Sweat And Tears by Bear Grylls is available now, published by Channel Four books