Idris Elba would've collapsed if he hadn't changed his diet while making his new TV documentary.

The 44-year-old actor spent months getting himself in peak physical condition to become a professional kickboxer for 'Idris Elba: Fighter' - but Idris has admitted he needed to make some major changes to his diet to avoid causing serious damage to himself.

He shared: ''My doctor said, at my age, I would've collapsed within four months of training if I'd not changed my diet.

''My body would have shut down, my immune system would have been compromised and I may have caught an infection, so I wouldn't have been able to train.

''I discovered I had huge allergies to foods I was eating every day, like wheat, corn and things like that, which were working against my metabolism.

''That was one of the hardest things because I love pasta, rice and beer and I had to cut all of that out. As soon as I did I lost huge amounts of weight, but I also gained a lot of energy.''

Idris admitted he was particularly drawn towards making the documentary because he relishes tackling his fears.

The London-born star told the Daily Mirror newspaper: ''I try to tackle everything with how fearful I am. Am I not doing something because I'm scared of it? And if the answer is yes then it entices me to want to get over that fear.''

However, Idris also confessed that there was a glimmer of a mid-life crisis about his latest on-screen challenge.

The 'Luther' star said: ''In this instance, I think there was part of my ego being tested at middle age, too.

''Some people go out and buy Ferraris or a motorbike; I was interested in testing if my body can withstand extreme training.''