Actress Blake Lively adopted a caveman diet to boost her strength throughout the grueling The Shallows shoot.

The 28-year-old plays a surfer left to fend off a shark in the gripping new action movie, and to transform her body to look like a health-obsessed medical student, she radically altered the types of food she was eating by adopting a form of paleo diet, which is thought to consist of the kinds of meals early humans ate while hunting and gathering thousands of years ago.

“It (filming The Shallows) was eight months after I had a baby," she tells news outlet Extra, referring to the birth of her daughter James in December, 2014. "It was such an undertaking, 'cause your body changes so much, not just for vanity's sake, but I had to be strong to keep up with it.

"It was my job to train. No gluten, no soy, and then no processed foods. I had butter, I had sugar, I had all that stuff. It was just making my own stuff. I guess it was close to paleo... it was nice to eat clean like that.”

Her physical training was extensive as well, so much so Blake felt as if she was preparing for some type of professional sporting competition.

“It was like being an athlete more than an actor,” she explains. “It was incredibly physically demanding. I had to do my own stunts six of the eight weeks, I had to look like someone who was strong enough to be a surfer.”

Fans can witness an extraordinarily fit Blake beating back the waves and fighting for survival in The Shallows when the film is released in theaters on Friday (24Jun16).