If you’ve seen Alicia Vikander turning heads on the red carpet or stealing scenes on the big screen (she is an Oscar winner after all), then you know the actress is pretty fit. But in order to prepare for all the action scenes in the new Tomb Raider film, she had to take things to the next level. Because when you’re stepping into a role originated by Angelina Jolie, you don’t want to mess around.

In addition to a vigorous weight-training program, Vikander’s trainer Magnus Lygdback says the A-lister also turned the ketogenic diet (or the keto diet for short) to help her get into the best shape of her life. According to a recent interview with PopSugar, Lygdback says they started the regimen two months before filming for Tomb Raider began.

So what exactly is this eating plan that has Hollywood buzzing? “The ketogenic diet focuses on eating foods that are low carbohydrate and high in fat,” says Megan Roosevelt, RDN and founder of Healthy Grocery Girl.

“Eating a low-to-no carb diet can put the body in a state of ketosis where it produces ketones and burns fat stores as a source of energy, versus using glucose from the diet,” Roosevelt explains. “This can help someone achieve weight and fat loss.”

Because Vikander was traveling so much, Lygdback told PopSugar they decided the keto diet would be best because it’d be an easier way to control her macronutrients. But that’s not to say this diet was a piece of cake (which definitely is not on the approved list of foods here). “It's always tricky the first couple of days, but [Alicia's] willpower is just unbelievable, and she didn't complain, even though I could see she was struggling a little bit. She kept on bringing it every day,” Lygdback said in the interview.

Want to give keto a shot? Roosevelt has a few quick tips: “Avoid processed or packaged foods which are typically higher in sugar,” she says, adding that incorporating real, nutrient-dense foods into your meal prep will help you stay on track. Vikander reportedly relied on a lot of fish and seafood for her protein, as well as avocado and coconut oil as a fat source.

Hey, if it’s good enough for Lara Croft...