How Cillian Murphy transformed for his role in 'Oppenheimer' — and the dangers of extreme dieting

• Cillian Murphy went to extreme lengths to lose weight for his role in "Oppenheimer."
• The Irish actor plays the creator of the atom bomb, J. Robert Oppenheimer, in the movie.

Christopher Nolan's movies are known for going to the extremes, and 2023's "Oppenheimer" is no exception. The biopic follows Cillian Murphy as J. Robert Oppenheimer as he builds the world's first atomic bomb during World War II — and the Irish actor took drastic measures to prepare for the film.

The star told The New York Times that he wanted to bring a physical performance to the role, saying: "I love acting with my body, and Oppenheimer had a very distinct physicality and silhouette, which I wanted to get right."

He pointed out that the scientist had a slim frame due to his diet, which meant losing weight. "I had to lose quite a bit of weight, and we worked with the costume and tailoring," he said. "He was very slim, almost emaciated, existed on martinis and cigarettes."

Emily Blunt plays Oppenheimer's wife, Kitty, in the movie and she recalled being surprised by Murphy's diet. She told Extra TV that he had "such a monumental undertaking," and that "he could only eat, like, an almond every day. He was so emaciated."

Murphy told The Guardian: "It's like you're on this f--king train that's just bombing. It's bang, bang, bang, bang. You sleep for a few hours, get up, bang it again."

He added: "I was running on crazy energy; I went over a threshold to where I was not worrying about food or anything… But it was good because the character was like that. He never ate."

The star also joked that he's had to smoke plenty of fake cigarettes over the course of his career. "I've smoked so many fake cigarettes for Peaky and this. My next character will not be a smoker," he said. "They can't be good for you. Even herbal cigarettes have health warnings now."

But despite his extreme efforts for the role, Murphy admitted that it's easy to become "competitive with yourself a little bit," something he said "is not healthy, I don't advise it."

Extreme dieting can be dangerous

While fasting for short periods of time has been linked to some health benefits and often contributes to weight loss, Murphy would not have survived for long if he really was only eating one almond every day.

While actors preparing for roles often undergo drastic steps and drop their food intake extremely low to achieve a certain look in a short time frame, crash dieting is not advisable if you want to achieve healthy, sustainably weight loss.

Losing weight requires being in a calorie deficit — consuming less energy than you're burning — but dropping your calories too low come with considerable risks. According to the Academy of Nutrition and Dietetics, low-calorie diets can lead to nutritional deficiencies, muscle loss, brain fog, and gallstones.

Severe restriction is also unsustainable for most people, so it can lead to yoyo dieting, falling into a "binge-restrict" cycle, and developing an unhealthy relationship with food.

Losing weight slowly generally means you're more likely to keep the weight off. Nutrition professionals recommend including a balance of protein, carbs, and fat in your diet, not over-restricting by enjoying all foods in moderation, and eating in a small calorie deficit.