Matt Damon and Emily Blunt say 'Oppenheimer' costar Cillian Murphy 'never' joined the cast for dinner because he was dieting for his role

• Matt Damon and Emily Blunt said Cillian Murphy skipped cast dinners while filming "Oppenheimer."
• "We invited Cillian to dinner every night, and he never went," Damon said.

Matt Damon and Emily Blunt said Cillian Murphy didn't attend cast dinners while filming "Oppenheimer" because he was dieting as part of playing the titular role.

"Oppenheimer," written and directed by Christopher Nolan, stars Murphy as J. Robert Oppenheimer, the American theoretical physicist known as the father of the atomic bomb. The high-stakes thriller is based on Kai Bird and Martin J. Sherwin's Pulitzer Prize-winning biography "American Prometheus."

The cast of "Oppenheimer" boasts plenty of notable stars, including Blunt as Katherine "Kitty" Oppenheimer, a biologist who is also the wife of Murphy's character, and Damon as Leslie Groves, the director of the Manhattan Project.

Just as the real-life scientists gathered in the middle of nowhere for their nuclear experimentation, the cast and crew of "Oppenheimer" filmed in a desert designed to emulate Los Alamos, New Mexico, in the 1940s.

Damon and Blunt recently told Entertainment Tonight that the experience of filming "Oppenheimer" was like being at summer camp — except with plenty of margaritas.

"We were in New Mexico together, so we had dinner every night," Damon said.

"It's unusual for all the actors to be on location together," Blunt said. "We were shooting in this really remote location in the middle of the New Mexican desert, and there was one hotel, and we were all together. That doesn't always happen. Normally, everyone's in their own place, or you're shooting at home, or everyone's got their own house, and you don't hang out."

There was only one restaurant nearby, so the cast gathered there for daily meals, the actors said.

"That was the only way to get food," Blunt said.

"It wasn't like the kind of restaurant that was open 24 hours," Damon said. "It was open for breakfast, lunch, and dinner. It was like the dining hall, you would go in, and you'd see everybody, so everyone would just end up sitting together."

Damon and Blunt said Murphy missed out on cast dinners because of his diet for the film.

"We invited Cillian to dinner every night, and he never went," Damon said, with Blunt adding that Murphy instead ate "one almond most nights or a little slice of apple."

"He was losing so much weight for the part that he just didn't eat dinner, ever," Damon said.

Nolan said in interviews that he wanted Murphy, a frequent supporting actor in his films, to play the lead role immediately after writing the draft for "Oppenheimer."

Damon and Blunt previously told People magazine that the weight and "responsibility" of the role prevented Murphy from joining in on the cast's fun activities.

"The sheer volume of what he had to take on and shoulder is so monumental," Blunt said. "Of course, he didn't want to come and have dinner with us."

"He couldn't," Damon said. "His brain was just too full."

"Oppenheimer" is set to theaters Friday.

Extreme calorie restriction can be dangerous

Without knowing the details of Murphy's diet and lifestyle, it's impossible to put his "dinners" into context.

Skipping dinner could have been an attempt of intermittent fasting, which is commonly carried out by eating all food for the day in an eight-hour window and fasting (or consuming only water and herbal tea) for the remaining 16 hours. However, even by having an almond or a slice of apple, he wouldn't technically be fasting.

While actors preparing for roles often undergo drastic steps and take diets to extremes, when it comes to sustainable weight loss and health advice for the general public, extreme calorie restriction is not recommended.

A calorie deficit is required for weight loss, but dropping your calories too low can backfire — you are at risk of nutritional deficiencies, muscle loss, brain fog, and gallstones, according to the Academy of Nutrition and Dietetics.

Trying to stick to a very low calorie diet is also hard to sustain, so can lead to yoyo dieting and a "binge-restrict" cycle for some people.

To lose weight healthy and sustainably, 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 calorie deficit — without dropping calories too low. To ensure you lose fat rather than muscle, resistance train and make sure you're eating enough protein.

Slow weight loss is generally more sustainable, so it's recommended to lose no more than one to two pounds per week on average, although people with more weight to lose can safely shed pounds at a faster rate (and the inverse for those without much excess body fat).