Gaining 35 Pounds, Here Is How Sydney Sweeney Transformed For Christy Martin

When Sydney Sweeney signed up to play former boxing champion Christy Martin in the upcoming biopic Christy, she knew it would be more than a costume change. The film chronicles Martin’s rise in the male-dominated world of boxing and her personal struggles, earning widespread attention ahead of its premiere. It has created quite a buzz for her jaw-dropping transformation into the legendary boxer.

To authentically step into Martin’s gloves, Sweeney underwent a striking physical transformation, gaining 35 pounds and following a rigorous training and nutrition program.

How Did Sydney Sweeney Transform for the Role

During the promotional tour, Sydney Sweeney opened up about her training regimen. 

In a conversation with Sports Illustrated, she said, “Physically, I started working with a weight trainer, Brent Roberts, and I started working with a boxing coach, Mima. I would work out morning. I would box for two or three hours in the middle of the day, and then I'd work out again at night.”

Explaining more about her diet, she further added, “I upped all my calorie intakes, protein shakes, supplements, creatine, just everything I possibly could. And I gained 35 pounds.” Through the training, she was the “happiest version” of herself. “I loved it so much. I was eating the best pastas and cheeseburgers. I mean, I eat all that anyways, but it was just like, instead of one, I was eating three in one sitting,” she added.

How Did Sydney Sweeney Learn Boxing For Christy?

Sweeney shared what made the process of learning to bix so unique. “Well, I actually grew up kickboxing and grappling. So I had been doing it for years beforehand. So it was kind of just like retraining my brain to box, which is actually harder than I thought it was going to be, because I'm so trained to stand and square up a certain way, and it's very different when it comes to boxing,” she said. The actress had to retrain her “brain not only to box, but then also to fight like Christy.”

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Why the Physical Shift Matters in a Biopic

Biopics that centre on athletes often hinge on authenticity in movement and physical presence. When the subject is a fighter like Christy Martin, viewers expect a credible connection to the ring. Sweeney’s transformation allows her to move like a boxer, look like a boxer, and carry the physicality of the role.

A related study published in Frontiers in Physiology found that significant weight gain, paired with intensive resistance and combat training, can help athletes build muscle mass. It can also increase explosive power and alter their body composition, which is key for physically demanding roles. The study suggests that a gradual increase in calorie and protein intake supports safe, effective adaptation in muscle and metabolic profile, provided recovery is prioritised.

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Conclusion

Sydney Sweeney’s impressive 35-pound transformation for Christy is a vivid example of what happens when preparation meets purpose. Her deep dive into boxing, nutrition and mindset creates a foundation for a role that is as much about strength as it is about story. For anyone looking to transform physically and mentally, whether for sport, film or life, her journey serves as proof that change is possible with the right commitment and tools.