The 28-year-old actress covers Shape's March issue and reveals that, during the first season filming her hit Amazon series, she began rapidly losing weight.
"My stay-healthy secret when I'm working 14 or 15-hour days on Mrs. Maisel is bone broth," she says. "For the first season of the show, I was feeling very tired. And because I have a fast metabolism, I felt like I was dropping weight in an unhealthy way."
"Bone broth has collagen and fat in it, along with all kinds of vitamins and minerals, and I really think it helped me," she continues. "I was the only person on our set who didn't get sick that season. I have to credit the bone broth."
In addition to learning new ways to keep her body healthy, Brosnahan's starring role as Midge Maisel has made her branch out in many areas, though her personal life remains steady.
"My life itself has stayed the same. I still live in the same apartment in New York," she reveals. "What has changed is that now I'm sitting in the same room with people I've long admired and been inspired by, like Steven Spielberg and Sarah Paulson. I have the opportunity to converse with and seek advice from them. Doors are opening for me that I never could have anticipated."
"What I had to do to play this part is something I never thought I could do -- namely comedy," Brosnahan adds of her role. "I'd been told I wasn't funny, and that just felt true. This job has been immensely intimidating, especially in that department. But I have learned from Midge that it's so important not to tell yourself you can't do things."
Brosnahan was no stranger to doing things her own way before Midge came into her life. Back in high school, the actress chose an unlikely activity after not being cast in the school musical.
"I was on my high school [wrestling] team. I'd always been curious about it," she says. "Then in my freshman year, when I didn't get a part in the musical Beauty and the Beast, I was devastated, and wrestling was there for me."
"I loved it! I won some matches; I lost some. I remember wrestling someone who was 119 pounds (I was 112) and getting pummeled," she continues. "But I enjoyed how difficult the training was and watching my body change because of the work I was putting in."
"When you lose someone you love, you search for ways to commemorate and honor them and keep their memory alive," she says. "This past year has obviously been very challenging for my family. Katy inspired me greatly as an artist and a human being, which is why I have partnered with Frances Valentine, the fashion brand she created, to celebrate her life and legacy and the impact that she and her work had on so many people."
"Spring was Katy's favorite season, and I love it too, so it felt like a perfect fit to be featured in the spring campaign wearing her beautiful collection," Brosnahan adds.
When she's not hard at work staying healthy or with her new Frances Valentine duties, Brosnahan is already thinking ahead to season three of The Marvelous Mrs. Maisel. When ET caught up with the actress last month, she revealed the guest she'd most like to see in the show's third installment.
"I love Sterling K. Brown so much," she raved of the This Is Us star, before making her pitch: "We treat our guest stars well!"
Watch the video below for more on Brosnahan: