While the NBC show's cast was surprised by Winfield's progress, they seemed shocked for a different reason when fellow contestant Rachel Frederickson stepped onto the stage.
It was at Tuesday's live finale, filmed weeks after the three finalists went home to continue their weight-loss journeys on their own, that 24-year-old Frederickson was crowned "The Biggest Loser."
In fact, she's the biggest loser in the show's history: She logged the highest percentage of weight loss of any winner in the series' 15-season run.
A former competitive swimmer who was an athletic 150 pounds in the second-to-last episode, Frederickson's 59.62 percent weight loss has some critics calling her too thin.
After starting the competition at 260 pounds, Frederickson lost a total of 155 pounds. At the finale, she appeared dramatically thinner than the last time viewers saw her as she won the "Biggest Loser" triathlon.
Viewers took to social media to express concern over Frederickson's appearance, and trainers Jillian Michaels and Bob Harper — who have yet to publicly react to Frederickson's win — appeared somber and shocked as Frederickson walked out onstage.
"I actually can't stop thinking about The Biggest Loser," one fan wrote on Twitter. "It was such an amazing night with a very weird ending. I hope Rachel is OK."
Some said the show should set new rules to prevent contestants from becoming underweight just to win the coveted title and $250,000 prize money.
"Perhaps #biggestloser should set a lower limit according to health guidelines and if they go below, no win," one fan wrote. Another agreed: "There needs to be a red line that disqualifies finalists for too much weight loss based on a minimum BMI threshold."
Other viewers were quick to point out that the criticisms about Frederickson's weight — and posts calling her anorexic — amount to body-shaming.
"#rachel from #BiggestLoser looks great!," a fan wrote on Twitter. "It's sad how unhealthy people post their #bully words of disgust. Proud of her!"
Watch Frederickson's "Biggest Loser" win.