The insider told Us Weekly that Affleck "has been trying to eat healthy, work out a ton, cut out carbs and eat lots of protein."
Affleck who struggled with alcohol addiction in the past also "isn't drinking."
Part of these diet changes are due to Ben's new girlfriend, Ana de Armas.
The source explained, "Ana also tries to eat healthy and has gotten Ben into it more. They are doing good. They are focused on Ben’s kids, work, making time for dates and just enjoying each other."
Ben's renewed focus on health has resulted in noticeable weight loss for the actor.
He showed off his leaner figure on Instagram in a video with longtime friend Matt Damon. The video offered fans a chance to hang out with the actors in Los Angeles to help fundraise for Eastern Congo Initiative and Water.org.
Although Affleck's new appearance might have been what stuck with fans the most.
One wrote, "What happened with Ben?"
Another commented, "Ben you look great! Happy to see that the road to recovery has been a transformative experience for you. Congrats to you!"