Actor Aamir Khan went from fit to fat—and then got very fit again. But he insists he did it without steroids.

Khan put himself through one of Bollywood’s biggest body transformations ever (check out 12 more major Bollywood transformations here) for his latest film, the wrestling drama Dangal, gaining some major attention for ballooning to 40% body fat—and then cutting down to less than 10% in a matter of months. In the film, he portrays Indian wrestler Mahavir Singh Phogat, who coached his daughter to India's first-ever medal in women’s wrestling at the 2010 Commonwealth Games. Khan portrays Phogat in his athletic prime and when he’s older, which is why he went through the stunning body transformation.

“The film has two phases,” Khan told the Hindustan Times. “For one part, I had to put on weight. So, I weighed 96 kgs with 38% body fat. I had to reduce to 9% body fat within five months. That was a huge task."

Just take a look at the body transformation detailed in this video:

Somewhat unsurprisingly, the Bollywood star was hit with some questions about how he turned himself into a ripped dude with a rock-hard six-pack after weighing over 200 pounds. The answer? He did it totally naturally.

“I have not used any substances," Khan told the Hindustan Times. "I think that the good rate to lose weight is roughly one pound a week. If you want to lose weight little faster, then you have more of a calorie deficit. So, if you have calorie deficit of 500 calories a day, you lose one pound a week. If you have the calorie deficit of 1,000 calories a day, you lose two pounds a week—that is considered to be on the higher side."

But for his transformation, Khan dropped weight even faster: “When I started the weight-loss process, I was doing double that. I was losing four pounds a week for the first three weeks. Then I brought it down to two pounds a week. So, I had 20 weeks to do that. If you do the math, you will understand that is how I got there. Immediately after PK [a 2014 movie starring Khan], I was in weight training mode for two years.”

Brutal? Yes. Effective? Definitely.

Dangal, directed by Nitesh Tiwari, will be released on December 23.