Usher has always sported enviable abs, but when he signed on to play Sugar Ray Leonard in Hands of Stone, the star adopted a new diet and hit the gym – hard.
"When you're playing a role like this, you can't act a boxer. The only way you know what it is to be a boxer is to be one," the entertainer – who stars opposite Edgar Ramirez in the biopic about champion fighter Roberto Durán – tells PEOPLE exclusively. "I began to see a transformation happen with my body."
Starting at 172 lbs. (he got down to 146 lbs. at his lowest), Usher, 37, cut out carbs by taking on Dr. Gundry's "Matrix" diet, which requires a "very specific regimented style of eating," he says.
The star ate five to six small meals a day – fish, blueberries and pistachios were often on the menu – and worked out a "three per day" pattern for his workouts, which included running, swimming, biking and fighting amateur boxers in the boxing gym (where he also trained with Leonard).
For much more on how Usher got into the best shape of his life, his new music and his family, pick up the latest issue of PEOPLE, on newsstands Friday.
"It was wonderful," he says of overhauling his diet and workout routine. "Once you clean your body out and really get accustomed to eating that type of food, you see the results and you feel better."
While he's still trying to stay healthy, the Grammy winner jokes he's now working on a "new role" called "Usher Ray" and has become a little more lax.
"The first 20 lbs. are the hardest in the world," he says. "When you get into that year of work and you created goals and met them, you don't want to go back and completely fall off the wagon. But Usher Ray likes to have cookies on occasion."