Pop hunk Nick Jonas became hooked on working out as he attempted to get in shape and tone up for a movie role.

Now buff and musclebound, the former Jonas Brothers star admits vanity made him get in shape because he didn't want to appear pudgy on the big screen.

He tells Men's Fitness magazine, "The initial decision came when I found out I was going to be doing Careful What You Wish For. I spend a lot of time in the movie without my shirt on, so I wanted to tone up and be in good shape.

"I started loving the lifestyle of being fit and feeling healthy, and the results I was starting to see were really great."

However, the diabetic pop star admits he had to be careful with his diet: "It's really about just staying on top of it and managing every detail. I had to work with my doctor, making sure all my numbers were good and that I wasn't putting too much stress on my body.

"When I work out, my blood sugar goes up. But then about an hour and a half later it drops down low. So post-workout I would have to not only have protein to rebuild but also carbs (carbohydrates) to sustain my blood sugar."