The 43-year-old actor has shed 38lbs for his role as real-life AIDS victim Ron Woodroof in the movie, and while he insists he is not suffering any health problems as a result of his dramatic weight loss, he knows he has to be careful when regaining his former physique as he could send his body into shock.
He said: "I've been checked. My levels are fine. I'm as healthy as can be. My blood pressure, everything's fine.
"The real health challenge is when you put it back on. It's very easy to create a form of diabetes if you don't do it right. You can't just start eating cheeseburgers and ice cream. Your body will go into shock and it just won't work."
Despite his dramatic weight loss, Matthew insists he feels fine and has shed the pounds in a healthy way, though he is fascinated by the way his body has responded to the changes.
He added in an interview with website The Daily Beast: "I feel fine. I really do. I got the means to lose the weight in a really healthy way. I'm eating fresh fish. I'm just eating small amounts. I'm not being starved. I'm sleeping well ... It's just really an exercise in discipline.
"If anything, it's as much a spiritual journey as it is physical ... It's really been enlightening to understand my anatomy. I don't have the leverage I used to. I have body soreness."