Even when Matthew McConaughey looked deathly skinny he was secretly plotting to immediately pack on the weight and regain his famous physique ... and TMZ has learned how he did it in under 4 months.
By now you've seen the pics -- McConaughey looking ripped again ... after dropping nearly 30 pounds last November to play an AIDS patient.
According to our sources, even though McConaughey looked like an extra on a Sally Struthers infomercial ... he was still pumping iron to keep his muscles intact.
So when it was time to get jacked for his next role ... we're told MM simply added weight by stuffing his face with lots of fish, asparagus, eggs, nuts and sweet potatoes.
Our source described the process as "very well planned out."
All right, all right, all right.