The Briton, back in the ring after 11 months out, was ground down by his opponent, who took the title by majority decision with scores of 114-114, 115-112 and 116-112.
DeGale, back in action following a serious shoulder injury, started brightly but began struggling from the middle of the second round onwards as he looked increasingly lethargic.
In the fifth, the 31-year-old found himself in a corner and, unable to slip out of trouble, was hit with a vicious uppercut, then a right, another uppercut and a short hook to the head.
DeGale managed to stay on his feet and was back in the lead by the eighth round, when another Truax uppercut landed flush.
The final rounds saw both men desperate to land blows and points, giving as good as they got, but upon the final bell, the Minnesotan was declared the victor.