The producers of the film claim that Christian Bale dropped from about 173 pounds in weight down to about 110 pounds in weight to make this film. They also claim that Bale actually wanted to drop down to 100 pounds, but that they would not let him go below 120 out of fear that his health could be in too much danger if he did. His diet consisted of one can of tuna and an apple per day. His 63-pound weight loss is said to be a record for any actor for a movie role. He regained the weight in time for his role in Batman Begins (2005).
Brad Anderson had not asked Christian Bale to lose such a vast amount of weight and was genuinely shocked when he saw Bale's appearance on the first day of shooting. He also confessed to being thrilled by the actor's dedication.
Christian Bale found that his weakened condition caused problems in the more demanding action scenes. He found running a particular problem as he simply had no leg muscles left.
Oddly enough, Christian Bale claimed that he actually enjoyed losing so much weight, oftentimes finding himself in a euphoric state due to the extreme conditions he subjected himself to. He was also able to deal with the very hot July and August temperatures in Barcelona much better than the rest of the crew.