At the Well blog at The New York Times, Gretchen Reynolds has a post on a Danish study that found that men who exercised 30 minutes a day lost more weight than those who exercised 60 minutes.
Before you decide not to do that Ironman, you might want to look at the considerable limitations of the study. The study lasted only 13 weeks, and although Reynolds doesn't say so, it involved only about 60 young men--about 20 each in the no-exercise, moderate-exercise, and high-exercise groups. And the men kept diaries of what they'd eaten--a way to track calories that is not entirely reliable.
Reynolds might have been more careful to note the study's small size and other limitations, but she concludes with a universal truth: the relationship between exercise and weight loss "remains complicated and tangled." Still, it's an interesting piece.