I was looking back at an older study that studied high-protein diets. It compared a fat-restricted ad libitum diet that was either high in protein or carbohydrates. Here is a graph from the study:
Now, looking at this, it seems like a clear-cut case: high-protein beats high-carb.
But thank goodness they did a follow-up study. Here is a graph from that study:
You can see right-around the six-month mark the high-protein diet begins to gain weight. At one year, the wheels come off and subjects start bailing on both diets. At the end of two years, the medium-protein diet actually has slightly better weight-loss results than the high-protein diet.
In the end, if you had to give credit to something, it appears that the fat restriction (set to 30% of calories) was probably the driving factor in weight loss, not the protein content.