Which diet is better for keeping weight off?

An international study just published in The New England Journal of Medicine provides the most robust evidence so far that a high protein + low glycemic index diet is best for weight maintenance. The authors observe:

"Studies of weight-control diets that are high in protein or low in glycemic index have reached varied conclusions, probably owing to the fact that the studies had insufficient power."

773 overweight adults from eight European countries who completed an initial low-calorie weight loss phase were randomized to follow one of five diets for weight maintenance: a low-protein and low-glycemic-index diet, a low-protein and high-glycemic-index diet, a high-protein and low-glycemic-index diet, a high-protein and high-glycemic-index diet, or a control diet. The diets were 'ad libitum', meaning they were allowed to eat freely within the constraints of their diet plan. What did the data show?

"...only the low-protein–high-glycemic-index diet was associated with subsequent significant weight regain. In an intention-to-treat analysis, the weight regain was 0.93 kg less in the groups assigned to a high-protein diet than in those assigned to a low-protein diet and 0.95 kg less in the groups assigned to a low-glycemic-index diet than in those assigned to a high-glycemic-index diet."

Moreover, it seems that the high-protein and low-glycemic-index groups were more comfortable:

"Fewer participants in the high-protein and the low-glycemic-index groups than in the low-protein–high-glycemic-index group dropped out of the study."

Thus the authors conclude:

"In this large European study, a modest increase in protein content and a modest reduction in the glycemic index led to an improvement in study completion and maintenance of weight loss."