On-again, off-again dieting makes weight control a much tougher proposition in the long run.

The Nutri-System Health & Fitness Information Bureau cites an on going medical project involving researchers from five universities around the country who are focusing on "yo-yo dieting," or repetitive large gains and losses in weight.

Experiments on both laboratory animals and humans have shown that excess weight is lost more quickly the first time than on the second or third cycles, while caloric intake remains the same.