Remember not too long ago it was all about low fat.  The food industry responded with gusto and it took out the fat and replaced it with refined carbs and sugars.  Yet, despite dropping the fat we were expanding our waistlines. 

Refined carb products. You know the ones Americans love donuts, cakes and cookies and yes the "low fat" varieties are making us fat.  These foods cause a yo-yo affect on blood sugar causing insulin tsunamis.  Excessive insulin, which converts blood sugar to fuel and fat tissue, causes your blood sugars to crash making you, you guessed it, more hungry.  Refined carb products are also very calorie dense so you can think of refined carbs as a great way to eat more calories and still feel hungry.

Low glycemic foods, such as most vegetables and fruits, cause a moderate and measured blood sugar elevation.  Without the yo-yo pattern of insulin release, blood sugar levels do not crash so you feel full longer and eat less.  In a study of overweight adolescents, participants who ate breakfast which included high-glycemic instant oatmeal later consumed 53% more calories at lunch than those fed low-glycemic steel-cut oatmeal.  The steel-cut oatmeal resulted in lower blood sugar and insulin levels so participants felt full longer and ate less for lunch.  Steel-cut oatmeal, which has its protective fiber intact, is less readily digested and converted into blood sugar than instant oatmeal, which has its protective fiber stripped away.

Whenever you go on a calorie restrictive diet your metabolism slows.  It does not matter what type of diet you go on your metabolism will slow despite what the late night infomercials would like you to believe.  However on a low glycemic diet your body does a better job of preserving metabolism and you stay less hungry compared to a high glycemic diet.  You also have the added anti-aging benefit of reducing inflammation and CRP by 50% in one study compared to no change with a high glycemic diet.