According to advocates of the glycemic index system, foods that are high on the GI scale such as rice cakes, carrots, potatoes, or grape juice are "unfavorable" and should be avoided because high GI foods are absorbed quickly, raise blood sugar rapidly and are therefore more likely to convert to fat or cause health problems.

While the GI does have some useful applications, such as the use of high GI foods or drinks for post workout nutrition and the strong emphasis on low GI foods for those with blood sugar regulation problems, there are flaws in strictly using the glycemic index as your only criteria to choose carbs on a weight loss program.

In addition, when you are on a diet program aimed at improving body composition (losing fat or gaining muscle), you will usually be combining carbs and protein together with each meal for the purposes of improving your fat to muscle ratio. When carbs are eaten in mixed meals that contain protein and some fat, the glycemic index loses even more of its significance because the protein and fat slows the absorption of the carbohydrates (as does fiber).

Rice cakes have a very high glycemic index, but if you were to put a couple tablespoons of peanut butter on them, the fat would slow the absorption of the carbs, thereby lowering the glycemic index of the combination.

I know many bodybuilders (myself included) who eat high glycemic index foods such as white potatoes every day right up until the day of a competition and they reach single digit body fat. How do they do it if high GI foods “make you fat?" It’s simple – high GI foods DON’T necessarily make you fat – choosing natural foods and burning more calories than you consume are far more important factors. Although it’s not correct to say that all calories are created equal, a calorie deficit is the most important factor of all when fat loss is your goal.

Eating Low glycemic index foods alone does NOT guarantee you will lose fat. You have to take in the bigger picture, which includes calories/energy balance, meal timing and frequency, macronutrient composition, choice of processed versus refined foods as well as how all these nutritional factors interact with your exercise program.