The glycemic index (GI) is a scale that measures how quickly carbohydrate foods are broken down into glucose. The original purpose for the glycemic index was to help diabetics keep their blood sugar under control. The glycemic index has recently attracted a lot of attention in the bodybuilding, fitness and weight loss world and has even become the central theme in numerous best-selling diet books as a method to choose the foods that are best for losing weight.
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.
Instead, we are urged to consume carbohydrates that are low on the GI scale such as black eye peas, old fashioned oatmeal, peanuts, apples and beans because they do not raise blood sugar as rapidly.
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.
For example, the glycemic index is based on eating carbohydrates by themselves in a fasted state. If you are following effective principles of fat-burning and muscle building nutrition, you should be eating small, frequent meals to increase your energy, maintain lean body mass and optimize metabolism for fat loss. However, since the glycemic index of various foods was developed based on eating each food in the fasted state, the glycemic index loses some of its significance.
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).
Mashed potatoes have a glycemic index near that of pure glucose, but combine the potatoes with a chicken breast and broccoli and the glycemic index of the entire meal is lower than the potatoes by itself.
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.
A far more important and relevant criteria for selecting carbs - as well as all your other foods, proteins and fats included - is whether they are natural or processed. To say that a healthy person with no metabolic disorders should completely avoid natural, unprocessed foods like carrots or potatoes simply because they are high on the glycemic index is ridiculous.
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.
The glycemic index is clearly not a "gimmick" and should not be completely disregarded, as it is a definitely a legitimate nutritional tool. Is it a good idea to eat low GI foods in general? Sure. Is eating high GI foods after your workouts a good idea? Absolutely. But diet programs which hang their hats on glycemic index alone as the "miracle solution" are just another example of how one single aspect of nutrition can be used as a "hook" in marketing and said to be the "end all be all" of fat loss, when it's really only one small piece of the puzzle.
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.