Not all carbs are evil is the motto of the low GI diet. As a measurement of how quickly a particular carbohydrate is used by the body, the glycemic index is already found on nutritional labels in Europe and Australia, but has yet to be endorsed any American organization. The GI classifies carbohydrates on a scale to indicate which ones are better to consume if you are trying to lose weight.

"That's what's so different about glycemic index," says David Grotto, RD, LD, spokesperson for the American Dietetic Association. "It's not low carbohydrate, it's more selective carbohydrate, if you will.

Keeping You Full
Begun as a diet to help people control their diabetes, the general rule of the GI diet is: the higher the glycemic index, the faster a food is metabolized by the body. If a food is digested and broken down by the body quickly, your blood glucose (commonly referred to as blood sugar) levels rise rapidly, leading to a quick insulin response, making you feel tired and hungry sooner. Foods with low GI are metabolized slower, meaning they sit in your digestive track longer and are gradually absorbed by the body. This leads to a more gradual blood glucose increase, keeping your hunger satiated longer.

While a few studies have shown that following a low GI diet will keep you feeling fuller longer, helping you lose weight, there is no conclusive understanding of how this diet works. Keeping track of the GI of foods can get tricky, but this early evidence suggests that it may prove to be a useful tool for dietary planning.

GI assigns a value to a food based on the average body's metabolism of a simple carbohydrate: usually table sugar or white bread, assigned a value of 100. All other foods are then ranked according to how 50 grams of it, as compared to how 50 grams of sugar or white bread, affects one's blood glucose levels after a period of time. A value below 55 would be considered a low GI; a value from 56 to 69 would be a moderate range; anything above 70 is considered to have a high glycemic index. While such values provide a handy guide, they have not been standardized, so two different charts may show a different GI value for the same food.

Related to GI, glycemic load is a measurement that not only takes into account the glycemic index of a food, but also how much of it is there. Since, no matter what, you still can't eat heaping portions of any kind of food, glycemic load may be a better dietary tool than GI alone. Glycemic load can be assigned to an entire meal or to the food eaten over an entire day, for example.

"The thinking is, if you have a high glycemic index food, but you have a smaller amount of it, it may have pretty much the same effect of having a low glycemic index food but a larger quantity of it," Grotto said. "So that gets back to looking at portion size."

White Potatoes vs. Sweet Potatoes
A diet based on GI or glycemic load acknowledges that not all carbs act the same way, a new thought in the world of weight loss. However, it says nothing about how much protein or fat people should consume on these diets. These forms of nutrition have been shown numerous times to have a large influence on body weight and, no matter how meticulous you are about sticking to a low–GI diet, may impede weight loss.

Making matters worse, different people, different eating situations and the foods you eat with your carbohydrates can provoke greatly varying glycemic responses, that is, how the body responds to the ingestion of carbohydrates.

"If you have a high glycemic-index food such as a baked potato, for example, but you also have something with protein and fat, and you eat that meal slowly and not under stress…that can actually slow up that glycemic response," Grotto said.

And to complicate things more, the variety of a particular food and the way it's prepared also greatly impact its GI. Depending upon the variety, a potato can have a GI ranging from 55, for a sweet potato, to 110, for a boiled white potato. And the GI of a potato can be decreased, for example, if you prepare the potato the night before and let it sit in the fridge before you eat it, because the starch becomes less digestible.

The Next Dietary Trend?
These complications, however, lead to a fairly flexible diet that is less about prohibitions and more about adapting a diet to your needs and cravings. If you like your bowl of cereal in the morning, that's fine, just make it a high fiber–type of cereal, which tends to have a lower GI. And if you like to eat fruit, that's great, too, just stick to non-tropical fruits that tend to have lower GIs.

Keeping to a low GI diet means that you can eat a carb-crammed bowl of pasta, if you keep the portion size reasonable and balance it out with low glycemic foods for the rest of the day. And cooking it al–dente will help to lower the overall GI of the pasta, making you digest it more slowly.

"Those that want to follow lower glycemic index, the good news is you can do that and still eat some higher glycemic index foods," Grotto said. "It's watching the quantity of them and having them as a smaller portion of what you're eating."

So there's the catch that you can't avoid in any diet: it's all about moderation.