Good carb, bad carb? Experts debate.

Macaroni faster than spaghetti? A greenish banana faster than a freckled one? Yes, say some of the top-tier nutritional experts.They are convinced that carbohydrates should be labeled good or bad, just the way fats are.

The debate involves an idea called the glycemic index. It is a way of rating how quickly carbohydrates are digested and rush into the bloodstream as sugar. Fast, in this case, is bad. In theory, a blast of sugar makes insulin levels go up, and this, strangely, leaves people quickly feeling hungry again.

Despite its detractors, the idea seems to be gaining momentum, in part because it is offered as scientific underpinning by the authors of a variety of popular diet schemes, mostly of the low-carb variety.

To believers, the glycemic index is a kind of nutritional Rosetta stone that explains much of what has gone wrong with the world's health and girth over the past two decades: Why diets so often fail. Why diabetes is becoming epidemic. Why mankind is growing so fat.

We overeat because we are hungry, the theory goes, and we are hungry because of what we have been told to eat, which is too much fast-burning food that plays havoc with metabolism by quickly raising blood sugar levels. All of that starch at the base of the food pyramid has had the unintended effect of making us ravenous.

"It's almost unethical to tell people to eat a low-fat, high-carbohydrate diet with no regard to glycemic index," says Janette Brand-Miller of the University of Sydney, one of the field's pioneers.

Controversy weighs on subject
The idea has already entered the scientific mainstream in much of the world and is endorsed by the World Health Organization, but it remains deeply controversial in the United States. It is dismissed by some of the country's weightiest private health societies, including the American Heart Association and the American Diabetes Association.

The fact that carbohydrates break down at different rates has been suspected for a long time. It is why diabetics were once (but no longer) told to studiously avoid sweets, since presumably sugary foods would quickly turn into sugar in the blood stream. About 20 years ago, scientists came up with the glycemic index, or GI, as a way to compare this.

The body converts all carbohydrates -- from starches to table sugar -- into sugar molecules that are burned or stored. The faster carbs are broken down by the digestive system, the quicker blood sugar goes up and the higher their GI.

The GI of at least 1,000 different foods has been measured, in the process knocking down many common-sense dietary beliefs. For instance, some complex carbohydrates are digested faster than the long demonized simple carbs. Foods such as white bread and some breakfast cereals break down in a flash, while some sweet things, like apples and pears, take their time.

In general, starchy foods like refined grain products and potatoes have a high GI -- 50 percent higher than table sugar. Unprocessed grains, peas and beans have a moderate GI. Nonstarchy vegetables and most fruits are low. For instance, overcooking can raise the GI. Ripe fruit is lower than green. A diced potato is lower than mashed, and thick linguini is lower than thin.

To make matters even more confusing, the glycemic index measures only the carbohydrate in food. Some vegetables, such as carrots, have quite high GIs, but they don't contain much carb, so they have little effect on blood sugar.

Therefore, some experts prefer to speak of food's glycemic load, which is its glycemic index multiplied by the amount of carb in a serving. Considered this way, a serving of carrots has a modest glycemic load of 3, compared with 26 for an unadorned baked potato. Metabolic havoc

Blood sugar levels may shoot twice as high after a high-GI meal as after a low one, and that unleashes metabolic havoc: The body responds with a surge of insulin, which prompts it to quickly store the sugar in muscle and fat cells. The high sugar also inhibits another hormone, glucagon, which ordinarily tells the body to burn its stored fuel.

Blood sugar plunges. So much is stored so fast that within two or three hours, levels may be lower than they were before the meal. Suddenly, the body needs more fuel. But because glucagon is still in short supply, the body does not tap into its fat supply for energy. The inevitable result? Hunger.

That, at least, is the theory. Experiments to prove this are difficult and time-consuming. Among those trying is Dr. David Ludwig of Boston's Children's Hospital, who has done several studies on overweight teenagers.

Ludwig says overweight people do not need to starve themselves. On a low-GI diet, they can eat enough to feel satisfied and still lose weight. In a pilot study, he tested this on 14 overweight adolescents. They were put on two different regimens -- a standard low-cal, low-fat, high-carb diet and a low-GI plan that let them eat all they wanted. After one year, the low-GI volunteers had dropped seven pounds of pure fat. The others had put on four. Now he is repeating the study on 100 heavy teenagers.

Even such small experiments have been rare. Most support for the idea comes from big surveys that follow people's health and diets over time. Some of these show that those who consistently favor low-GI fare are less likely to become overweight or to get diabetes and heart disease.