Should people really care that they digest potatoes faster than carrots?

Macaroni faster than spaghetti? Rice Krispies faster than Special K? A greenish banana faster than a freckled one? A Snickers bar faster than a Twix?

Yes, say some of the country's top-tier nutritional experts. They are convinced that carbohydrates should be labeled good or bad, just the way fats are, and that some of the carbs people love most — velvety puddles of mashed potatoes, lighter-than-air white bread — are dietary evil, to be avoided like the nastiest artery-choking trans-fats.

No, contend other equally respected nutritional experts. Potatoes and other starchy standbys are perfectly respectable. A carb is a carb is a carb.

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.

The debate over whether every person who puts food in his mouth should know about this is fervid even for the field of dietary wisdom, where fierce opinions based on ironclad beliefs and sparse data are standard.

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. However, some painstakingly argue that the glycemic index is just as important for the carbohydrate-loving brown rice aficionado as it is for the most carbo-phobic, double-bacon-cheeseburger-hold-the-bun Atkins follower.

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.

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.

To some of the skeptics, this is just another half-baked mishmash of dietary arm-waving, cobbled together to justify the high-fat, low-carb schemes that dietitians love to hate.

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.

While it seems reasonable that chewy, whole-grain bread is digested more slowly than a French baguette, some of the results are less obvious. 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.

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.

In one, he tested the idea that a high-GI breakfast makes people hungrier at lunch. A dozen obese boys were fed three different breakfasts, all with the same calories — a low-GI vegetable omelet and fruit, medium-GI steel-cut oats or high-GI instant oatmeal.

At noon, they could eat as much as they wanted. Those who started the day with instant oatmeal wolfed down nearly twice as much as those getting the veggie omelet.

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.

The evidence is strong enough for authors of some popular diet books, who use the glycemic index as one of their primary rationales. "It's a new unifying concept that brings nutritional habits out of the dark ages and says it's all about the numbers," says Barry Sears, author of the Zone series of diet books. "It says diet does not have to be based on philosophy. It can be based on hard science."

Major U.S. health organizations are less impressed. Ludwig expects this to change, in part because paying attention to the glycemic index can help everyone choose healthier carbs, whether they go low-fat or high.

But that seems unlikely any time soon at the heart association. The head of its nutrition committee, Dr. Robert Eckel of the University of Colorado, says the theory that high-GI foods make people hungry is "ridiculous" and argues that a scientific case can be made for just the opposite.

Dietitians generally encourage a balanced, varied diet emphasizing unadulterated whole foods, and they cringe at a classification that puts ordinary baked potatoes and white rice on a taboo list.

"It's an artificial system of classifying foods as good and bad," says JoAnn Carson, a nutritionist at the University of Texas Southwestern Medical Center.

Others worry that the whole business is just too hard to keep straight.

"We are putting before the public an extraordinarily complicated message, which I don't think they will follow or be very happy with," says Dr. Xavier Pi-Sunyer of St. Luke's Roosevelt Hospital Center in New York City.

Not necessarily, responds Harvard's Dr. Walter Willett. "I do think this is an important concept for people to understand, but I don't think they need to worry about specific numbers."

His advice: Go light on the white bread, white rice, potatoes pasta and sugary foods.

On the Net:

GI tables:

https://diabetes.about.com/library/mendosagi/ngilists.htm