Not all carbohydrates are created equal.

For years, food companies have disclosed the amount of carbohydrates in their products according to a simple formula mandated by the Food and Drug Administration: subtract the protein, fat, moisture and ash content (minerals like calcium and phosphates) from the weight of the food. Whatever is left over is a carb, listed on food packages as "total carbohydrates."

But now that carbs are a dietetic dirty word in many quarters, new definitions are surfacing. Just as there is good cholesterol and bad cholesterol, nutritionists are increasingly distinguishing between good carbs and bad carbs. Food manufacturers are eager to use definitions that shed the best possible light on the carbohydrates in their products.

The key is how the body responds to certain carbs. For example, a potato, once digested, quickly raises blood sugar -- it's a bad carb. But the fiber in fruit is indigestible and doesn't raise blood sugar levels. Good carb. Other carbs, such as sugar alcohol or glycerin, common in a host of processed foods, digest in the body but don't turn into blood sugar. Nutritionists see those as neutral carbs.

So far, however, FDA rules call for labeling products only on the basis of a carbohydrate's chemical makeup, not its physiological effect on the body. Chemically, carbohydrates are molecules made of carbon, hydrogen and oxygen that operate as a simple or complex sugar and provide energy to the body. But in the new low-carb era, food manufacturers are eager to give detailed explanations of why their carbs are more benign than those of others: Products that are high in fiber, or products that use soy or a sugar replacement, are likely to benefit from this spin.

With both the food companies and consumers pushing, the FDA is preparing to come up with a better definition for carbohydrates. Lester Crawford, deputy FDA commissioner, says the commission's Obesity Working Group is examining the role of carbohydrates in the diet. He says when the dietary guidelines were laid out, "Carbohydrates were not an issue. There was little or no understanding of them as contributors to overweight or whatever." In the last meeting, in 1991, to determine dietary guidelines, "We did not discuss carbohydrates."

The famous Atkins diet that helped fuel the whole low-carb trend didn't distinguish between kinds of carbohydrates either. In his first two "diet revolution" books, the late Dr. Robert Atkins counseled dieters to eat only 20 grams of carbohydrates a day during the diet's initial two-week period, which left out even fruit and carrots because of their carb count.

Officials at Atkins Nutritionals, which grew out of the diet empire and was recently acquired by Parthenon Capital and Goldman Sachs Capital Partners, say research into carbohydrates has evolved since then. It now uses a "net carb" calculation, essentially subtracting the amount of fiber and sugar substitutes that don't digest easily (such as maltitol, lactitol or sucralose, sold as Splenda) from the total carbs, leaving only the sugars, syrup or honey that cause a big spike in blood sugar.

"We needed to help our user base understand the difference between good carbs and bad carbs," says Matt Wiant, chief marketing officer for Atkins Nutritionals. "The FDA doesn't recognize the difference between different types of carbohydrates, and people are confused."

The big natural-food company Hain Celestial Group Inc. recently launched a line of "Carb Fit" pastas and snacks, also flagging the "net carb" content on the front of the box. A one-cup serving of Carb Fit rotini pasta has 17 grams of carbohydrates, but only 9 grams of net carbs (the other 8 grams are fiber).

But the FDA is skeptical, and is looking at more comprehensible definitions. "I think something like net carbohydrates ... less than 1% of people would understand what that was, and that's not what we want to have out there," says Dr. Crawford.

BRAN OVER BANANAS   Foods with a high-glycemic index contain carbohydrates that break down quickly, causing spikes in blood-sugar levels. Low-glycemic foods cause a more gradual increase.   High glycemic Low glycemic Potatoes Most legumes Bananas Whole fruits (apples, oranges) White bread Bran White rice Brown rice French fries Couscous Refined cereals Whole-grain cereals Soft drinks Bulgar, barley Sugar Whole wheat, oats Source: Harvard School of Public Health

Another evolution in carb counting is the glycemic index, a ranking of carbohydrates on a scale from 0 to 100, depending on how much they raise blood sugar levels after eating. Foods with a high glycemic index are rapidly digested and absorbed and result in big fluctuations in blood sugar levels; foods with a low glycemic index, by virtue of their slow digestion and absorption, produce gradual rises in blood sugar and insulin levels.

The index has two scales, valuing either pure glucose or white bread as 100 and then compares other foods to it. According to the University of Sydney's Web site, www.glycemicindex.com³, a Golden Delicious apple from Canada rates a 39. Boiled lentils from the U.S. rate a 28. A baked russett Burbank potato ranks a 111. The glycemic index is already used in Australia on food packages.

But the glycemic index, while cited widely by some nutritionists, is highly controversial because it doesn't account for how people actually eat. Foods eaten together often react differently than when eaten alone; fiber, for example, slows the absorption of some nutrients; fat and protein, when consumed along with carbs, will slow the release of carbohydrates into the blood. Although the index has long been used by people with diabetes to regulate their diets, the American Diabetes Association doesn't endorse it, saying that people should watch the amount, not the source, of carbohydrates.

The FDA's Dr. Crawford says the glycemic index "has some chemical backing," but he stops short of endorsing it as a method to evaluate carbohydrates. Both the popular South Beach Diet and the latest iteration of the Atkins regime, "Atkins for Life," published in January, use the glycemic index, which has raised the index's profile in recent months.

Even the old distinction of complex carbohydrates and simple carbohydrates is falling away. Complex carbohydrates were long seen as healthier than their simpler counterparts. "But there are cases like potatoes where it is a complex carbohydrate and it is also a bad carbohydrate," says Eric Rimm, associate professor of epidemiology and nutrition at the Harvard School of Public Health. On the other hand, "fructose [found in fruit] is a simple carbohydrate but it is not necessarily a bad carbohydrate."

Amid the confusion, some big companies are opting for unwieldy explanations on their labels. Hershey Foods Corp., which this year launched a line of sugar-free candies for people with diabetes, declares 23 grams of carbohydrates for every five pieces of its miniature sugar-free Reese's Peanut Butter Cups. The label notes, however, that 19 grams of that comes from lactitol (a sugar substitute). "Lactitol is a slowly metabolized carbohydrate that generally causes only a small rise in blood glucose levels," the label reads.

Write to Sarah Ellison at sarah.ellison@wsj.com⁶

Updated November 24, 2003