The net carb myth...

In an effort to cash in on the low-carb craze, food manufacturers have invented a new category of carbohydrates known as "net carbs," which promises to let dieters eat the sweet and creamy foods they crave without suffering the carb consequences.

The problem is that there is no legal definition of the "net," "active," or "impact" carbs popping up on food labels and advertisements. The only carbohydrate information regulated by the FDA is provided in the Nutrition Facts label, which lists total carbohydrates and breaks them down into dietary fiber and sugars.

Any information or claims about carbohydrate content that appear outside that box have not been evaluated by the FDA.

"These terms have been made up by food companies," says Wahida Karmally, director of nutrition at the Irving Center for Clinical Research at Columbia University.

"It's a way for the manufacturers of these products to draw attention to them and make them look appealing by saying, 'Look, you can eat all these carbs, but you're really not impacting your health, so to speak.'"

The concept of net carbs is based on the principle that not all carbohydrates affect the body in the same manner.

Some carbohydrates, like simple or refined starches and sugars, are absorbed rapidly and have a high glycemic index, meaning they cause blood sugar levels to quickly rise after eating.

Other carbohydrates, such as the fiber found in whole grains, fruits, and vegetables, move slowly through the digestive system, and much of it isn't digested at all (insoluble fiber).

In the U.S. and some other countries, fiber is shown as a sub-category of carbohydrate. Non-digestible types of fiber don't count as a "net carb."

So, anyone buying a product labelled in the US will need to subtract the fiber figure from the total carbohydrate value to work out the 'net carbs'.

In the United Kingdom, the rest of the European Union and some other countries, fiber is shown separately. In other words, the total carbohydrate value is the 'net carb' count.

For example, a slice of bread sold in the U.S. but made in the United Kingdom might say on the nutrition label that it contains 10 grams of total carbohydrate and 4 grams of fiber. But it would be a mistake to conclude that it contains only 6 grams of net carbs. Because it's made in the United Kingdom, the total carbohydrate value is the 'net carb' count.

The same product made in the U.S., however, would contain 6 grams of Net Carbs. That's because it follows U.S. practice, where the fiber is included in the carbohydrate.

Also in this category of largely indigestible carbohydrates are sugar alcohols, such as mannitol, sorbitol, xylitol, and other polyols, which are modified alcohol molecules that resemble sugar.

Sugar alcohols are structurally similar to their carbohydrate cousins but have an increased number of hydrogen atoms, which, in chemical terms, renders them "hydrogenated" or "reduced"; as a result, they are processed somewhat differently by the body.

In calculating net carbs, most manufacturers take the total number of carbohydrates a product contains and subtract fiber and sugar alcohols because these types of carbohydrates are thought to have a minimal impact on blood sugar levels.

For example, the label on PowerBar's new double chocolate flavor "ProteinPlus Carb Select" bar says it has "2 grams of impact carbohydrates." The Nutrition Facts label on the product says it has 30 grams of total carbohydrates.

Just below the nutrition facts box, the "impact carb facts" box provided by the manufacturer explains, "Fiber and sugar alcohols have a minimal effect on blood sugar. For those watching their carb intake, count 2 grams." That's 30 grams minus the bar's 27 grams of sugar alcohols and 1 gram of fiber.

Although sugar alcohols have been used in small amounts in items like chewing gums for years, researchers say little is known about the long-term effects of consuming large amounts of these substances. They can also have a laxative effect.

Maltitol, for example, is not absorbed in the small intestine as regular carbohydrates are; instead, it passes undigested into the large intestine where it is fermented by intestinal bacteria, producing carbon dioxide, hydrogen, and small fatty acids.

Research published in the journal Nutrition Research Reviews shows that maltitol can raise both blood sugar and insulin levels.

Sugar alcohols do contain calories and do raise blood sugar levels. While maltitol, for example, may contribute fewer calories than regular carbs do — approximately 3 rather than 4 calories per gram — there are still calories in there.

Atkins Nutritionals Inc. says it will replace the words "net carbs" and the seal now found on its packages with a new phrase, "net Atkins count," and a new seal. The new term is supposed to reflect a more precise method of measuring the carbohydrates in its foods.

Atkins derives the "net Atkins count" for a product by testing blood-sugar levels of people who have just eaten it. The technique borrows heavily from the glycemic index, which rates a food according to its impact on blood sugar levels.

The bottom line is that products with a low "net carb" count often contain a lot of calories. If you want to lose weight, rather than focus on the little c" of carbohydrates, focus on the "big C" — calories.

Reference
Livesey, G. (2003). Health potential of polyols as sugar replacers, with emphasis on low glycaemic properties. Nutrition Research Reviews, 16, 163-191