When a Carb's Not a Carb: The Net Carb Debate

Will counting net carbs help or hurt weight loss efforts?

By Jennifer Warner
WebMD Weight Loss Clinic - Feature

Reviewed By Michael Smith, MD

When is a carb not a carb? That's the question many carb-conscious dieters are facing as they struggle to keep their carb counts within the strict limits recommended by Atkins and other low-carb diets.

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.

But 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, DrPH, RD, 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.'"

Although the number of products touting "net carbs" continues to grow, nutrition experts say the science behind these claims is fuzzy, and it's unclear whether counting net carbs will help or hurt weight loss efforts.

What's in a Net Carb?

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. Excess simple carbohydrates are stored in the body as fat. Examples of these include potatoes, white bread, white rice, and sweets.

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).

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. These substances are commonly used as artificial sweeteners.

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.

The Skinny on Sugar Alcohols

But researchers say the impact of sugar alcohols on blood sugar levels and the body is not fully understood, and they may also cause problems in some people.

"There are some sugar alcohols that can raise your blood sugar," says Karmally. "Certain sugar alcohols do have a higher glycemic index, and they still are not counted as carbohydrates by these companies."

"When you tell a person 'net carbs' or 'impact carbs,' it's very confusing," says Karmally. "A person with diabetes may think, 'It's fine for me to have as much as I want.'"

People with diabetes are advised to closely monitor their intake of carbohydrates because their bodies can't produce enough insulin to keep blood sugar levels within a safe range.

"I think we should not misguide people and make them aware that these sugar alcohols also contribute calories," says Karmally. "Too much of them can actually have a bad effect, and some of them can also have a laxative effect."


"Terms like net carbs shouldn't trick dieters into thinking, 'This is a free lunch, and I can have as much as I want.'"

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.

Registered dietitian Jackie Berning, PhD, says she steers her patients against products containing sugar alcohols for those reasons.


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