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.'"
"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."
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.
"I just don't know how they're going to react. We've never put that much in," says Berning, an associate professor of nutrition at the University of Colorado at Colorado Springs. "Some are going to get diarrhea, and some are going to have gastrointestinal problems."
Calories vs. Carbohydrates
Berning says the larger issue she has with products that tout a low "net carb" count is that they also often contain a lot of calories.
"It's my guess that most people are restricting carbohydrates because they want to lose weight," Berning tells WebMD.
"The point I think they're missing is that you may have 2 net carbs in this bar but you've also got 260 calories," she says referring to double chocolate Powerbar. "I don't care that it's only 2 net carbs. The thing is, have you done enough exercise, have you balanced the rest of your diet to put in 260 calories in that bar -- whether it has 30 grams of carbohydrates or 2?"
Rather than focus on what she calls "the little c" of carbohydrates, Berning says people interested in weight loss should focus on the "big C"-- calories.
Karmally agrees and says terms like net carbs shouldn't trick dieters into thinking, "This is a free lunch, and I can have as much as I want," just because a food company says the impact or net carbs are only so much.
"Our report concludes that there is no substitute for the simple formula that 'calories in must equal calories out' in order to control weight," says FDA Acting Commissioner Lester Crawford in a news release announcing the report.
In addition, the report recommended that the FDA respond to requests to define terms such as "low," "reduced," and "free" carbohydrates as well as provide guidance on use of the term "net carbs." Several industry and consumer groups as well as food manufacturers have petitioned the FDA to set official "low carb" levels as well as take action on "net carb" claims.
Until the agency takes action on the carbohydrate claim issue, experts say carb counters are probably better off eating foods that are naturally low in refined carbohydrates, such as fruits and vegetables, rather than highly processed foods like snack bars, pastas, and sweets that have had their natural carbohydrates stripped away.
"Whole foods, like whole grains, fruits, and vegetables, should be the foundation of diet," says Karmally. "Because if you miss out on these foods, then you end up missing out on a whole bunch of nutrients and antioxidants that have a potential benefit on reducing the incidence of chronic, degenerative diseases."