SACRAMENTO, Calif. -- The low-carbohydrate craze is creating a lot of questions for people watching their waistlines.

But how can a low-carb food product label read 3 grams of "net" carbs on the front label and 22 grams of "total" carbs on the back?

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The Food and Drug Administration has not outlined what a net carb actually means, so food manufacturers can make virtually any claim they want on their labels, reported KCRA-TV in Sacramento.

"When you eat a food that contains carbohydrates -- let's take a regular piece of bread -- blood-sugar levels, or glucose levels, go up and the hormone insulin responds to that," said Elizabeth Applegate, a nutrition expert at the University of California, Davis. "Now, in a food that says it has a lower number of net carbs, what they are saying is the carbohydrates they have in there don't cause an insulin response."

Fiber doesn't affect insulin, so the theory is that it's the carbs minus the fiber that's in the product that makes up the net carbs.

For example, a bread label might say 6 grams of net carbs on the front, but 9 total carbs and 3 grams of fiber on the back. And many low-carb dieters say it works.

"This is an example of a good outcome of this whole fad because people are buying this bread. And you know what? They're eating fewer calories, and they're getting more fiber," Applegate said.

But there are some exceptions. KCRA-TV found a chocolate bar claiming to have 3 net carbs, 22 total carbs and 10 grams of fiber. So by the math, it should have 12 net carbs, not 3.

Applegate says manufacturers aren't counting sugar alcohols like sorbitol, manitol and glycerin, which are listed in the fine print. Those ingredients might not affect insulin much, but they can still make the consumer gain weight.

"What consumers really need to know is that food still has calories, and it still counts, and don't be disappointed if you don't lose weight, because you need to take in fewer calories than you're burning," Applegate said.

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