Calculating Net Carbs

Counting Net Carbs is pretty straight forward except when sugar alcohol is involved. Most low-carb protein bars contain sugar alcohol in sweeteners. Examples of sugar alcohols include:

• Sorbitol
• Xylitol
• Mannitol
• Isomalt
• Maltitol
• Lactitol
• Erythritol
• Hydrogenated Starch Hydrolysates

Typically, you take the total carbs minus the fiber to get Net Carbs. This nutritional label tells us that this bar contains 20g Net Carbs (24 g total carb - 4 fiber).

20g Net Carb = 24 g Total Carb - 4g Dietary Fiber

Carb counting becomes more complex when sugar alcohol is involved. Take this label for example, just looking at the total carbs and fiber, the Net Carbs is 22g... but with the sugar alcohol ingredient, this bar contains 10g Net Carbs. Or does it? 

10g Net Carb = 24 g Total Carb - 2g Dietary Fiber - 12g Sugar Alcohol

The Atkins website specifically states "to calculate Net Carb count with sugar alcohols, simply subtract grams of sugar alcohols (including glycerin), as well as fiber, from total grams of carbs." Their formula is Net Carbs = Total Carb - Dietary Fiber - Sugar Alcohol. So according to Atkins, they claim that, their Caramel Nut Chew Bar is 2g Net Carbs. 

2g Net Carb = 17 g Total Carb - 6g Dietary Fiber - 9g Sugar Alcohol

However American Diabetes Association, NIH, and UCSF disagrees that the entire carb content in sugar alcohol is subtracted from total carbs. 

American Diabetes Association points out in their How do sugar alcohols affect blood levels? article that "if a food has more than 5 grams of sugar alcohols: Subtract ½ the grams of sugar alcohol from the amount of total carbohydrate in a serving of that food."

NIH stated in their Carbohydrate Issues: Type and Amount article that "only about half of the carbohydrate grams from sugar alcohols and half or less from dietary fiber are metabolized to glucose whereas almost all "other carbohydrate" (mainly starch such as amylose and amylopectin) becomes blood glucose.

USCF illustrated in their Counting Sugar Alcohols article that "sugar alcohol is INCOMPLETELY absorbed. Estimate that only half of the sugar in sugar alcohol will be absorbed and impact your blood sugar."

Thus the formula should be: 
If food has more than 5 grams of Sugar Alcohol, 
then Net Carb = Total Carb - Dietary Fiber - 1/2 Sugar Alcohol. 

Therefore the correct amount for the Caramel Nut Chew Bar is 6.5g Net Carbs, not 2g as marketed on their packaging.

6.5g Net Carb = 17 g Total Carb - 6g Dietary Fiber - 4.5g Sugar Alcohol

So what? What's the difference between a mere 2g and 6.5g Net Carbs? The difference is 4.5g Net Carbs per bar. If you eat two bars a day, that is an extra 9g Net Carbs consumed. And if there are other meals that contain sugar alcohol, the wrong amount will be logged in your diet journal - which could impact weight loss. Bottom line is being informed when reading nutrition labels will keep your carb count accurate and not go over your daily budget... especially in the Atkins Induction Phase, where you can only consume 20g Net Carbs a day.

Additional source:

http://livehealthy.chron.com/figure-sugar-alcohol-carbs-6694.html