Food labels can be tricky to understand, but they are necessary if you want to eat healthily. Some nutritional information is not directly available on the nutrition facts label you find on packaged foods, but you can figure it out by doing a simple calculation. For example, you can subtract grams of fiber -- both soluble and insoluble -- from grams of total carbs to obtain what is called available or net carbs. Diabetics and low-carb dieters often use net carbs instead of total carbs when tracking their dietary intake.
Total Carbohydrates
The figure for grams of total carbs listed on the nutrition facts table includes all types of carbohydrates that a food contains, including sugar, sugar alcohols, starches, soluble fiber and insoluble fiber. When you add up all of these types of carbohydrates, you get the total carb content of a food. However, your body does not metabolize all carbs in the same way, so the total carb content of a food does not give you a clear idea of how a food affects your body.
Fiber
Fiber can be either soluble or insoluble. Most food labels simply indicate the total fiber content, while others specify soluble and insoluble fiber. Both types of fiber prevent constipation and promote a regular intestinal transit time. In addition, soluble fiber appears to benefit blood sugar control in diabetes and can improve blood cholesterol levels. Insoluble fiber is found mainly whole grains, while soluble fiber is abundant in oats, barley, legumes, nuts, psyllium, flaxseeds and in fruits and vegetables, especially eggplant, okra, apples and citrus.
Total Vs. Available Carbs
By deducting the soluble and insoluble fiber from the total carbs in a food, you separate out the carbs that your body does not digest. Humans do not absorb calories from soluble or insoluble fiber. The remaining carbs in a given food, which consist of starches and sugars, enter your bloodstream once digested to give energy to your body. Calculating the available carb content tells you how much the food will raise your blood sugar levels and provide calories. Simply knowing the total carb content of a food does not give you a lot of information.
Calculating Available Carbs
You can calculate the available carbs by deducting both the soluble and insoluble fiber content. All food labels indicate the total fiber content of food, which is the sum of the soluble and insoluble fiber. You can use the total fiber content to make your calculation easier. For example, if a slice of oat bread contains 15 g total carbs, 2 g total fiber, 1 g insoluble fiber and 1 g insoluble fiber, you can calculate its available carb content two ways. Either deduct the 2 g total fiber, or deduct 1 g insoluble fiber and 1 g soluble fiber from the 15 g of total carbs. In this example, the available carb content of the slice of oat bread is 13 g.
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