Carbs, Net Carbs, and How to Count Them

What Are Carbohydrates?

Carbohydrates are one of many sources of fuel for your body. They are primarily found in fruits, vegetables, legumes, seeds, and some dairy products. While carbs are a necessary fuel, you don’t actually need to eat any — your liver can make all the carbs you need from protein and fat.

To understand how your body deals with carbs, it is important to compare the different types: sugar, starch, and fiber — as well as a related molecule, sugar alcohol. Each is digested differently and, in turn, affects your blood sugar, insulin response, and metabolism differently. Counting all carbs equally doesn’t give an accurate picture of what’s happening in your body; this is the idea behind net carbs, which takes these differences into account.

Sugar

Sugar molecules are rings made of five or six carbons with conjoined hydrogens and oxygens. There are many kinds of sugars — lactose, for instance, is the sugar found in milk while fructose is the sugar found in fruits. Both glucose and fructose are found in honey and cane sugar. In the latter, glucose and fructose combine to form sucrose.

For the most part, we convert the sugar we consume into glucose. Other sugars (other than fructose) are converted to glucose in your small intestine and then absorbed through the intestinal wall into your blood. This absorption is fast, causing spikes in blood sugar levels and triggering an insulin response. Because of this fast absorption and insulin response, sugars count in full towards net carbs.

Starch

Starch is produced by most green plants as a way to store energy, analogous to fats in animals. It is made of long chains of glucose and is the most common carbohydrate in most human diets — large amounts of starches are found in staples such as potatoes, wheat, corn, and rice.

Starch digestion occurs throughout your digestive system: enzymes in your saliva and later in your small intestine slowly break off each glucose molecule. Just like other sugars, the molecules are then absorbed into your blood, raising blood sugar levels and triggering an insulin response. Likewise, starches count in full towards net carbs.

Fiber

Fiber is a structural component of all plants. Similar to starch, fiber is made of long chains of glucose — though the way in which glucose molecules are bonded to form fiber prevents our enzymes from breaking the chain like they do for starch. Instead, fiber remains undigested until it reaches the colon, at which point it is broken down by bacteria for nutrition. Since glucose cannot pass through the colon wall, the individual molecules are not absorbed into the blood and therefore result in no blood sugar or insulin response — which is why fiber doesn’t count toward net carbs.

Sugar Alcohol

Sugar alcohol — neither sugar, alcohol, nor carbohydrate — is naturally occurring in fruits. It is also industrially produced for use as sweeteners in many “diet” foods. Common sugar alcohols are erythritol, mannitol, sorbitol, xylitol, lactitol, maltitol, and isomalt.

Used as a substitute, sugar alcohol contains fewer calories that sugar — 1.5–3 calories per gram compared to 4 calories per gram. This is because it is converted to glucose and absorbed much more slowly than sugars and starches. As a result, it causes a slower spike in blood sugar and generates a much lower insulin response. To account for this, only half of carbs from sugar alcohol are counted towards net carbs.

Insulin and Metabolism

As your body digests carbohydrates and sugar alcohols, your blood sugar levels rise. This, in turn, triggers a release of insulin from the pancreas. Insulin transports the glucose in the blood to cells that need fuel. Once the cells are satisfied, the extra glucose is stored in the liver and muscles as glycogen — a readily-available energy source. Any remaining glucose is then converted to fat. If you are overeating carbs, though, some of the glucose can end up in fat regardless of the energy needs of your cells.

A ketogenic diet limits or removes carbs and sugar alcohols from your diet. As a result, blood sugar levels don’t spike and insulin responses are limited. With no energy coming from carbs or sugar alcohols, glycogen is used instead — and since glycogen is bound to water molecules, using up your glycogen storage results in an initial loss of water weight. It is only when your glycogen is depleted that the body actually begins to burn fat for energy.

Counting Net Carbs

When counting carbs for your macros, the goal is to track only the carbs that could trigger blood sugar and insulin responses.

Since fiber is not digested and since sugar alcohols are digested and absorbed slower, the basic formula for calculating grams of net carbs is:

Total Carbs - Fiber - ½ Sugar Alcohol = Net Carbs

Let’s look a nutrition table as an example. First, we find the total carbs, which in this case is 29g — remember that total carbs includes sugars, so don’t count it twice!

Next, we subtract the fiber, which is 2g, to get 27g. Finally, we subtract half of the sugar alcohol. Half of 18g is 9g, leaving us with a grand total of 18g of net carbs.

29g Total Carbs - 2g Fiber - ½ (18g Sugar Alcohol) = 18g Net Carbs