What are Net Carbs and why do they matter?

In reference to ketogenic diets, “Net Carbs” are the carbohydrates that will affect your ability to maintain ketosis.  Be wary though, as “Net Carbs” is defined by the people who market diet foods. 

Previously, “Net Carbs” referred only to the total carbohydrates in a product minus the fiber.  Now some things are not considered a “Net Carb”, but can still kick you out of ketosis.  Sugar alcohols are a prime example. But first, let’s define a couple of things. 

There are multiple types of carbohydrates found in food.  Some are digested and affect your insulin levels, while others don’t. Simple sugars and starches are the most common carbohydrates in our diet and both substantially affect blood glucose and insulin levels. 

Please see the Enhancing Ketosis FAQ page for information on how insulin is tied to ketosis. Fiber, while still a carbohydrate, isn’t digested, so it doesn’t have any effect on insulin. 

Here is where it gets sketchy.  Sugar alcohols are also excluded from “Net Carbs”, but some of them DO have a substantial impact on insulin.  When dealing with carbohydrates, one of the things you need to understand is the Glycemic Index (or GI). 

Glycemic Index is a scale that ranges from 0-100 and measures the impact of food on insulin levels as compared to glucose, with 0 being no impact on insulin and 100 being the same impact on insulin as glucose. 

However, even though glucose is 100, there are still sugars that have a greater impact on insulin and blood sugar than glucose, so their GI is higher than 100. 

It is important to note that most of us don’t eat pure glucose, so to give a better idea of the effect on insulin, we need to compare to something we are familiar with: table sugar. 

When we think of sugar, you are probably thinking about granulated table sugar.  As a reference, table sugar has a GI of around 65.  We will use this as a reference point to compare sugar alcohols to. 

Luckily, there are quite a few sugar alcohols with a GI <10, but even they have some issues.  We will be looking at the most common sugar alcohols and their pros and cons.  We’ll also look at a couple of other carbohydrates to look for.         

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Erythritol is one of the very few 0 GI sweeteners, primarily because it is absorbed by the body, but isn’t digested.  It has a 0 GI, 0.21 kcal/g (table sugar is 4 kcal/g) and a low risk of laxative effect.  It does have a cooling sensation when eaten, which some people find undesirable.  It is keto safe.

Mannitol is also 0 GI, but it is more likely to have a laxative effect than Erythritol.  However, in very low doses, it is acceptable.  It is keto friendly.

Glycerol (glycerin or glycerine) has a GI of 3, so is still very low.  It is used in a lot of products to help maintain a soft texture or help keep moisture in a product. 

The one drawback is that it has as many calories as sugar (most sugar alcohols, except erythritol are around 2 kcal/g).  However, at the amounts found in most products, it shouldn’t affect weight loss.  It is keto friendly.

Isomalt, Sorbitol and Xylitol round out the rest of the keto friendly sugar alcohols.  They are all in the 3-10 range for GI (although Xylitol is around 12), but all have relatively high incidence of laxative effects.  Taken in small doses, these should all be fine on keto.  WARNING, Xylitol is highly poisonous to dogs. 

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Polydextrose (a synthetic polymer of glucose) has a GI of around 7.  It isn’t an alcohol, but it is keto friendly. 

Maltitol is the one sugar alcohol most likely to kick you out of ketosis.  It has a GI of around 52 (depending on the preparation, ie syrup) and has a large laxative effect.  Want to guess which sugar alcohol is used the most in sugar-free candy?  Yep, maltitol. 

That is one of the reasons we can’t say sugar alcohols are all keto friendly.  This is one sugar alcohol (along with the rarer hydrogenated starch hydrolysates, or HSHs like polyglycitol) that should be avoided on keto.  It is nearly as bad as table sugar in regards to breaking ketosis.

As you can see, it isn’t appropriate to completely remove sugar alcohols from the carb count.  This is especially the case for maltitol and polyglycitol, which should be counted as full carbs in my opinion.  I personally only remove sugar alcohols with a GI <10 from my Net Carbs count.  The formula I follow is:

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Another thing of note is allulose. This is a rare sugar found in figs, raisins and other fruits in small amounts. The part that is interesting (besides being a zero calorie, zero carb, 0 GI sweetener) is that it is not required to be on the nutrition facts label as a carbohydrate.

So far, it is the only sweetener in this category. It is keto safe and should be subtracted from Net Carbs as well.

Total Carbohydrate – Dietary fiber – Sugar Alcohols (<10 GI) allulose = Net Carbs

One thing related to Net Carbs and GI that needs to be pointed out are carbohydrates that are commonly ignored by diet food producers. 

The worst offender is maltodextrin.  It is used extensively and is one of the reasons a lot of processed foods are not keto friendly.  The GI of maltodextrin is between 85 and 110 (depending on the study), meaning it affects your blood sugar and insulin levels (thus breaking ketosis) more than table sugar and close, if not more so than pure glucose. 

Be careful reading nutrition labels because many companies make their serving size small enough to keep the total carbohydrate per serving low.  This allows a legal “low carb” label, even if there is a large insulin response to the food.  My best advice it to read labels like someone is trying to hide carbs and never assume “sugar-free” means keto safe.

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*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure or prevent any disease.