The ketogenic diet shifts the body’s primary fuel source from carbohydrates to fat, a metabolic state known as ketosis. Maintaining ketosis requires strictly limiting carbohydrate intake, typically below 50 grams per day. This constraint makes finding effective, low-carb sweeteners a high priority. Sugar alcohols, a class of polyols like erythritol and xylitol, are common in low-carb products, providing sweetness without the caloric load of traditional sugar. The central question is whether these substitutes compromise the daily carbohydrate limit or disrupt the metabolic balance of fat-burning.
Calculating Net Carbs from Sugar Alcohols
Sugar alcohols are technically carbohydrates but are treated differently in carb counting because they are only partially digested. Keto dieters use the concept of “net carbs” to represent carbohydrates that are fully absorbed and converted into glucose. To find the net carb count of a packaged food, start with the total carbohydrates listed on the nutrition label.
The standard calculation involves subtracting dietary fiber from total carbohydrates, as fiber is indigestible. A further adjustment is made for sugar alcohols because their molecular structure prevents full absorption in the small intestine. For most sugar alcohols, the common practice is to subtract half of the total sugar alcohol grams from the total carbohydrate count. This partial subtraction accounts for the portion that is not absorbed and therefore not metabolized for energy.
Erythritol is a notable exception to the half-subtraction rule. It is almost completely absorbed in the small intestine and excreted in the urine without being metabolized. Because of this unique metabolic pathway, erythritol is generally subtracted in full when calculating net carbs, yielding zero net carbs per gram. This difference in absorption, resulting from varying chemical structures, is crucial for accurately tracking daily carbohydrate intake.
Glycemic Response and Maintaining Ketosis
While net carb calculation is a starting point, the true metabolic concern is how the absorbed portion of a sugar alcohol impacts blood glucose and insulin levels. Ketosis is primarily regulated by insulin; a significant spike signals the body to stop burning fat and revert to using glucose for fuel. Therefore, a sweetener’s Glycemic Index (GI)—a measure of how quickly it raises blood sugar—is crucial.
Sugar alcohols have a wide range of GI values, reflecting varying rates of absorption and metabolism. Erythritol, for instance, boasts a GI of zero, causing no measurable increase in blood glucose or insulin, making it safe for ketosis. However, other polyols are partially metabolized into glucose in the liver after absorption, triggering an insulin response. This partial metabolism means a low net carb count may still push an individual out of ketosis, especially if consumed in large quantities.
For example, maltitol has a GI ranging from 35 to 52. While lower than table sugar (sucrose) at around 65, this is still high enough to cause a substantial glucose and insulin response in some people. Consuming foods sweetened with higher-GI sugar alcohols risks exiting the ketogenic state, regardless of the net carb label. The physiological response is the ultimate determinant of a sweetener’s suitability for maintaining ketosis.
Specific Sugar Alcohols: Keto-Friendly vs. Non-Keto Options
Based on glycemic impact, sugar alcohols can be categorized to guide the ketogenic lifestyle. Erythritol, with its negligible caloric value and zero glycemic index, is the gold standard among polyols for keto dieters. It is widely used in commercial keto products and is an excellent substitute for baking due to its favorable metabolic profile.
Xylitol is a common polyol often found in gums and mints for its dental benefits. It has a low glycemic index, typically ranging from 7 to 13, and is acceptable in moderation for most keto followers. However, it is absorbed more readily than erythritol and can affect blood sugar if consumed excessively. Pet owners must exercise extreme caution with xylitol, as it is highly toxic to dogs, even in small amounts, causing a rapid release of insulin.
Maltitol is the most problematic sugar alcohol for maintaining deep ketosis. Its glycemic index, potentially reaching over 50 for maltitol syrup, is too high for reliable use by those aiming for strict carbohydrate restriction. Foods listing maltitol should be avoided, as the glucose and insulin spike can negate the day’s carb efforts. Isomalt (GI of about 2) and sorbitol (GI of around 9) fall into the moderate category, requiring careful monitoring and sparing consumption.
Digestive Side Effects and Consumption Limits
The incomplete absorption that makes sugar alcohols beneficial for carb counting is also the source of their most common drawback: gastrointestinal distress. Since polyols are not fully broken down in the small intestine, they travel intact to the large intestine. There, gut bacteria ferment the unabsorbed sugar alcohols, producing gas. This fermentation leads to uncomfortable symptoms like bloating, flatulence, and abdominal discomfort.
Furthermore, these compounds have an osmotic effect, drawing water into the large intestine. This influx can result in a laxative effect or diarrhea, especially when large quantities are consumed quickly. Tolerance varies significantly among individuals and depends on the specific sugar alcohol. Erythritol is usually the best tolerated because the majority is absorbed before reaching the large intestine, minimizing fermentation.
Individuals should establish personal tolerance limits by starting with small amounts and observing their body’s reaction. For most polyols, consuming more than 10 to 20 grams in a single sitting can trigger noticeable side effects. Even for erythritol, excessive intake can lead to digestive issues. Moderation is necessary, not just to maintain ketosis, but also to prevent uncomfortable digestive symptoms.