The ketogenic diet requires drastically restricting carbohydrate intake, typically limiting digestible carbohydrates to 20 to 50 grams daily, to induce a metabolic state called ketosis. This process switches the body from burning glucose to burning stored fat for fuel. Confusion often arises when evaluating packaged foods that use alternative, low-calorie sweeteners. Sugar alcohols are frequently used in “keto-friendly” products, and understanding their unique metabolic impact is necessary for accurate carb tracking and maintaining the daily carb limit.
Defining Sugar Alcohols
Sugar alcohols, scientifically known as polyols, are carbohydrate molecules structurally resembling both sugar and alcohol, though they do not contain ethanol. While found naturally in fruits, commercial versions are manufactured from sugars and starches.
Manufacturers favor polyols because they provide sweetness with significantly fewer calories than standard sugar. Unlike regular sugar, which is quickly absorbed, sugar alcohols are only partially absorbed in the small intestine.
Their molecular structure resists complete breakdown by digestive enzymes. This incomplete absorption means a large portion moves into the large intestine, which is why they are treated differently when calculating a daily carb allowance.
Applying the Net Carb Rule
The concept of “net carbs” is central to ketogenic dieting, representing the portion of total carbohydrates the body digests and converts into glucose. The standard formula is to take total carbohydrates and subtract the grams of dietary fiber and the grams of certain sugar alcohols. This acknowledges that fiber is indigestible and many sugar alcohols minimally affect blood sugar.
Food labels often list total sugar alcohol grams without differentiating their metabolic impact, necessitating a conservative calculation. For most polyols, the general practice is to subtract only half of the listed sugar alcohol grams from the total carbohydrates. This conservative approach treats the remaining half as a digestible carb load that contributes to blood glucose levels.
For example, consider a product with 20 grams of total carbohydrates, 5 grams of fiber, and 10 grams of non-erythritol sugar alcohols. Subtract the full 5 grams of fiber, plus 5 grams (half of the 10 grams) of the sugar alcohols. This calculation results in 10 grams of net carbohydrates, the figure tracked against the daily carb limit.
Differential Metabolic Impact of Common Types
The complexity in counting sugar alcohols stems from the fact that not all polyols are metabolized the same way. Their effect on blood sugar is measured using the Glycemic Index (GI), which varies widely among different types. Understanding these differences is necessary for making informed food choices that support ketosis.
Erythritol is considered the gold standard for keto-friendly sweeteners because it has a Glycemic Index of zero. Approximately 90% of ingested erythritol is rapidly absorbed in the small intestine but is not metabolized for energy; instead, it is excreted unchanged in the urine. This unique pathway means erythritol contributes virtually no calories or net carbs, allowing for its full subtraction from the total carbohydrate count.
Conversely, polyols like maltitol, xylitol, and sorbitol have a much more significant metabolic impact. Maltitol, in particular, has a GI ranging from 35 to 52, which is more than half that of table sugar. About 40% of ingested maltitol is absorbed and fully metabolized, meaning it can cause a noticeable rise in blood glucose and insulin levels.
For products containing these higher-GI sugar alcohols, dieters should adhere strictly to the rule of subtracting only half of the listed grams, or consider avoiding them entirely. Xylitol and sorbitol fall in the middle, with GIs around 12 and 9, respectively, and a partial caloric load.
Their partial metabolism means they still contribute a measurable amount of glucose to the bloodstream. Subtracting only 50% of these sugar alcohols remains the most prudent strategy to prevent an unexpected increase in blood sugar that could disrupt ketosis. Choosing products that explicitly use erythritol provides the most reliable path to maintaining a low net carbohydrate intake.
Practical Considerations and Intake Limits
Beyond the metabolic impact, the unabsorbed portion of sugar alcohols presents practical considerations for digestive health. Since polyols are not fully digested in the small intestine, they travel to the large intestine where gut bacteria ferment them. This fermentation produces gas, which is the primary cause of common side effects like bloating, flatulence, and general gastrointestinal discomfort.
Sugar alcohols also have an osmotic effect, drawing water into the colon. This can lead to loose stools or a laxative effect, especially when consumed in large quantities. The severity of these digestive issues often correlates with the polyol’s absorption rate; less-absorbed types like sorbitol and maltitol cause more distress than highly-absorbed erythritol.
To maintain digestive comfort, moderate the intake of all sugar alcohols, regardless of their net carb calculation. A common guideline suggests limiting consumption of the more fermentable polyols to under 10 to 15 grams per day. Even for erythritol, exceeding 30 to 50 grams daily might still lead to mild digestive symptoms in sensitive people.