Is Sourdough Toast Low in Carbs? A Complete Nutritional Breakdown

November 11, 2025 •

3 min read

Despite common misconceptions, sourdough toast is not inherently a low-carb food. Its carbohydrate content is similar to other breads, but the key difference lies in how its carbs are digested due to the natural fermentation process.

Quick Summary

This article explores the nutritional profile of sourdough toast, examining how its fermentation process affects carbohydrate content and its impact on blood sugar. It offers a detailed comparison to other breads and provides strategies for incorporating it into low-carb diets.

Key Points

Not a Low-Carb Food: Standard sourdough toast contains a similar number of total carbohydrates as regular bread.
Lower Glycemic Index: Due to its fermentation process, sourdough has a lower glycemic index, leading to a slower and more gradual rise in blood sugar.
Better Digestion: The fermentation process breaks down starches and gluten, making sourdough potentially easier to digest for some individuals.
Increased Nutrient Bioavailability: Fermentation reduces phytic acid, which increases the body's absorption of key minerals like magnesium and zinc.
Pairing is Key: To manage blood sugar spikes, always pair sourdough with proteins and healthy fats.
Whole-Grain Offers More Fiber: Opting for whole-grain sourdough varieties provides more fiber, further aiding in blood sugar management.
Not Keto-Friendly: For strict ketogenic diets, traditional sourdough is too high in carbohydrates and should be avoided.
Look for True Low-Carb Loaves: If your goal is to minimize carbs, seek out specialty low-carb breads made with alternative flours.

Sourdough Fermentation: How It Changes Carbohydrates

Sourdough bread is made using a unique, slow fermentation process with a natural starter of wild yeast and lactic acid bacteria, unlike conventional breads that use commercial yeast. While the total amount of carbohydrates in a slice of sourdough is generally comparable to regular bread, the way your body processes them is significantly different.

The Impact of Lactic Acid Bacteria

The bacteria in a sourdough starter produce lactic acid, which lowers the dough's pH. This acidic environment has two main effects on carbohydrates. First, it alters the starch structure, making it more resistant to digestion. Second, the acid itself slows down the rate at which your body digests and absorbs the carbohydrates. The result is a lower glycemic index (GI), meaning it causes a slower, more gradual rise in blood sugar compared to standard white bread.

Resistant Starch and Gut Health

The fermentation process also increases the amount of resistant starch in the bread. Resistant starch functions more like fiber in the body, providing food for beneficial gut bacteria rather than being immediately broken down into glucose. This prebiotic effect supports a healthier gut microbiome and can contribute to overall metabolic health.

Sourdough vs. Other Breads: A Carbohydrate Comparison

While sourdough has distinct advantages, it is important to compare its carbohydrate profile to other popular breads, particularly for those on low-carb diets like keto. The comparison below illustrates that while sourdough is not low-carb, it offers different nutritional benefits.


Feature	White Sourdough (per 1 large slice)	Regular White Bread (per 1 large slice)	Whole Wheat Sourdough (per 1 large slice)	Low-Carb Bread (per 1 slice)
Total Carbs	~18.5g	~20.5g	~16.8g	~3-10g
Net Carbs	~17.7g	~19.5g	~14.6g	~1-6g
Fiber	~0.8g	~1g	~2.2g	~2-7g
Glycemic Index	54 (Low)	71 (High)	54 (Low)	N/A (Generally very low)

How to Incorporate Sourdough on a Lower-Carb Diet

For individuals closely monitoring carbohydrate intake, standard sourdough toast is still not an ideal choice. However, several strategies can help make it work within a modified plan:

Portion Control: Simply reducing your portion size can significantly lower carbohydrate intake. Instead of a full slice, consider a half-slice. Remember to always factor it into your daily carb count.
Pair with Protein and Fats: The overall glycemic load of a meal is crucial. To minimize blood sugar spikes, pair sourdough toast with protein and healthy fats. Examples include avocado and eggs, or butter and cheese. This slows down digestion and the release of glucose into the bloodstream.
Seek Whole-Grain Sourdough: Choosing a sourdough made with whole-grain flour increases fiber content. This further helps with blood sugar management and adds beneficial micronutrients.
Try Long-Fermentation Sourdough: Homemade sourdough or loaves from artisanal bakeries often undergo longer fermentation. This process allows the microorganisms more time to break down starches, potentially leading to slightly lower total carbohydrate levels and a greater reduction in phytic acid.
Look for Low-Carb Alternatives: Many specialty bakeries and brands now offer dedicated low-carb, keto-friendly breads made with alternative flours like almond or coconut flour. These provide the familiar texture of bread with a significantly reduced carbohydrate count.

Sourdough for Overall Health

Beyond its carbohydrate profile, sourdough offers other health benefits that are worth noting. The fermentation process breaks down phytic acid, an anti-nutrient found in grains that can inhibit mineral absorption. This makes minerals like magnesium, iron, and zinc more bioavailable. Furthermore, some individuals with a mild gluten sensitivity find sourdough easier to digest, though it is not a gluten-free product and is not suitable for those with celiac disease.

Conclusion

So, is sourdough toast low in carbs? The answer is no, not in the same way that a true low-carb bread alternative is. However, the fermentation process results in a lower glycemic index, improved digestibility, and increased nutrient availability compared to standard breads. For those on moderate low-carb diets, enjoying sourdough in moderation and pairing it with protein and healthy fats can be a viable option. For very strict low-carb or ketogenic diets, it's best to consider other alternatives. Ultimately, while it may not be a low-carb food, it is a better-carb food for many individuals.

Additional Resources

For those interested in the nutritional science behind sourdough and other fermented foods, authoritative sources like the National Institutes of Health provide detailed research. A study in the Journal of Agricultural and Food Chemistry further explains how lactic acid fermentation modifies nutrient profiles and lowers the glycemic response in baked goods.

Here is a useful study on sourdough fermentation

Frequently Asked Questions

No, fermentation does not remove all carbohydrates from sourdough. While the process involves microorganisms consuming some of the starches and sugars, a significant amount of carbohydrates remain. It does, however, change the structure of the remaining carbs, affecting how your body processes them.

Traditional, long-fermented sourdough bread typically has a lower glycemic index (around 54) compared to standard white bread (around 71). This means it causes a less dramatic spike in blood sugar after consumption.

For those on a strict ketogenic diet, traditional sourdough bread is not suitable due to its carbohydrate content. However, some specialty bakeries create keto-friendly sourdough using alternative flours like almond or coconut flour, which are acceptable for keto.

Yes, for many people, sourdough is easier to digest than regular bread. The long fermentation process helps to break down complex carbohydrates and some gluten, which can be beneficial for those with mild digestive sensitivities.

To reduce the glycemic impact of your sourdough toast, pair it with healthy fats and proteins, such as avocado, eggs, or nut butter. This slows digestion and moderates the blood sugar response.

Whole-grain sourdough toast is generally not significantly lower in total carbohydrates than white sourdough. However, its higher fiber content provides a lower net carb count and further moderates blood sugar levels.

Some commercially produced sourdough bread uses faster fermentation methods and may contain additives to achieve the signature sour flavor. This can negate some of the nutritional benefits, such as a lower glycemic index, associated with a traditional, slow fermentation process.