How Does Sourdough Bread Affect Blood Sugar Levels?

December 10, 2025 •

4 min read

Research has consistently shown that sourdough bread has a lower glycemic index (GI) compared to conventional bread, meaning it causes a more gradual and lower rise in blood sugar. This unique property is a result of the natural, long fermentation process that sets it apart from mass-produced loaves.

Quick Summary

The fermentation in sourdough breaks down starches and produces organic acids, leading to a slower release of glucose into the bloodstream compared to regular bread. This makes it a gentler option for managing blood sugar levels and avoiding sharp spikes, though portion control remains crucial.

Key Points

Lower Glycemic Index: The fermentation process reduces sourdough's GI, leading to a slower and more gradual increase in blood sugar compared to standard white bread.
Slower Digestion: Organic acids produced during fermentation, especially acetic acid, delay gastric emptying, which slows carbohydrate absorption.
Whole Grains are Better: Sourdough made with whole wheat or rye flour provides more fiber, further enhancing blood sugar control.
Pair with Protein and Fat: Combining sourdough with protein and healthy fats helps to stabilize blood sugar levels and prolongs satiety.
Not All Sourdough is Equal: True, long-fermented sourdough from an artisanal bakery offers more benefits than commercial 'sourdough-flavored' breads.
Portion Control is Key: Despite its benefits, sourdough is still a source of carbohydrates, and portion control is essential for managing blood glucose.

The Science Behind Sourdough and Blood Sugar

The secret to sourdough's gentler effect on blood sugar lies in its preparation. Unlike commercial breads that use baker's yeast for a rapid rise, sourdough relies on a symbiotic culture of wild yeast and lactic acid bacteria (LAB) called a 'starter'. This slow, natural fermentation process is what fundamentally alters the bread's composition, impacting how the body processes its carbohydrates.

Fermentation's Impact on Carbohydrates

During the long fermentation, the LAB and wild yeasts feast on the carbohydrates present in the flour. This process has several key effects on the final product and its glycemic response:

Lowering the Glycemic Index (GI): Fermentation actively breaks down starches, essentially pre-digesting them. This results in a bread with a lower GI, which is a measure of how quickly a food raises blood glucose levels. While white bread often has a high GI (around 71), genuine sourdough can have a moderate GI (around 54) or even lower.
Producing Organic Acids: The fermentation creates organic acids like lactic acid and acetic acid. Acetic acid, in particular, has been shown to slow down gastric emptying, meaning food leaves the stomach more slowly. This delayed digestion leads to a more gradual absorption of glucose into the bloodstream, preventing the rapid spike seen with other breads.
Resistant Starch Formation: The process of baking, especially when followed by freezing and thawing, can increase the formation of resistant starch in sourdough. Resistant starch is a type of fiber that resists digestion in the small intestine and instead feeds beneficial gut bacteria in the large intestine. This fermentation process further moderates the blood sugar response.

Sourdough vs. Other Breads: A Blood Sugar Comparison

Not all bread is created equal when it comes to blood sugar impact. The choice between sourdough and other types of bread can make a significant difference, especially for those managing conditions like diabetes or insulin resistance.

Comparison of Common Bread Types


Feature	Sourdough Bread	Commercial White Bread	Whole Wheat Bread	Gluten-Free Bread
Glycemic Index (GI)	Lower (~54)	High (~71-90)	Medium-High (~71)	Varies, can be high due to starches
Digestion Speed	Slows digestion and glucose absorption due to fermentation and organic acids.	Rapidly digested, causing quick blood sugar spikes.	Fiber helps slow digestion compared to white bread, but GI can still be high if made with commercial yeast.	Can be rapidly digested depending on the starch content of the flours used.
Key Benefit	Lower GI for more stable blood sugar levels. Enhanced nutrient absorption.	Readily available and inexpensive, but lacks nutritional benefits for blood sugar control.	Contains fiber, but the impact on blood sugar may not be as gentle as true sourdough.	Offers an alternative for those with gluten intolerance, but not necessarily a low GI choice.
Best For...	Individuals seeking better blood sugar management, improved gut health, and enhanced nutrient absorption.	Best to limit, especially for those monitoring blood glucose.	A good source of fiber, but pairing with protein and fat is important for blood sugar control.	Celiac disease or gluten-intolerance; requires label scrutiny for added sugars and starches.

Maximizing Sourdough's Blood Sugar Benefits

To get the most out of sourdough while managing your blood sugar, consider the following strategies:

Opt for Whole Grains: Not all sourdough is made the same. Sourdough made with whole-grain flour (like whole wheat or rye) contains more fiber, which further slows digestion and promotes better blood sugar control. Always check the ingredient list to ensure it's made with whole grains, not just refined flour.
Pair with Protein and Fat: Eating sourdough as part of a balanced meal is key. Combining it with protein (like eggs or salmon) and healthy fats (like avocado or nut butter) can help slow down the absorption of carbohydrates even further, leading to a more stable blood sugar response.
Practice Portion Control: Even with a lower GI, sourdough is still a carbohydrate source. Moderation is important for blood sugar management. Paying attention to portion sizes helps keep total carbohydrate intake in check.
Try Freezing and Toasting: Some research suggests that freezing and then toasting sourdough can further reduce its glycemic response by increasing resistant starch. This simple trick can provide an additional layer of blood sugar management.

Conclusion: A Smarter Choice for Blood Sugar

Sourdough bread's traditional, slow fermentation process makes it a healthier choice for blood sugar management compared to many other breads, including conventional white and some whole wheat varieties. The production of lactic and acetic acids and the breakdown of starches lead to a lower glycemic index, causing a more gradual release of glucose into the bloodstream. By opting for whole-grain sourdough and pairing it strategically with other foods, individuals concerned with blood sugar levels can enjoy bread as part of a balanced diet. However, it is always important to manage portion sizes and consult a healthcare professional for personalized dietary advice, especially for those with diabetes.

Frequently Asked Questions

Yes, people with diabetes can eat sourdough bread. Due to its lower glycemic index and slower impact on blood sugar compared to white bread, it can be a better choice, but portion control is still important.

Sourdough's lower GI is a result of its long fermentation process. The wild yeast and lactic acid bacteria break down starches and produce organic acids that slow down the absorption of glucose into the bloodstream.

Yes, whole-grain sourdough is often a better choice for blood sugar control. The added fiber in whole grains further slows digestion and contributes to a more stable blood sugar response.

Acetic acid, one of the organic acids produced during fermentation, delays gastric emptying. This means food moves more slowly from your stomach to your small intestine, which slows down the rate of glucose absorption.

Some studies suggest that freezing and then toasting sourdough bread can further reduce its glycemic response. This process increases the amount of resistant starch, a type of fiber that slows digestion.

Look for a sourdough with a simple ingredient list, typically just flour, water, and salt. Avoid products with added yeast or dough conditioners, as these often indicate a faster, less beneficial fermentation process. Opt for loaves made with whole-grain flour when possible.

No, sourdough bread is still a source of carbohydrates. While it can be a healthier choice than other breads due to its lower GI, it is not a 'no-carb' food, and moderation is essential for effective blood sugar management.