What Eating Sourdough Does to Your Blood Sugar, According to Nutritionists

Sourdough is often perceived as a "healthier" alternative to standard bread, especially when it comes to blood sugar. But is this necessarily true? While there are clear benefits of eating sourdough (particularly in place of regular white bread), there are many factors that affect your blood sugar response—including the way you consume sourdough. To learn more, we asked dietitians to share how eating sourdough affects blood sugar, as well as who might benefit most.

How Blood Sugar Responds to Bread

First, let's explore how blood sugar responds to bread in general, particularly standard white bread. 

A quick primer: "Blood sugar, or blood glucose, is the amount of glucose circulating in the blood," says Luciana Soares, DCN, RDN, LDN, FAND, registered dietitian and professor at Johnson and Wales University. Bread is a source of carbohydrates, which are broken down into glucose during digestion, then absorbed into the bloodstream, she notes.

This means any type of bread containing digestible carbs will raise blood sugar levels to some degree, according to Soares. However, "for overall metabolic health, the goal is to avoid excessive post-meal glucose fluctuations while maintaining a steady supply of glucose to the body's cells to support normal metabolic functions," notes Soares.

White bread is made from refined flour, which is digested quickly by the body. This causes a significant spike in blood sugar, especially when compared to whole wheat bread (which has more fiber, a nutrient that slows down the digestion of carbs). "However, not all white or whole wheat breads are created equal, and their effects on blood sugar can vary depending on factors such as ingredients, type of flour, fiber content, processing methods, and fermentation," says Soares. All that being said, sourdough bread has a different effect on blood sugar that's certainly worth considering.

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How Does Sourdough Affect Blood Sugar

"Sourdough has a lower glycemic index than white or wheat bread, meaning it results in a gradual rise in blood sugar rather than a sharp spike," says Roxana Ehsani, MS, RD, CSSD, LDN, sports dietitian and adjunct instructor at Virginia Tech. In fact, one study found sourdough lowered the rise in blood sugar compared to other types of bread. "The strongest effect was seen in sourdough using whole wheat flour, which makes sense as whole wheat flour contains dietary fiber," Ehsani says.

According to experts, this effect is primarily due to the fermentation process used to make sourdough. "This fermentation process alters the characteristics of the dough and can influence how carbohydrates are digested and absorbed in the body," Soares says. Specifically, fermentation produces lactic and acetic acids, which slow starch absorption in the digestive tract, according to Ehsani. "This causes glucose to enter the bloodstream more gradually, rather than all at once," says Ehsani.

Plus, "fermentation may also change the structure of starch and the physical characteristics of the bread, making some carbohydrates less accessible to digestive enzymes," says Soares. This could further contribute to the gentler rise in blood sugar after eating sourdough.

Who Does This Benefit the Most?

When it comes to blood sugar control, certain people may benefit more from choosing sourdough over white bread. This includes individuals who have prediabetes, diabetes, insulin resistance, or hypoglycemia, according to Ehsani.

However, "everyone can benefit from more stable blood sugar, as it supports steady mood and energy balance throughout the day," says Ehsani. Similarly, Soares says many people may benefit from a steadier glucose response, particularly if they're trying to improve satiety or moderate post-meal blood sugar fluctuations. 

Other Factors That Affect Blood Sugar When You Eat Bread

When you eat bread of any kind, your blood sugar response will depend on many factors, not just the bread itself. According to Soares, this includes individual metabolism, insulin sensitivity, age, body composition, physical activity, and underlying medical conditions.

It matters how you incorporate bread into your diet, too. Here are several aspects to consider:

• The food you eat with bread: When you eat white bread on its own, your blood sugar will rise quickly—and drop just as fast, says Ehsani. However, when it's eaten with something else, blood sugar will be influenced by the entire meal, Soares notes. For example, pairing bread with protein (like eggs), healthy fats (like avocado), and fiber (like vegetables or seeds) can yield a slower blood sugar response, as these nutrients slow the digestion of carbohydrates. 
• The serving size: "It also comes down to how much bread you're eating in a single sitting," Ehsani says. As Soares explains, even sourdough bread can significantly raise blood sugar if you consume it in large amounts, as total carb intake matters.
• Physical activity: Taking a short 10- to 15-minute walk after eating can help your muscles utilize glucose more efficiently and reduce the post-meal rise in blood glucose, says Soares.

"Ultimately, nutrition is about the overall dietary pattern and lifestyle, not a single food in isolation," says Soares. "Sourdough can be a nutritious bread choice and may offer advantages for blood sugar compared with some conventional breads, but as always, what you eat with it, how much you eat, and your overall lifestyle habits are what matter most for long-term health," she notes.