Is fermented food low GI? The surprising truth about fermentation and blood sugar

November 26, 2025 •

3 min read

According to research, many fermented foods often have a lower glycemic index (GI) than their unfermented counterparts, which helps support more stable blood sugar levels. The transformation that occurs during the fermentation process can significantly alter how the body digests and absorbs carbohydrates.

Quick Summary

Fermentation typically lowers a food's glycemic index by creating organic acids and increasing resistant starch, which slows carbohydrate digestion and absorption. This process helps prevent rapid blood sugar spikes after eating.

Key Points

GI Reduction Mechanisms: Fermentation lowers a food's GI by producing organic acids like lactic acid and increasing resistant starch content.
Slower Digestion: Organic acids created during fermentation can slow down gastric emptying, leading to a more gradual absorption of glucose.
Resistant Starch Formation: The low pH created by lactic acid bacteria promotes the formation of resistant starch, which is not digested and absorbed by the body.
Probiotic Support: The live bacteria (probiotics) in fermented foods can improve overall gut health and insulin sensitivity, further aiding blood sugar regulation.
Food-Specific Effects: The GI-lowering effect varies depending on the food. Real sourdough bread has a low GI, while not all fermented milks have an equally low insulinemic index.
Ingredient Matters: Be aware that added sugars in fermented foods like commercial yogurts can counteract the GI-lowering benefits.

What is the Glycemic Index (GI)?

The glycemic index (GI) is a numerical scale that ranks carbohydrate-containing foods based on their effect on blood sugar levels over a period of time. Foods are ranked on a scale from 0 to 100:

Low GI (55 or less): These foods are digested and absorbed slowly, causing a gradual and sustained rise in blood sugar.
Medium GI (56–69): These have a moderate effect on blood sugar.
High GI (70 or more): These are quickly digested and absorbed, leading to a rapid spike and subsequent crash in blood sugar.

Following a low-GI diet is particularly beneficial for people managing conditions like diabetes, but it can also help with weight management and overall energy levels by providing a more consistent supply of energy.

How Fermentation Influences the Glycemic Index

The act of fermentation fundamentally changes the chemical composition of food, impacting its effect on blood sugar. Microorganisms like bacteria and yeast interact with the food's carbohydrates, resulting in several key changes that lower the GI.

Production of Organic Acids

One of the primary mechanisms is the creation of organic acids, such as lactic and acetic acids, during fermentation. These acids have a direct impact on digestion by slowing down how quickly food leaves the stomach and enters the small intestine, delaying the absorption of carbohydrates into the bloodstream. The acids can also interact with starches, particularly in bread products, making them harder for digestive enzymes to break down.

Increase in Resistant Starch

Fermentation can increase the proportion of resistant starch (RS) in a food, a type of starch that is not fully broken down by the body's digestive enzymes and thus doesn't raise blood sugar. The low pH environment created by lactic acid bacteria promotes retrogradation, where starches recrystallize into a less digestible structure. Freezing fermented products can further enhance the resistant starch content.

Microbial Metabolism

During fermentation, the microorganisms consume some of the available sugars and starches in the food as their fuel source. This reduces the total amount of digestible carbohydrates in the final product. For instance, the long fermentation of sourdough bread breaks down and consumes much of the starch, contributing to its lower GI compared to conventional bread.

Examples of Fermented Foods and their GI Impact

Sourdough Bread: True sourdough, with long fermentation, has a significantly lower GI (around 53) than standard white bread (over 70).
Fermented Dairy (Yogurt, Kefir): Plain yogurt (10–19) and kefir have lower GIs than milk (20–34), partly due to lactic acid production.
Kimchi and Sauerkraut: Fermented vegetables are rich in fiber and organic acids, contributing to a lower GI.
Tempeh and Natto: These fermented soy products have lower glycemic indices due to microbial activity.

Fermented vs. Unfermented Food: A GI Comparison


Food Type	Fermented Version (Example)	Typical GI	Unfermented Version (Example)	Typical GI	Difference Explained
Bread	Sourdough (long-fermented)	53 (Low)	Conventional White Bread	72 (High)	Lactic acid and microbial action break down starch, slowing absorption.
Milk	Yogurt (plain, natural)	10-19 (Low)	Milk (skim or whole)	20-34 (Low)	Fermentation and lactic acid production further reduce GI and slow gastric emptying.
Soybeans	Tempeh/Natto	Low	Soybeans (boiled)	Moderate	Microbes consume carbohydrates and produce beneficial metabolites.
Cabbage	Sauerkraut/Kimchi	Low	Cabbage (cooked)	Low	While cabbage is low GI, fermentation adds probiotics and organic acids that support gut health and blood sugar control.

Important Considerations

While many fermented foods are low GI, the final GI can be affected by ingredients and preparation. Sweetened yogurt has a higher GI than plain. Pairing fermented foods with other ingredients can also affect the meal's overall glycemic response. Pairing a high-GI food with fiber, fat, or acidic food can help lower the overall GI.

Conclusion

Fermentation effectively modifies the nutritional profile of foods, often resulting in a lower glycemic index. Organic acid production and increased resistant starch during fermentation lead to a slower release of glucose into the bloodstream. This makes foods like true sourdough bread, plain yogurt, and kimchi beneficial for managing blood sugar and promoting metabolic health. Incorporating authentically fermented foods offers sustained energy and gut-health benefits.

For more detailed information on glycemic load and specific food values, consult resources like the Linus Pauling Institute.

Frequently Asked Questions

No, not all fermented foods automatically have a low glycemic index. While fermentation can lower the GI of many foods, the final value depends on the original ingredients, the specific fermentation process, and any added ingredients like sugar.

True sourdough bread, made with a long fermentation process, has a significantly lower GI than conventional bread. The long fermentation consumes more starches and produces organic acids that slow digestion, resulting in a more moderate effect on blood sugar.

Plain, unsweetened yogurt and kefir are considered low GI foods. However, flavored yogurts with added sugars can have a higher GI. It is important to check the label for added sugars.

Yes, incorporating certain fermented foods into a diet can aid in diabetes management. Their lower GI and ability to improve insulin sensitivity can help control blood sugar spikes. However, they should be part of a balanced diet and portion sizes should be monitored.

Resistant starch is a type of carbohydrate that is not digested in the small intestine. Fermentation can increase the amount of resistant starch in a food, which slows carbohydrate digestion and contributes to a lower GI.

Yes, kimchi and sauerkraut are typically low GI due to their high fiber content and the organic acids produced during fermentation. These components slow down the absorption of any carbohydrates present.

Yes, consuming fermented food can influence insulin levels. The lower GI of many fermented foods means a slower and lower insulin response. However, some fermented dairy products may have a disproportionately high insulinemic index compared to their GI, due to other components.