What Sourdough Fermentation Actually Changes About Bread

Sourdough fermentation uses wild yeast and lactic acid bacteria that break down starches, gluten structures, and phytic acid during the extended fermentation process, altering the bread's digestibility and how quickly its carbohydrates convert to blood sugar.

The glycemic index difference

Sourdough bread carries a glycemic index around 54, notably lower than white bread (71) and standard whole wheat (68).¹

What the 2024 review found

A 2024 systematic review in Critical Reviews in Food Science and Nutrition found sourdough bread produced lower postprandial glucose responses compared to commercially (yeast) fermented bread, with the effect most pronounced when made with whole wheat flour rather than refined white flour.²

Why the label alone isn't proof

Not all "sourdough" on grocery shelves is true long-fermented sourdough — some commercial "sourdough" bread uses flavoring rather than genuine wild fermentation, so the glycemic benefit isn't guaranteed by the label name alone.

The practical takeaway

• Look for bakery sourdough with a real, extended fermentation process — not just a "sourdough" label on mass-produced bread
• Whole wheat sourdough shows the strongest effect in the review, more than white sourdough
• Individual blood sugar responses to any bread vary significantly person to person