Sourdough fermentation of wholemeal wheat bread reduces glycemic responses in subjects with insulin resistance

Intake of wholegrain foods, as well as foods with slow glycemic response, is associated with reduced risk of chronic diseases. Since glycemic responses to breads made of wholemeal flour are high, retarding the glycemic response of a wholemeal bread would further increase its health benefits. The aim was to study postprandial glucose and insulin responses of wholemeal wheat breads in relation to baking technology. Breads were baked using 100% flour from peeled (2%) wheat kernels by straight dough or sourdough fermentation method, and with or without addition of xylanase during mixing of dough. Standard white wheat bread was used as reference. Eleven subjects with insulin resistance and features of the metabolic syndrome were served the breads in random order. Each test bread portion contained 50g of available carbohydrate. Blood samples for measuring glucose and insulin concentrations were drawn at eight time points over four hours. Nutrient composition and the content of soluble protein of the test breads were determined. In vitro hydrolysis of protein and molecular weight of the protein hydrolysates were also performed. The wheat bread produced using sourdough fermentation had the lowest postprandial glucose and insulin responses. Plasma glucose concentration was lower at time points 45, 60, 90 and 120 min (p<0.05) and higher at 240 min (p<0.01) as compared with the reference. Furthermore, serum insulin concentration was lower at the time point 90 min (p<0.05). The postprandial responses were not improved by addition of xylanase. Sourdough fermentation and xylanase treatment increased the amount of water-extractable arabinoxylan, while the latter also led to increased depolymerisation of water-extractable arabinoxylan. In the sourdough fermented bread, the content of soluble protein was the highest and the MW of the hydrolysed proteins the smallest. In conclusion, sourdough fermentation resulted in a bread with the most favourable postprandial glucose and insulin responses among the three tested wheat breads made of peeled kernels. Possible mechanisms by which fermentation produces this effect are low pH, increased arabinoxylan solubilisation, and proteolysis. The reduced postprandial response achieved by sourdough fermentation may further encourage the use of wholemeal wheat bread by persons with insulin resistance.