Structural differences between rye and wheat breads but not total fiber content may explain the lower postprandial insulin response to rye bread

BACKGROUND: Rye bread has a beneficial effect on the postprandial insulin response in healthy subjects. The role of rye fiber in insulin and glucose metabolism is not known. OBJECTIVE: The aim of the study was to determine the effect of the content of rye fiber in rye breads on postprandial insulin and glucose responses. DESIGN: Nineteen healthy postmenopausal women aged 61 +/- 1 y, with a body mass index (in kg/m(2)) of 26.0 +/- 0.6, and with normal glucose tolerance participated in the study. The test products were refined wheat bread (control), endosperm rye bread, traditional rye bread, and high-fiber rye bread; each bread provided 50 g available carbohydrate and was served with breakfast. Plasma glucose, insulin, glucose-dependent insulinotropic polypeptide, glucagon-like peptide 1, and serum C-peptide were measured in fasting and 8 postprandial blood samples. In vitro starch hydrolysis and the microscopic structure of the breads were also determined. RESULTS: Postprandial insulin, glucose-dependent insulinotropic polypeptide, and C-peptide responses to the rye breads were significantly lower than the response to the control; no significant differences in insulin and C-peptide responses to the rye breads were found. Glucose and glucagon-like peptide 1 responses to the rye breads were not significantly different from those to the control, except at 150 and 180 min. In vitro starch hydrolysis was slower in all rye breads than in the control, and the structure of continuous matrix and starch granules differed between the rye and control breads. CONCLUSION: Total fiber content does not explain the lower postprandial insulin response to rye bread than to wheat bread, but structural differences between rye and wheat breads might.