OBJECTIVE Wholegrain foods vary in the extent of processing. We investigated whether wholegrain particle size in bread influences postprandial glycemia in type 2 diabetes.

RESEARCH DESIGN AND METHODS Postprandial glycemia (incremental area under the blood glucose curve [iAUC]) was measured after consumption of three breads made with roller-milled wholegrain flour and added grains and a fourth made with stoneground flour. All flours and grains were 100% wholegrain wheat. Breads were nutrient matched.

RESULTS Fifteen adults (64 ± 10 years, HbA_1c 58 ± 13 mmol/mol) completed the study. iAUC for the three breads made with roller-milled flour ranged from 376 to 641 mmol⁻¹min⁻¹, inverse linear trend for grain particle size P = 0.039. The iAUC for stoneground wholegrain bread (503) was smaller than predicted from mean particle size.

CONCLUSIONS Wholegrain structural integrity in bread is a determinant of glycemic response. These findings have implications for dietary advice and the definition of the term “‘wholegrain.”