Beneficial glycaemic effects of high-amylose barley bread compared to wheat bread in type 2 diabetes

The rising prevalence of type 2 diabetes (T2D) is largely attributed to factors like obesity, sedentary lifestyles, and unhealthy diets. Developing foods that mitigate T2D risk and improve its management is crucial. Diets low in glycemic index (GI) and glycemic load have shown promise in T2D prevention and management, as well as in reducing cardiovascular disease risk and mortality. Cereals are staple foods worldwide, and their fiber content and starch composition significantly impact glucose metabolism in both diabetic and non-diabetic individuals. Starch comprises amylose (a linear α-glucan) and amylopectin (a branched polymer). Most cereals contain 15-30% amylose. Barley, with its potential benefits, offers several advantages. Genetically modified high-amylose barley (AmOn), with over 99% amylose, has demonstrated lower GI values in vitro and potentially beneficial effects on glucose metabolism. Hulless barley also exhibits lower GI than wheat and positive effects on gut microbiome, hormones, and appetite regulation. This study aimed to investigate the acute effects of breads containing different proportions of AmOn and hulless barley on glycemic responses in individuals with T2D, also examining insulin, glucagon, incretins, and lipid responses.

Existing research highlights the potential of diets low in glycemic index (GI) and glycemic load to prevent and manage type 2 diabetes (T2D) and associated cardiovascular risks. Studies have shown that dietary fiber content and starch composition in cereals influence glucose metabolism. High-amylose starches, due to their slow digestion, have demonstrated a reduction in postprandial glucose. Barley, in comparison to wheat, has shown a lower GI in humans and positive effects on gut health and hormone responses. Previous studies on high-amylose barley (AmOn) and hulless barley, specifically regarding their impact on glucose metabolism in T2D patients, are limited, prompting the current investigation.

This study employed a single-blinded, randomized, crossover design. Twenty adults with T2D (18 completed) were randomly assigned to consume one of four types of bread at four separate visits: 100% wheat bread, bread with 50% hulless barley, bread with 75% hulless barley, and bread with 50% AmOn. The primary outcome was the 4-hour postprandial glucose response (incremental area under the curve, iAUC). Secondary outcomes included postprandial insulin, glucagon, triglycerides (TG), free fatty acids (FFA), glucagon-like peptide-1 (GLP-1), and gastric inhibitory polypeptide (GIP) responses (iAUC or total AUC). Blood samples were collected at various time points. Bread composition was analyzed for moisture, total carbohydrate, fiber, and starch content. Mixed model ANOVA was used for statistical analysis. All participants received a standardized dinner the night before each visit. Anti-diabetic medications were paused 24h before each visit. The four test days were separated by at least a six-day washout period.

Bread made with 50% AmOn reduced the 4-hour postprandial glucose iAUC by 34%, 27%, and 23% compared to 100% wheat bread, 50% hulless barley bread, and 75% hulless barley bread, respectively. The 75% hulless barley bread reduced the iAUC by 11% compared to the wheat bread. The 50% AmOn bread also reduced postprandial insulin iAUC compared to 100% wheat bread and 50% hulless barley bread. Postprandial glucagon responses were similar across all bread types. All barley breads resulted in lower postprandial GIP iAUC compared to wheat bread; however, no differences were seen in GLP-1. Analysis of TG and FFA was complicated by variations in fasting values among the groups. The total carbohydrate content (g/100g) did not significantly differ between the breads, but the fiber content did. The 50% AmOn bread had the highest fiber content, followed by the 50% and 75% hulless barley breads. The total carbohydrate content per serving was higher for 50% AmOn and 50% hulless barley bread compared to 100% wheat and 75% hulless barley bread.

The study demonstrates that replacing wheat flour with high-amylose barley flour, either AmOn or hulless barley, significantly reduces postprandial glucose responses in individuals with T2D. The lower glycemic response observed with AmOn bread aligns with in vitro studies. The reduction in glucose and insulin response with AmOn is likely due to the slow gelatinization and digestion of amylose, increased resistant starch (RS3), and reduced enzyme accessibility. Lower GIP levels observed with AmOn bread may be attributed to the higher fiber content and lower glucose response. Differences in fasting TG and FFA levels among groups complicate the interpretation of postprandial lipid responses, highlighting a potential limitation of the study design. The differences in fiber content between bread types may contribute to some of the glycemic differences observed. While this study provides valuable insights into the acute effects of barley bread, long-term studies are needed to evaluate the impact on weight, satiety, lipid metabolism, and gut microbiome.

Replacing wheat flour with 50% AmOn or 75% hulless barley flour in bread lowers postprandial glucose responses in individuals with T2D. The beneficial effects likely stem from the increased fiber content and the unique properties of high-amylose starch, which slows digestion and reduces glucose absorption. Future research should focus on long-term studies incorporating these breads into dietary interventions to further clarify the effects on diabetes management, body weight, lipid metabolism, satiety, and gut microbiota. Improving barley's baking qualities is also crucial for wider adoption.

The study's acute design limits conclusions regarding long-term effects. Variations in fasting lipid values among groups affected the interpretation of postprandial lipid data. The use of AmOn was not commercially feasible in Denmark due to regulations, limiting the generalizability of results related to this specific type of barley. The study included a relatively small sample size. Further research is needed to investigate the long-term effects and address the limitations of the present study.