Efficacy of a low glycemic index diet and effectiveness of oat β-glucans on cardiometabolic risk factors: Results from randomized controlled trials

Doctoral thesis, 2024

Background: Cardiometabolic diseases (CMD) are major causes of death and morbidity world-wide. Adhering to a healthy diet, such as the Mediterranean diet (MED), has proven effective for prevention of these conditions. Aspects of carbohydrate quality, including glycemic index (GI) and dietary fiber also play an important role for prevention of CMD. Low-GI foods improve glycemic response and have potential beneficial effects on glycemic control. Dietary fiber, gut microbiota, and short chain fatty acids (SCFAs) have been linked to cardiometabolic risk. The oat-derived β-glucan fiber has shown benefits for short-term blood glucose response.

Aim: This thesis aimed to evaluate the effects of GI and dietary fiber on metabolic health among individuals at elevated cardiometabolic risk. The aim was further to evaluate long-term effects of low GI on cardiometabolic risk factors in non-diabetic individuals; identify individual postprandial glucose responses associated with metabolic effects; investigate relationships between SCFAs, type 2 diabetes (T2D) risk factors, and gut microbiota composition; and to assess the impact of β-glucan-enriched bread on long-term glucose control in individuals at T2D risk.

Results: The low-GI diet improved postprandial glucose control compared to the high-GI diet. The MED lowered daily blood glucose in both groups, but only the low-GI diet reduced daily glycemic variability. Two distinct postprandial glucose response clusters were identified and associated differently with T2D risk markers, glycemic control, and gut microbiota. No association was found between SCFAs and T2D risk factors or glycemic control. However, significant positive association was found between acetate concentrations and blood pressure. In a pragmatic study, β-glucan-enriched bread did not improve long-term glycemic control in high-risk individuals.

Conclusion: Low-GI foods within a Mediterranean diet improved postprandial glucose control and reduced glycemic variability, emphasizing GI as a key dietary component for glycemic regulation. The association between acetate and blood pressure suggests a possible role for SCFAs in CMD risk. Although β-glucans have shown benefits for acute glycemic responses, their effectiveness for long-term glycemic control was limited in real-world settings. Findings on individual differences in glucose responses and gut microbiota profiles highlight the need for dietary strategies considering individual variability, i.e., precision nutrition, for cardiometabolic health.