Whole grains have a low glycemic index and contain fiber, nutrients and phytochemicals. The consumption of whole grain foods is associated with reduced risk of cardiovascular disease, type 2 diabetes and certain cancers. Eating whole grains replaces refined grains, which have a higher glycemic index and glycemic load and are stripped of many of their beneficial nutrients during the refining process. Eating a diet high in whole grains can significantly reduce blood cholesterol, low-density lipoprotein (LDL or “bad”) cholesterol and triglycerides and lower fasting glucose levels and insulin levels.
The goal of this study was to examine the effects of rye wholemeal bread with and without added fat on postprandial blood glucose and gastric emptying in healthy adults. The glycemic responses of the two types of bread were assessed in relation to the dietary fibre content and botanical structure of the wheat flour from which the respective bread was made.
A total of 81 healthy adults participated in the randomized, controlled trial. Half of the participants stayed on a weight-maintenance diet and the other half switched to a whole grain diet for eight weeks. The diets consisted of the same food items and were nutritionally equivalent. Blood and stool samples were collected at baseline, after four weeks, after six weeks and at the end of the trial.
Subjects consumed either the white bread or the rye wholemeal bread with added fat on three occasions each week for 8 weeks. Both breads contained a similar amount of total calories, available carbohydrates and dietary fibre. In addition, a proportion of the wheat flour used in each bread had been ground to make it coarser and more like wholegrain flour.
No significant differences were observed in blood glucose response and gastric antral cross-sectional area 15 min after ingestion of the white or rye bread. However, the incremental areas under the glucose curve were significantly smaller after ingestion of the rye wholemeal bread than the white bread.
The consumption of whole grain rye bread was associated with a reduced postprandial blood glucose response and rapid gastric emptying. This is probably due to the increased content of non-digestible components, including cross-linked arabinoxylan and cellulose that resist starch digestion. These substances also have a positive effect on the gut microbiota, increasing the number of butyrate-producing bacteria, such as Roseburia and Eubacterium rectale, and decreasing the number of pro-inflammatory bacteria, such as Enterbacteriaceae.
A higher intake of whole grain is associated with a healthy gut microbiota and improved immune functions. This is likely a result of the increased butyrate production by the gut microbiota, which can improve host health by reducing inflammation and enhancing immune function. Additionally, whole grain containing RS may have beneficial effects on gut microbiota through its indigestible components that support colonic mucus barrier function and regulate gastrointestinal pH.