Gut microbiota can determine the health effects of whole grain

Whole grain has been shown to decrease the risk of many diseases, such as type 2 diabetes and certain cancers, but less is known about the effects of whole grain on gut microbiota and the gut-brain axis. Gut microbiota means trillions of microbes, mainly bacteria, found in the human gastrointestinal tract. The gut-brain axis is a two-way communication route between the gut and the brain in which gut microbiota plays a central role.

Gut bacteria break down whole grains in different ways

Whole grain is rich in dietary fiber and plant bioactive compounds that gut microbiota can use as a nutrient source. Microbes can also produce new compounds, metabolites, from these dietary compounds. For example, gut microbiota produces short-chain fatty acids from dietary fiber and these fatty acids can play an important role in the gut-brain axis.

– However, there is a large inter-individual variation in gut microbiota composition, which can affect the production of metabolites, says Laura Pirkola.

In her doctoral thesis, Laura Pirkola has investigated the effects of whole grain oat bread, whole grain rye bread, and low-fiber refined bread on microbiota and the levels of short-chain fatty acids and other metabolites were studied using a so-called fecal fermentation model. The effect of microbiota composition was investigated using fecal samples from two different donors in the model.

Study subjects ate whole grain rye bread for three weeks

Laura also studied the effects of whole grain rye bread on gut microbiota and the gut-brain axis were studied in a dietary intervention study, where the study subjects consumed a high amount of whole grain rye bread or a control bread daily for three weeks.

– We saw differences in short-chain fatty acids and other metabolite levels and the utilization of dietary fiber during fermentation between the samples with microbiota from different donors. This indicates that microbiota composition can affect how nutrients from bread are used by gut microbiota, says Laura.

Rye increases the production of the fatty acid butyrate

The levels of a specific short-chain fatty acids, butyrate differed between the two donors, especially in samples with rye. Butyrate is considered a key metabolite in the gut-brain axis. No significant changes in fecal microbiota composition or the gut-brain axis were observed in the three-week intervention study.

– However, we observed a small increase in the abundance of two microbes that produce butyrate in the participants consuming whole grain rye. Altogether, the results presented in this thesis indicate that rye may have the potential to increase butyrate production, likely depending on gut microbiota composition, says Laura.

Rye has been shown to increase butyrate levels in previous human studies, but the effects of whole grain rye on the gut-brain axis remain unclear.