Yuandi Zhang, Emma Feeney*

School of Agriculture and Food Science, Institute of Food and Health, University College Dublin

The rise of Sourdough bread

Walk into any supermarket in Ireland today and the bread aisle looks very different from a decade ago. Sourdough is everywhere: on shelves, in coffee shops, on social media. Its increasing popularity is attributed not only to the taste, texture of sourdough, but people also find it to be more ‘gentle’ on their gut than other breads. We wanted to test whether this was merely a perception, or if there was anything scientific that could support this.

Our research crosses aspects of both food science and human nutrition. We are interested in both the science of how food tastes and how food behaves once it enters the body. For years, we have been exploring how individual biology, including genetic variation, can affect the way people perceive and respond to food. Much of that work has focused on dairy including fermented cheeses, and fermented milk powders.

When the opportunity arose to examine whether the fermentation process in sourdough bread makes any meaningful difference to digestion, glycaemic responses and gut-related wellbeing, in collaboration with Teagasc, we were keen to get involved. Some evidence already suggests that different bread types can influence blood sugar responses and gastric emptying. But the evidence for sourdough specifically was scarce and somewhat inconsistent, with just a few studies examining the gut microbiome over a meaningful period, and incorporating sensory measures.  We wanted to use this opportunity to find out more about how sourdough’s flavour and texture might also be playing a role in bread digestion.

Why sourdough might be different

Sourdough is leavened by living community of wild yeasts and lactic acid bacteria, a ‘starter’ that ferment the dough over many hours, in contrast to traditional breads that are leavened using commercial baker's yeast. While the composition of the starter is unique to each sourdough, it is this extended fermentation that produces the characteristic tangy flavour, and it may also alter how the bread behaves during digestion, potentially influencing the rate at which starch is broken down and what ultimately reaches the gut microbiome as substrate.

Our gut microbiome is formed by the food and nutrients we eat, and by the physical form that food arrives in. The structure of that matrix may affect how it is processed along the digestive tract, and in in conjunction with Teagasc we hope to answer some of these key questions.

Our research

We have designed a series of complementary studies at UCD and with our collaborators in Teagasc to test whether the unique characteristics of sourdough bread give rise to differences in the overall digestion, glycaemic function, differences in gut microbiota, and in measures of comfort. 

The main study is a randomised controlled trial in which 50 adults with mild to moderate digestive discomfort will consume either approximately 180g of sourdough bread or 180g of white bread daily for eight weeks, within a design that includes a baseline period and a washout phase.

We are measuring a broad range of outcomes. Participants will wear continuous glucose monitors throughout the study and will also use portable exhaled hydrogen devices at home, which provide a non-invasive measure of fermentation activity in the gut.

Stool samples, collected before and after the intervention, will be analysed by shotgun metagenomic sequencing to characterise changes in the microbiome, including its diversity, abundance, and composition, in conjunction with complementary in-vitro digestion studies at Teagasc. Together, these measures allow us to follow the journey of bread through the body: how quickly starch is released into the bloodstream, how the gut ferments what remains, and whether the microbial community changes over time.

Alongside this longer-term trial, a single-day crossover study in 30 volunteers will capture the immediate postprandial response to each bread type: blood glucose, exhaled hydrogen, appetite, and satiety over eight hours.

The roles of genetic differences

One aspect of our design takes us further into individual biology. We are collecting saliva samples from participants to measure a genetic variant called AMY1 copy number variation. The AMY1 gene encodes salivary amylase, the enzyme that begins breaking down starch the moment bread enters your mouth. People vary naturally in how many copies of this gene they carry, which can influence how much amylase their saliva contains. This variation may mean that two people eating the same slice of bread are already beginning to digest it differently before they swallow.

Whether AMY1 copy number interacts with bread type in ways that matter for gut health outcomes or for overall glycaemic function is still unknown, and we hope to answer that, at least partly, in this overall project.

Connecting bread structure to digestion

Our preliminary work so far in characterising a range of different breads show clear differences in the physical structures and texture, with sourdough standing out as quite unique. We hope that the combination of continuous metabolic monitoring, microbiome sequencing, and individual biological characterization in this project will give us a more complete picture of what happens after bread is eaten, contributing to the knowledge base in this area, as well as helping to identify important bread parameters in the future for optimal digestion.

Bread is one of the most widely consumed foods in the world. Understanding how bread production can affect the gut microbiome, has the potential to inform dietary guidelines in ways that are practical and accessible for most people.

The results, once our studies are completed, will be published in open-access journals and the trial is registered on a public clinical trials database (ISRCTN44284096).