Study: Nutrition has greater influence on gut microorganisms than defensins

Stockholm [Sweden], May 6 : A multitude of variables impacts the makeup of the gut microbiota, including diet and the body's synthesis of the intestinal defence molecule defensins, which was found by researchers at Ume University in Sweden. Instead, scientists determined that these molecules may have a role in preventing blood sugar levels from increasing after ingesting a high-calorie "Western-style diet."
"While the effect of defensins on shaping the adult microbiota composition is rather minor when compared to diet, defensins still have a very important role in protecting us against microbial infections; and our research highlights their protective role against metabolic complications that can arise after the intake of a high-fat and high-sugar Western-style diet," said Fabiola Puertolas Balint, PhD student at Umea University's Department of molecular biology.
She is working in Bjorn Schroder's research group, which is affiliated with the Umea Centre of Microbial Research, UCMR, and The Laboratory for Molecular Infection Medicine Sweden, MIMS, at Umea University.
The gut microbiota refers to the community of trillions of microorganisms that live inside everyone's gut. Over the past decades, the abundance of specific bacteria in this community has been extensively studied due to its connection to many diseases, including inflammatory bowel diseases, obesity and diabetes, and even psychological disorders. The microbial community is seeded during birth, after which several internal and external factors help shape the community to its final composition. These factors include, among others, diet (especially fibre), genetics, medication, exercise, and defence molecules, the so-called antimicrobial peptides.
Antimicrobial peptides can be regarded as the body's own naturally produced antibiotic molecules. In particular, the largest group of antimicrobial peptides - the defensins - is produced by all body surfaces, including the skin, the lungs and the gastrointestinal tract. Defensins are considered the immune system's first line of defence against infections but at the same time they have also been thought to be essential in shaping the microbiota composition in the small intestine. However, it was so far unclear how big their effect was as compared to diet, which is known to have a major impact.

To investigate this, the researchers from Bjorn Schroder's lab used normal healthy mice and compared their microbiota composition in the small intestine to mice that could not produce functional defensins in the gut, and then both mouse groups were fed either a healthy diet or a low-fibre Western-style diet.
"When we analysed the microbiota composition inside the gut and at the gut wall of two different regions in the small intestine, we were surprised - and slightly disappointed - that defensin had only a very minor effect on shaping the overall microbiota composition," said Bjorn Schroder.
However, the intestinal defensins still had some effect directly at the gut wall, where the defensins are produced and secreted. Here, a few distinct bacteria seemed to be affected by the presence of defensins, among them Dubosiella and Bifidobacteria, likely due to the selective antimicrobial activity of the defensins.
"To our surprise, we also found that the combination of eating a Western-style diet and lacking functional defensins led to increased fasting blood glucose values, which indicated that defensins may help to protect against metabolic disorders when eating an unhealthy diet," said Bjorn Schroder.