Individuals with type 2 diabetes do not produce enough insulin, or their cells do not respond to it appropriately. As a result, cells do not absorb sugar efficiently, and blood sugar level rises. Over time, this can cause damage to internal organs.
The Western diet, which is high in saturated fats and refined sugars, increases the risk of developing type 2 diabetes. Recently, scientists set out to identify which specific gut bacteria species might play a role in this association between diet and diabetes.
The gut microbiome includes hundreds of species of bacteria. Scientists have shown that an imbalance in the microbiome, or dysbiosis, has associations with adverse health outcomes.
One 2019 study suggested that a disturbance in the gut microbiome might contribute to the development of type 2 diabetes.
A recent paper, which appears in Nature Communications, suggests that a small number of specific bacteria might be pivotal.
Scientists from Oregon State University in Corvallis, OR, the University of Vienna in Austria, the National Cancer Institute, and the National Institutes of Health in Bethesda, MD, carried out the research.
This paper is important as it shows that specific bacteria that are not “keystone” influencers of the whole microbiome may still have an important individual impact on health.
“The analysis pointed to specific microbes that potentially would affect the way a person metabolizes glucose and lipids. Even more importantly, it allowed us to make inferences about whether those effects are harmful or beneficial to the host. And we found links between those microbes and obesity.”
– Study co-leader Andrey Morgun
The researchers combined experiments on mice with the analysis of large quantities of data from previous research in mice and humans. The scientists gave mice either a regular diet or food equivalent to a Western diet.
As the researchers expected, mice fed a Western diet developed glucose intolerance and insulin resistance, which are contributing factors to type 2 diabetes.
They also noted a significant change in the composition of the gut microbiome. Researchers then applied a “Transkingdom Network” analysis, which is a data-driven approach that models interactions between the microbes and the body to identify which gut bacteria contributed most to the changes in metabolism.
They managed to narrow down the list to four bacteria that appeared to play a key role in reducing or intensifying the harmful effects of a Western diet: Lactobacillus johnsonii, Lactobacillus gasseri, Romboutsia ilealis, and Ruminococcus gnavus.
Natalia Shulzhenko, an associate professor of biomedical sciences in OSU’s Carlson College of Veterinary Medicine and the study’s other co-leader, said:
“The first two microbes are considered potential ‘improvers’ to glucose metabolism, the other two potential ‘worseners.’ The overall indication is that individual types of microbes and/or their interactions, and not community-level dysbiosis, are key players in type 2 diabetes.”
An analysis of data from human research showed that the four bacteria identified in mice also correlates with the body mass index (BMI) of people on a Western diet.
People who had higher levels of the two “improvers” had a lower BMI; people with more “worseners” were more likely to have higher BMI. They also found that R. ilealis was present in more than 80% of people with obesity, which suggests that this microbe might contribute to obesity.
The authors of the study now wanted to know what happens to the mice’s metabolism when they receive “improvers” and “worseners” to see if the bacteria could improve the metabolism of people with type 2 diabetes.
Different strains of Lactobacillus occur in many fermented foods, including certain dairy products, such as yogurt. Mice on a diet that contained R. ilealis showed a reduced glucose tolerance level and insulin production, which suggests a diabetes-like condition.
Interestingly, while R. ilealis did not affect adiposity (the amount of body fat), L. gasseri and L. johnsonii helped reduce it.
As expected from the earlier analysis in the study L.gasseri and L.johnsonii improved glucose tolerance in mice fed on a Western diet. In addition, providing evidence of potential therapeutic benefit, L.gasseri improved established glucose tolerance in these mice.
The authors noted that “minimal alterations in microbiota induced by L.gasseri and L.johnsonii supplementation did not explain the restoration of glucose metabolism promoted by these bacteria.”
Next, the researchers looked at target organs that the Lactobacilli might affect, such as the gut and the liver.
Scientists have previously shown that reducing fat in the liver is important for recovery from type 2 diabetes. The authors of the recent study found that genes that controlled liver cell mitochondrial function, which have links with lipid metabolism and overall glucose control, were upregulated.
The mitochondrial health in the liver of mice that received L. gasseri or L. johnsonii improved, reducing “bad lipids.” Scientists think this is important for achieving better glucose tolerance and metabolism.
The authors of the study believe that, in the future, these findings might help develop treatments for type 2 diabetes. As Morgun comments:
“Our study reveals potential probiotic strains for treatment of type 2 diabetes and obesity as well as insights into the mechanisms of their action. That means an opportunity to develop targeted therapies rather than attempting to restore ‘healthy’ microbiota in general.”