It is well known that we have to be careful with what foods we eat, remembering to stay healthy and eat our fruits and vegetables. Diets high in fat can create serious health issues such as obesity, high cholesterol, and possibly Type 2 diabetes. Also on that list of related health problems is cardiovascular disease, which is characterized by blood clots, due to fat and plaque build-up in blood vessels, and can lead to a heart attack or stroke. Previous research has implied a connection between the microbiome and cardiovascular disease, due to the microbiome’s effect on production of a molecule called trimethylamine N-oxide (TMAO). As of yet, no research has been done to track the association between the microbiome and lipid (fat) build-up, so this is precisely what researchers published in Circulation Research set out to do.

The scientists located in The Netherlands, Poland, and Massachusetts, collected blood cholesterol measurements from 1500 LifeLines-DEEP subjects. LifeLines-DEEP is a collection of subjects used for assessing various health issues. Ethnic outliers and genetically related participants were removed from the study. Fecal samples were collected from 1180 participants, and sequenced. By the end of the data collection, 99 participants were excluded for reasons such as antibiotic use, or use of potentially microbiome-altering medications. In total there was a final number of 893 participants (380 men and 513 women) for which cholesterol samples, microbiome samples, and genotypic information was obtained. The participants included a wide range of age, BMI, and blood lipid levels.

The researchers found that gut microbiome species richness was significantly higher in women, and increased with age. Microbial richness was positively correlated with high density lipoproteins (HDL, the 'good cholesterol'), not correlated with low density lipoproteins (LDL, the bad cholesterol), and negatively correlated with body mass index (BMI). For example, the study confirmed that lower abundances of kingdom Archaea, families Christensenellaceae and Rikenellaceae, class Mollicutes, and genus Dehalobacterium are associated with high BMI. It was estimated that the microbiome could explain 4.57% to 65 of variation in BMI, triglyceride and HDL. No link was found between the gut microbiome and genetic predisposition to obesity of high blood lipid levels.

One hypothesis raised by the researchers is that bacteria potentially try to correct lipid imbalances, thereby helping to prevent cardiovascular disease. The strong associated between the gut microbiome and BMI and blood lipid levels – regardless of age, sex, and genetics – suggests that the microbiome does indeed play a role, if indirectly, in cardiovascular disease and other fat-related issues. 34 gut bacteria were found to be associated with BMI and blood lipids. There is a real potential for the utilization of this information in health therapies, such as blood clot and stroke prevention.

Type 1 diabetes (T1D) is a disease in which your immune system attacks and destroys your insulin-producing cells. There is a known genetic risk factor in developing T1D, but there are also significant non-genetic components to getting the disease. Previous research in mice has established the microbiome's connection with the development of diabetes, but the link in humans has not been studied as closely. Researchers from various institutions in the U.S. and Finland recently assembled a cohort of infants genetically at-risk for diabetes, and tracked the changes in their microbiomes. They discovered that the microbiomes of those individuals that were eventually diagnosed with diabetes underwent characteristic shifts leading up to diagnosis, and that these changes were not observed in healthy infants. They published the results of their study in Cell Host and Microbe.

The researchers sampled the stools of 33 infants in Finland and Estonia that were genetically at-risk for diabetes. Their first major discovery was that even though the bacterial composition of the microbiome grew, changed, and became more diverse with age, the types and number of genetic pathways that were expressed by the microbiome, as well as the metabolites produced by the microbiome remained stable. They also found many similar bacterial species between infants, however these infants usually had different strains of said bacterial species. In most of these cases, once a particular strain established itself in the gut it remained stable and would not be displaced.

The scientists tracked the microbiome changes that occurred with diet as well. During breast feeding Bifidobacterium and lactobacillus predominated, and Lachnospiraceae decreased. After cessation of breast feeding the addition of eggs barley and soy seemed to have a direct influence on the microbiome. One of the biggest factors in the developing microbiome was actually geography, as the Estonian infants had significantly higher levels of Bacteroides and Streptococcus species.

The researchers then compared the microbiome samples between those infants that were eventually diagnosed with diabetes and those that were not. They discovered that a few bacterial species were much more abundant in those infants that got diabetes: Blautia, the Rikenellaceae, and the Ruminococcus and Streptococcus genera, including Ruminococcus gnavus and Streptococcus infantarius. Interestingly, each of these bacteria are ‘pathobionts’, or bacteria which exist in many healthy peoples’ microbiomes but have the potential to become pathogenic. Also, certain bacteria such as Coprococcus eutactus and Dialister invisus were non-existent in the diabetics' guts. In addition, the researchers discovered that the expression of specific genes, like those associated with sugar transport and the biosynthesis of amino acids, underwent shifts prior to the onset of diabetes. Finally, many of these bacteria that were associated with T1D appeared right before the onset of the disease, and these bacteria were linked to the presence and absence of certain metabolites in the stool.

These results provide exhaustive evidence for an association between the microbiome and diabetes. It links specific bacteria in the microbiome and the expression of certain genes by the microbiome to the disease. The next step is to study the mechanisms by which the microbiome induces diabetes, and then therapeutics can be developed.

Associations between the microbiome and blood lipids

The infant microbiome changes before the onset of type 1 diabetes