Drugging the microbiome to decrease atherslcerosis

TMAO is sometimes found in fish.

Trimethylamine N-oxide is a metabolite produced by the microbiome from foods such as red meat and fish.  This metabolite has been independently linked to atherosclerosis, among a host of other diseases.  Researchers at the Cleveland Clinic have been investigating the relationship between the microbiome and this molecule for many years, and we have written about a few of their publications previously.  (Click the TMAO tag below to learn more.)  Most recently, they have researched various compounds that could possibly decrease the production of TMAO by the microbiome.  Last week they published the results of this study in the journal Cell.

The researchers identified a molecule, 3,3-dimethyl-1-butanol (DMB), which inhibited the production of TMAO by gut bacteria.  DMB is a natural product that is commonly found in balsamic vinegar and olive oil.  This molecule was able to shift the microbiome towards bacteria that did not produce TMAO, and importantly, it did not strictly act as an antibiotic and broadly decrease the abundance of microbiome bacteria.  The scientists tested this molecule in mice and showed that it decreased the plasma levels of TMAO in mice that ingested choline.  Moreover, the mice that received DMB had less arterial plaque (i.e. less atherosclerosis).  In addition, the DMB did not appear to have any toxic effects on the mice. 

These researchers hope that the DMB or other agents that lower TMAO levels could possibly be used as therapeutics.   Beyond atherosclerosis, TMAO has been implicated in a number of diseases, ranging from certain cancers to inflammatory diseases.  These diseases are complex though, and their etiologies are not completely understood, so it remains to be seen if this microbiome approach will be successful.  In the mean time, a little less red meat and a little more balsamic vinegar probably won’t hurt.  

Associations between the microbiome and blood lipids

Cholesterol molecule

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.

Eating more vegetables appears to improve microbiome-mediated health indicators

There are many diets that have been rigorously shown to decrease metabolic syndrome (obesity, diabetes, etc.) and are generally associated with a healthy lifestyle, such as vegetarian, vegan, and Mediterranean diets.  The one thing they share in common is a high consumption of plant material, and a low consumption of meat.  There are mechanistic reasons for why high veggie - low fat diets should improve health, and many researchers now believe this is partly due to the gut microbiome that these diets create.  In order to help demonstrate the microbiome-mediated health benefits of a high vegetable – low meat diet, a team of researchers from Italy recently measured the microbiome and specific metabolites produced by the microbiome in 153 individuals.  They then compared these results with the diet that the individual had consumed prior to the measurements, and confirmed that these ‘healthy’ diets were creating ‘healthy’ microbiomes.  They published their results in the journal Gut.

The scientists asked 51 vegans, 51 vegetarians, and 51 ominivores individuals to self-declare their eating habits over the past seven days, and then sampled their stool and urine for bacteria and metabolites.  They learned that amongst the different types of diet the individuals’ overall microbiome diversities were relatively similar.  However, they did show that Bacteroidetes were more prevalent in vegetarians and vegans than in ominvores, and that a higher Firmicutes to Bacteroidetes ratio existed in the guts of ominvores than in vegans and vegetarians.  In addition, the abundance of Prevotella, which is normally associated with health, was positively correlated with overall vegetable intake, and on the contrary Ruminococcus was negatively associated with a high vegetable diet.

The scientists also measured specific metabolites in the individuals.  They discovered that short chained fatty acids (SCFAs), which are normally implicated with health, were associated with the consumption of fruits, vegetables, and legumes.  In addition, there were positive associations between SCFAs and specific populations of bacteria, such as Prevotella.  On the other hand, the metabolite trimethylamine oxide (TMAO), which is a microbiome metabolite whose concentration is directly related to atherosclerosis and other diseases, was significantly lower in vegetarian and vegan diets compared to omnivore diets. It was also directly associated with the abundance of the aforementioned Ruminococcus. 

These relationships between SCFAs and veggies are unsurprising, because SCFAs are the byproducts of bacteria breaking down the complex glycans found in fiber.  In addition, the TMAO is produced by gut bacteria from carnitine and choline, two molecules that exist in red meat and eggs, among other things.  Regardless though, this study should remind us that our diet can shape our microbiome and have lasting health effects.  This study only reinforces that a diet high in veggies that feeds the microbiome is probably a healthy choice.

New study suggests gut microbiome directly influences BMI, triglyceride, and HDL levels

Molecular structure of cholesterol

The microbiome has long been associated with cardiovascular disease, especially after studies showing differences between the gut microbiomes of obese and slim individuals.  The mechanisms by which the microbiome may be influencing heart disease are still unknown, but there are a few mechanisms that have been identified.  For example, as has been previously discussed on this blog, trimethylamine N-oxide (TMAO) in the blood is an independent risk factor for atherosclerosis, and is produced by gut bacteria from choline and carnitine.  In addition, systemic, chronic inflammation is associated with heart disease, and our avid readers will know that the microbiome can cause chronic inflammation in the vagina, gut, and mouth.  Overall though, a direct relationship between specific bacteria and heart disease has not been shown.  A recent epidemiological study though, did just that.  The researchers, mostly from the Netherlands, were able to identify specific species that were associated with higher BMIs, as well as those that were directly correlated with HDL cholesterol levels.  They published their results in the journal Circulation Research.

The scientists measured the genomes, microbiomes, BMI, and blood lipids of 1500 adults.  Their results showed that higher overall diversity and richness of the gut microbiome was associated with a lower a lower BMI (healthier state), lower triglycerides (healthier state), and higher level of HDL cholesterol (healthier state).  The diversity was not, however, associated with total cholesterol nor LDL levels.  The researchers then identified specific bacteria associated with these health indicators.  There are too many to list in this blog, so we encourage interested readers to take a look at the article.  Some examples though: Akkermansia, Christensenellaceae, and Tenericutes were each associated with low BMI, low triglycerides, and high HDL (all healthy states), while Eggerthella was associated with high BMI and high triglycerides, and Butyricimonas was associated with high BMI, high triglycerides, and low HDL (all unhealthy states).  Finally, the researchers sought to determine just how important the microbiome was to overall BMI, triglyceride levels, and HDL levels by incorporating the host genetics, age, and gender into their calculations.  They showed the 4.5% of the variance in BMI, 6% of the variance in triglycerides, and 4% of the variance in HDL is directly attributable to the microbiome.

These study results reaffirm the importance of the microbiome to our overall health, and even quantitatively show its influence on specific health indicators.  The authors do not attempt to explain why specific bacteria would cause variation in these metrics, although as previously mentioned some mechanisms have already been demonstrated.  To check to see which other diseases these bacteria have been associated with, use the search tool, or click the tags below to see all the blog articles that mention them.

Gastric bypass surgery alters microbiome which possible contributes to weight loss

Schematic of Roux-en-Y anastomosis.

Roux-en-Y gastric bypass surgery and vertical banded gastroplasty are two types of bariatric weight loss surgeries that are highly effective in promoting weight loss.  The mechanisms for their efficacy are complex and not completely known, but both surgeries are shown to reduce caloric intake, suppress hunger and increase gastric emptying.  Little is known about how the microbiome changes during these surgeries, and how this change may effect subsequent weight loss.  A team of Swedish scientists investigated this topic and showed the gut microbiota undergo important changes.  They published their results in the journal Cell Metabolism.

The researchers compared the microbiomes of women that were obese and hadn’t had surgery with those who were of similar BMI presurgery, but had undergone surgery at least nine years earlier.  They observed some major differences in the women’s microbiomes, with the post-operative women had much higher levels of Gammaproteobacteria and lower levels of Firmicutes.  When the scientists looked at actual genetic variations they found many differences.  Some notable differences were a decrease in short chain fatty acid (SCFA) and in increase in trimethylamine N-oxide (TMAO) creation in women who had surgery.  As we have written about in this blog before, SCFAs are often associated with health, while TMAO is a risk factor for some cardiovascular diseases.  Interestingly, when they took the microbiomes from both groups of women and transferred them into germ-free mice, the mice receiving microbiomes of women that had undergone surgery gained less weight than the mice that received microbiomes of obese women.

Gastric bypass surgery is often a last resort for folks that have severe obesity.  While not normally considered, the microbiome is drastically affected by this procedure. The microbiome is certainly altered by the procedure, and it appears that it may even be helping keep the weight off.  However, there may be some negative microbiome-mediated consequences as well, derived from alterations to micrbiome metabolism, such as an increased level of TMAO.  Like all surgeries, folks undergoing this one need to balance the risks and rewards of the procedure, and hopefully after this study, the microbiome will be considered.

Microbiome metabolite, TMAO, associated with increased risk of chronic kidney disease and atherosclerosis

Our microbiome is responsible for producing many of the molecules and metabolites that end up in our bloodstream.  Here, these molecules can serve many important therapeutic functions, like in the case of short chained fatty acids (SCFAs).  However, some of these metabolites are beginning to be linked to various diseases.  The best known example of this is microbiome mediated production of trimethylamine-(N)-oxide (TMAO).  TMAO is produced by the microbiome from molecules called carnitine and choline, which come from foods like red meat and eggs.  A lot of literature is now being published that is establishing a connection between TMAO and atherosclerosis, and more recently between TMAO and chronic kidney disease (CKD).  In a new study published last week in the Journal of the American Society of Nephrology, researchers from the University of Kansas assessed the connection between each of these things, TMAO, atherosclerosis, and CKD.

The researchers hypothesized that patients with chronic kidney disease would show increased levels of atherosclerosis because circulating TMAO cannot be cleared from the blood by their injured kidneys.  They measured the degree of CKD and plasma TMAO levels in over 100 patients and confirmed their hypothesis: they noticed a direct relationship between severity of their CKD and circulating TMAO levels.  In addition, the researchers noticed that those people who received kidney transplants had improved TMAO levels, showing that the relationship between CKD and TMAO was most likely due to loss of kidney function.  Finally, the researchers measured biomarkers for atherosclerosis in these patients and correlated the amount of TMAO with the amount of atherosclerosis, even showing that the long term survival of these patients was related to the amount of circulating TMAO levels.

This paper is one of the first to make a connection between circulating TMAO levels and kidney function, which itself is related to atherosclerosis and mortality.  The microbiome is ultimately responsible for the creation of TMAO in the blood, by breaking down things like meats, eggs, and nuts.  It appears that a dietary shift, or a modulation of the microbiome that decreases its ability to perform this transformation should be considered for patients with CKD who are at risk for atherosclerosis.