Gut microbiota or gut flora represents the population of complex and dynamic microorganisms in the human gastrointestinal tract. These microorganisms include bacteria, fungi, archaea having a symbiotic relationship with the host, and influence during disease and homeostasis. Gut microbiota starts to develop during infancy and is controlled by multiple factors, among which diet is one of the most contributing factors throughout life. These microorganisms play a vital role in maintaining metabolic homeostasis, immunity, and protection against pathogens. Alterations in gut microbiota have been associated with the pathogenesis of many diseases, including heart diseases, obesity, diabetes, cancer, and other inflammatory conditions.
Role of Gut Microbiota
The microorganisms in the gut form an intricate and mutually beneficial relationship with the host (human), also known as a symbiotic relationship. The development of gut microbiota is started from birth, and after birth, it is subjected to alterations due to factors including illnesses, diet, and medicines. Gut flora has three primary roles:
● Act as a Defence Against Pathogens
Gut microorganisms play a role in directly inhibiting invading organisms by producing cytokines that initiate an inflammatory response.
● Development of Immunity
Gut microbiota also plays a role in the development of the human immune system by regulating the production of antibodies. These antibodies also act outside of the gut like lungs or other body tissues. Gut flora also produces certain metabolites that can alter the immune response. For example, gut microorganisms produce short-chain fatty acids (SCFA) through fermentation that stimulate the production of innate immune cells (basophils, neutrophils, and eosinophils). These innate immune cells are vital to limit the spread of infection.
Without gut microbiota, the human body will not be able to digest some of the essential nutrients from the food eaten. These organisms have enzymes that the human body lacks, for example, breakdown and digestion of certain polysaccharides, fiber, starches, sugars, and oligosaccharides. Gut flora also synthesizes folate and biotin and facilitates the absorption of certain nutrients like calcium, magnesium, and iron.
Alterations in Gut Microbiota by Diet
Dietary habits have been linked by alterations in gut microbiota contributing to heart diseases, cancer, obesity, metabolic disorders, and diabetes. It is estimated that around 30 to 40 percent of an adult’s gut microbiota can be altered during their lifetime, and diet is one of the major contributing factors. For example, Bacteroides enterotype is found in individuals consuming protein-rich and fatty diet, and Prevotella enterotype is seen in individuals consuming fiber-rich diets. It is seen that high-fat diets are linked to low SCFAs and low Bifidobacterium concentrations. Clinical studies have also shown the relationship between high protein intake and increased risk of kidney disease. Intakes of nutrients below the minimum recommendations are associated with undernutrition, and intake above that level are associated with overnutrition illnesses.
Consumption of Red Meat and Alterations in Gut Microbiota
Diet has been a major modulator of gut microbiota alterations. Dietary factors such as proteins, fats, polyphenols, and fiber from food intake can alter the composition of gut microbiota and certain microbial metabolites that impact human health. Diet rich in animal proteins or excessive consumption of red meat is strongly associated with negative alterations in gut microbiota and the production of harmful metabolites that lead to various diseases, including cancer (colorectal) and heart diseases. In western diets, red meat is frequently consumed, and long-term consumption is associated with an increased risk of illnesses.
Red Meat and Role of Neu5Gc
Red meat is rich in N-glycolylneuraminic acid (Neu5Gc), a type of sialic acid. It is a nine-carbon backbone acid sugar present at the outermost end of glycan chains found in all cells of non-human mammals. Sialic acids are involved in various pathological and physiological processes. Humans cannot produce Neu5Gc due to the evolutionary loss of enzyme CMAH (CMP-Neu5Ac hydroxylase). When humans consume red meat containing Neu5Gc, it becomes incorporated into cell-surface glycans. The researchers believe that the body recognizes it as a foreign body and stimulates the immune system to form antibodies against Neu5Gc. It induces an inflammatory response when anti-Neu5Gc attacks Neu5Gc containing glycans. This process is termed as xenosialitis.
To study host-microbe interactions and to know how a Neu5Gc rich diet can affect gut microbiota and bacterial metabolism, a team led by Dr. Karsten Zengler of the University of California, San Diego conducted a study on genetically engineered mice. The genetically engineered mice cannot synthesize Neu5Gc on their own, and the only source of Neu5Gc is by diet, just like humans. Half of the mice are then fed on a Neu5Gc rich diet and a half with a soy-based diet. It was seen that mice with a Neu5Gc rich diet contain fewer bacterias in the gut as compared to the soy-based diet fed. However, several types of bacterias are found in abundance in mice consumed the Neu5Gc diet. Among them, Bacteroides are significant, known to use sialic acid (family of sugars including Neu5c). The researchers identified the increases in Bacteroides enzymes, sialidase using DNA sequencing in the mice that consumed red meat. They concluded that bacteria use this sialidase enzyme to release Neu5Gc from the cells.
To confirm the similar findings of increased levels of sialidase in human gut microbiota who consumed red meat by examining the data from unique hunters-gathers living in remote areas of Tanzania. They consume meat only during the dry season (hunting season), and in the wet season, they eat berries and honey. The Bacteroides with similar sialidase activity was present in the gut during the dry season.
To further confirm the findings, the researchers found that sialidases could strip Neu5Gc from store-bought pork and beef sausages in the laboratory upon cutting up and crushing. These findings also help find ways to remove Neu5Gc from the meat using bacterial sialidases to reduce the risk of inflammatory diseases.
After all these studies, the researchers concluded that increased xenosialitis (anti-Neu5Gc antibody response to glycan containing Neu5Gc) is seen in the gut of humans harboring fewer bacteria with Neu5Gc preferring sialidases leading to increased risk of inflammatory diseases including colorectal cancer. This study was also published on September 23, 2019, in Nature Microbiology.
Red Meat and Role of Trimethylamine N-oxide (TMAO)
People who consume a lot of red meat are also at higher risk of developing cardiovascular disease, probably due to atherosclerosis triggered by red meat. L-carnitine present in red meat contributes to altering the gut microbiota making the person prone to heart diseases. It is seen that people eating red meat have increased levels of TMAO (trimethylamine N-oxide) as compared to those who are consuming white meat. L-carnitine in red meat increases the bacteria in gut microbiota that metabolizes it and produces great amounts of TMAO. High levels of TMAO have been associated with atherosclerosis, a disease-causing buildup of plaque in arteries leading to cardiovascular diseases. TMAO is thought to be the risk predictor of heart diseases. It raises the risk of thrombosis by altering platelets and making them respond differently to clotting triggers. Research from the University of Leicester, United Kingdom, suggested that higher levels of TMAO make acute heart failure fared worse.
The research team led by Robert A Koeth and Dr. Stanley L. Hazen of the Cleveland Clinic in Ohio studied the relationship between red meat and heart diseases and came up with the two possible mechanisms. The study and its results were also published in the European Heart Journal. The study suggested that red meat consumption enhances gut bacterial production of TMAO and reduces its elimination through kidneys.
The researchers asked people (participants) to consume red meat and carnitine supplements. Those eating red meat have more levels of TMAO than vegans or vegetarians. When participants took antibiotics to suppress gut microbes, less TMAO is produced after eating carnitine, suggesting that gut microbes play a role in the production of TMAO. They also found the differences in gut microbiota between meat and non-meat eaters. The researchers also found the association between carnitine and TMAO levels suggesting that dietary habits may alter gut microbiota and the ability to form TMAO from carnitine. The researcher also saw that the kidneys of the participants on red meat are less efficient in removing TMAO from the blood. And after ceasing the red meat consumption, the levels of TMAO falls in the blood. Gut production of TMAO was lower, and kidney elimination was higher when the individuals consumed a non-meat or white meat diet.
The teams also evaluated 2600 cardiac patients and found that patients with higher carnitine levels are at higher risk of developing serious cardiac events such as stroke, heart attack, and death. This higher risk is only seen in patients with higher levels of plasma TMAO suggesting that TMAO is a link between heart diseases and carnitine.
The team further investigated by studying genetically engineered mice with no pre-existing gut microbiota and found that these mice do not make TMAO when fed with carnitine. But after some time, when they can develop gut microbiota, they start to make TMAO upon feeding with carnitine. They also studied mice who fed with carnitine supplemented diet for several months showed changes in gut microbiota. It leads to increased production of TMAO and an increased risk of atherosclerosis. Another group of mice on the same diet but also given an antibiotic to suppress gut microbiota showed lower levels of TMAO and reduced risk of atherosclerosis. They further researched that TMAO affects major pathways responsible for clearing up cholesterol from the body, making it accumulate and promote atherosclerosis.
At the end of all these experiments and studies, the team leader Dr. Hazen concluded that the gut microbiome is altered by diet. A diet rich in carnitine shifts our gut microbiota to like carnitine making the meat-eaters more susceptible to forming TMAO, while vegans and vegetarians are less likely to synthesize TMAO from carnitine.
Gut Microbiota and Metabolic Syndrome
Gut microbiota alterations have been found to be linked with obesity and metabolic syndrome, primarily due to dietary factors. Studies were carried out on cafeteria fed diet rats and concluded that consuming such a diet for 12 weeks resulted in obesity, insulin resistance, dyslipidemia, and hepatic steatosis. These changes were seen in relation to decreases in gut microbiota diversity. An increase in Proteobacteria and Actinobacteria has been seen, which are thought to produce endotoxemia and alterations in metabolism.
Diet is an important contributing factor in altering the gut microbiota of humans. It is the reason that consuming certain diets puts an individual at an increased risk of developing cancer, stroke, cardiovascular diseases, and many more. Red meat is thought to be significantly linked with changing gut microbiota mainly due to higher amounts of Neu5Gc and carnitine. Both of these change the microorganisms in the gut leading them to initiate an inflammatory response of atherosclerosis. Inflammation is the underlying cause of many medical conditions. And atherosclerosis is one of the leading causes of cardiovascular diseases. Studies are still ongoing to use these findings to use it for the prevention of developing diseases. However, by changing our dietary patterns, we can keep our gut microbiota healthy and use it to prevent illnesses. Vegan and vegetarian diets do not alter the gut flora negatively; that is why these diets are protective and beneficial for health compared to an animal-based diet.