By Don Lincoln, Ph.D., University of Notre Dame
A slightly overweight person might be delighted to hear that they could soon bid goodbye to their daily jogging routine and binge on desserts of their choice. Of late, medical researchers have been working on the cause and effect of the gut microbiome on human metabolism. A number of health conditions from ulcers to obesity seem to be linked to the microbiomes inside the body.
In fact, the entire human body is a germy mess, meaning there are entire ecosystems of microorganisms living inside your body. Not just externally on the skin or in the mouth but even deep inside the digestive tract. Though it is a known fact that the immune system is constantly battling some of these microbes, what is intriguing is that there are some microorganisms in the gut that are crucial to maintaining good health.
The microbiome is the ecosystem of millions of microbes living in and on the human body. It consists of thousands of different microorganisms including bacteria, archaea, protists, fungi, and viruses. These microbiomes are found in the digestive tract as well, where they live, compete, and die for the resources inside the body. The ecosystems of these microorganisms affect the metabolism and immune system of the body in addition to regulating the hormones. However, the composition of the microbiomes varies from person to person, and this variation explains the different responses to the same food.
Sources of Microbiomes
A study in 2014 estimated that the human body housed roughly ten times more microbiome cells than the number of human cells. However, another study in 2016 suggested that there were 1.3 microbial cells for every human cell, with an uncertainty factor of about 25 percent. Though the ecosystem of microbiomes is unique to each human, this ratio is considered as a decent modern estimate.
The microbiomes that reside in our gut reach there from different sources. Some of them are picked up from the foods that we eat while others from our surroundings. But the first place we inherit them is from our mother at birth. Babies born through a vaginal delivery inherit more of the microbiomes than those born through cesarean sections.
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Ulcers and Gut Microbiomes
In earlier times, it was believed that ulcers were caused by stress, and they were treated using tranquilizers, antacids, mood elevators, or antidepressants. However, in 1984, a gastroenterologist Barry Marshall solved a medical mystery by experimentally infecting himself. He proved that a bacterium, helicobacter pylori, was the reason behind acute gastritis, which in turn caused ulcers. He used antibiotics to kill the bacteria and to treat the disease. Today, we know that approximately 75 percent of ulcers are caused by helicobacter pylori.
The existence of bacteria in the human gut has been known for at least 30 years now. But, it’s only recently that there has been some interesting research focusing on the gut microbiome.
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The Gut Microbiome Connection to Obesity
Obesity is one of the growing concerns of the western world, especially in America. A third of Americans are obese and two-thirds are overweight. It is a fact that obesity is associated with diabetes, stroke, heart disease, and the chances of premature death. In September 2013, Professor Jeffrey Gordon of Washington University in St. Louis and his colleagues spearheaded the research on understanding the human gut microbiome. They published their paper in the journal Science, the flagship journal of the American Association for the Advancement of Science. Their study on the microbiomes of heavy and skinny people revealed that the gut microbiome of lean people was more diverse compared to those of obese people. However, Gordon and his team needed more proof to correlate the microbiome and obesity.
So, they tried to establish that the correlation between the microbiome and obesity was true in the case of mice. Gordon and his team found that gut microbiota or intestinal microbes from human women, especially twins, could transfer characteristics of obesity or leanness to sterile mice. Researchers observed that all the sterile mice metabolized the same way when fed with the same amount of food. However, when the microbes from thin and obese women were transferred to these mice, their metabolic activity changed. The mice that were inoculated with microbes from obese women gained more weight compared to the ones that had received microbes from thin women.
Researchers also conducted genetic sequencing techniques on the poop of obese and thin people. The study established that obese individuals had more bacteria from the phylum firmicutes and nearly 90% less bacteroidetes than thin people. Also, when obese people lost weight, they had higher amounts of bacteroidetes but this was still much less than the bacteroidetes ejected by the naturally thin people.
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The Complicated Ecosystem of Microbiomes
Our understanding of digestion is that once chewed, the acids in the stomach work to break down the food, which is then split into nutrients that are absorbed by the small intestine. The larger particles that are not absorbed by the small intestine are sent to the large intestine. However, there is growing evidence to suggest that the process is much more complicated. There are microorganisms in the gut that are simultaneously breaking down more food than the teeth or stomach can. This means that two people eating an identical amount of food can absorb different amounts of nutrients from the food depending on the amount of bacteria in the gut. The ecosystem of these microorganisms in the gut depends on the type of food one consumes. For instance, a typical western diet, which is high on fat and low on fiber, creates a less congenial environment for many microbes and reduces the number of different species of gut microbes.
Thus, despite all the doubts and biases of medical publications, scientific literature suggests that the gut microbiome studies do look promising, and it can be safely assumed that they are past the stage of providing an accidental positive result.
Common Questions About the Gut Microbiome
Both these terms are often used interchangeably but there is a subtle difference. While microbiota refers to all microorganisms found in a specific environment, microbiomes are specific to each organism.
The idea of the existence of a microbiome ecosystem in the human gut was considered foolish as it was believed that the acid concentration in the stomach was sufficient to kill most of the microbes.
In the summer of 1984, Barry Marshall drank an infectious broth of lukewarm beef extract with some bacteria mixed in it. Three days later, he developed bad breath and started vomiting. A few days later he took antibiotics to kill the bacteria and eventually won the Nobel Prize in medicine 20 years later.
Microbiomes can enter your body when you shake hands, kiss, touch doorknobs, and many other ways.