Let’s face it. We don’t hold the contents of our intestine in high regard. They are unsightly, malodorous, and when not disposed of appropriately in areas with high concentrations of people, they can lead to disease. The less time spent in their presence, the better. In fact, the whole philosophy behind toilets seems to revolve around giving us the power to immediately make our droppings disappear with the flip of a handle. Our disgust with what comes out of the business end of our intestine is even reflected in our language where we have a large number of epithets to equate worthless objects or persons we find to be truly despicable with, well…excrement. All this negative focus is a great shame as the lowly turd is a vital part of the exams that clinicians perform to diagnose a host of diseases, as was pointed out (with some exaggeration) in the famous “Poo Song” in the television series “Scrubs”.
However, our dung has very important functions in both health and disease beyond serving as mere diagnostics. With some hindsight this concept seems obvious, considering that the number of bacteria in our bowels is the same as the number of cells in our bodies, and that all these bacteria and other microorganisms actively metabolize foodstuffs in close proximity to the lining of our intestines. But, only recently have scientists begun performing in depth studies of the functions of what is formally called the “intestinal microbiome”. What they are discovering is amazing.
For example, the US and other industrialized nations are currently dealing with an obesity epidemic, and scientists have found that the intestinal microbiome plays a role. As it turns out the bacterial makeup of the intestinal contents of obese and non-obese people is different. Obese people seem to have bacteria that promote obesity! Scientists working with germ-free mice have found that these animals are resistant to obesity caused by a high-fat diet. Furthermore, by recolonizing these mice with specific strains of bacteria, scientists have found that not only do some bacteria promote obesity, but others protect against the effect of the obesogenic bacteria. The mechanisms by which this happens are not yet clear but gut bacteria may modulate the levels of satiety hormones released from the intestine or may affect the immune response and the physiology of adipose tissue by means of bacterial components that leak through the lining of the intestine into the blood. This suggests that we can reduce a person’s propensity for obesity by populating their intestinal tract with the right type of bacteria.
Another finding from studies with germ-free animals is that these animals have changes in their behavior compared to animals with an intact intestinal microbiome. That’s right: the levels of several brain molecules that regulate mood and cognition can be affected by the makeup of the bacterial content of the gut! In fact, in humans there are various conditions involving changes in gut bacteria such as irritable bowel syndrome that are accompanied by feelings of anxiety and depression, and certain psychiatric conditions are also believed to be affected by the makeup of the bacteria of the gut. This again suggests that if we find the right bacterial combination to introduce into the gut, we may be able to positively regulate brain function.
But the effect of gut bacteria doesn’t stop here. Gut bacteria also are modified in people suffering from several maladies such as cardiovascular disease, diabetes, cancer, and others. Gut bacteria may even hinder or enhance the effects of certain drugs. The influence of the intestinal microbiome on the human body has led some scientists to claim that it should be considered an independent organ just like the liver or the pancreas.
Unfortunately the intestinal microbiome is a fiendishly complicated association of thousands of strains of bacteria and other microorganisms interacting with one another and with the cells of the intestine, and the specific bacterial makeup of the microbiome varies from one person to another and can change with diet. Despite all the claims made by pre- and pro-biotic companies we still do not have a widely applicable way of modifying the intestinal microbiome to achieve specific effects on human health, but this is an active area of investigation.
Photo of Escherichia Coli bacteria from the Rocky Mountain Laboratories, NIAID, NIH, is the public domain.