Most of us pay less attention to the many colonies of bacteria living in our intestinal tract than we do to the possibility that there may be colonies living on Mars. Recent research suggests it is time to do so. Our gut contains densely packed colonies of bacteria that affect not only digestion and diseases of the intestinal tract, but also our immune system and, according to some research, perhaps hunger, weight, mood, and athletic performance as well.
According to an article by Drs. Zhang and Yang, our intestinal tract contains more than 1000 or more bacterial species. These varieties of bacteria, of which we are usually unaware unless we have “tummy troubles,” function to assist in digestion, particularly of high fiber fruits and vegetables. They break down the chemical composition of fiber, thus transforming the indigestible carbohydrate to substances, as well as short chain fatty acids, which are used for energy. Our bacteria are also gatekeepers, helping the intestinal immune system deal with foreign antigens or proteins when they enter our body. Intestinal bacteria contain enzymes that make vitamin K, a critical component in the formation of blood clots. Intestinal bacteria also synthesize other vitamins: biotin, vitamin B12, folic acid, and thiamine.
That the bacterial flora can change is well known to anyone who has taken antibiotics for several days, and then confronted with less than optimal intestinal functioning. The antibiotic wipes out so-called healthy bacteria, and sometimes it takes several days or longer to restore normal functioning.
Zhang and Yang’s report asserts that diet also causes significant changes in gut bacteria. Consumption of a high-fat, high-sugar diet causes unhealthy bacteria to flourish. Conversely, diets low in those nutrients, but high in fiber, bring back a better class of bacteria. For somewhat obvious reasons, most of these studies are done on laboratory animals, since they require samples of intestinal bacteria found in the feces, and it is hard to find human volunteers for these studies.
Is it possible that our bacteria can affect our mood? Some scientists suggest that we can reduce anxiety and depression if we have the good kind of bacteria. This is based on evidence that intestinal bacteria make neurotransmitters, the chemicals that transmit messages in the brain. But it seems unlikely that our gut will control our mood since neurotransmitters made in the gut never get into the brain. (But interestingly, we do talk about our gut having feelings, i.e., our language is full of expressions that suggest our gut has moods: a feeling in my gut, gut response, gut reaction, etc…)
Intestinal microbes may affect the amount of ghrelin, a hormone that tells the brain whether we are hungry. But if so, no one has figured out what species of intestinal bacteria may do this—or whether they will make us feel so full we will eat less. Now athletes are allowing their intestinal bacteria to be analyzed to see if they differ from those of sedentary folk. According to an article in a recent issue of Outside magazine, some super-fit athletes do have varieties of bacteria not found in non-athletes. However, since they tend to follow extremely healthy, low-fat diets, is it their diets or their incredible athletic feats that change the bacteria? (Or, do the bacteria contribute to their athletic success?)
Much more research has to be carried out to show that intestinal bacteria are having a direct effect on hunger, athletic performance, or obesity before we can start manipulating our bacterial colonies to bring about certain desirable health effects. To be sure, there are some studies now gaining interest, that have tested the effects of fecal transplants in which bacteria from healthy volunteers may be transplanted to the intestines of those suffering from an intestinal disease like irritable bowel syndrome. These studies are showing promise in helping people whose intestinal disorders do not respond to conventional therapies.
In the meantime, while we are waiting for more science to support some of the claims that our gut bacteria can alter our heath for better or for worse, we are told to load our intestinal tract with good bacteria. Supposedly, these good bacteria can be eaten if we consume fermented foods such as sauerkraut (fermented cabbage), miso and tempeh (fermented soybeans), kimchi (a Korean dish of fermented cabbage with hot spices), kombucha tea (a fermented drink made with tea, sugar, bacteria, and yeast). and kefir (a fermented yogurt drink). These foods contain probiotics, or living bacteria, that when ingested populate our intestinal tract with good microbes. Pasteurization will kill the microbes, both good and bad, which is why many yogurts and canned sauerkraut are not on the list.
But there is a problem. Although scientists can identify many of the species of good intestinal bacteria, they are not listed on a package of tempeh or a bottle of kombucha. Moreover, how many bacteria are we actually eating? Probiotics may differ in their content of something called CFUs, or colony-forming units. CFUs describe the density of viable bacteria in a product. According to Dr. Shekhar K. Challa, a gastroenterologist who wrote Probiotics For Dummies, quantitating the CFUs of probiotics in most food products is almost impossible. CFUs are not listed under calories or any other place on the food label.
So will eating unpasteurized sauerkraut make enough good bacteria to make you thin (that is, if good bacteria will make you thin)? Probably not. But sauerkraut contains almost no calories, and chopping a cabbage, mixing it with salt and watching it turn into sauerkraut is something to do on a snowy afternoon. And after you eat it, its bacteria will have a happy home in you.
(“Effects of a high fat diet on intestinal microbiota and gastrointestinal diseases,” World Journal of Gastroenterology 2016, Oct 28; 22(40): 8905–8909) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5083795/