The answer lies in our shared evolutionary history with bugs, lots of them. These are the bugs who share our bodies with us. Indeed, for every 1 of our big human cells, roughly 100 to 1000 little bugs live alongside and inside of us. They number in the tens of trillions, with approximately 1 million of these microbes living within every square centimeter of our skin. They contribute to our good health as well as to our sickness. As our species and theirs evolved, we established some rules to govern our cohabitation, and most of the time everything works out fine; however, like an unpredictable roommate, these bugs can turn against us, and their impact can be profound.
The most important thing to appreciate is that they usually do very good things for us. They help digest our food and extract nutrients from vegetables and fruits. They constantly wage a silent and deadly war with invading bugs that might do harm; sometimes they lose this battle and then we suffer the consequences. Because our species and theirs co-evolved we need them; we cannot live without them. In addition, we need them to be in harmony with each other and with our big human cells. When we induce an imbalance in their numbers we inadvertently disturb their ability to protect and feed us. Scientists are beginning to understand how a shift in the mixture of these microbes (for example there are hundreds of different types of bugs that live in our intestines) can lead to poor health, including heart disease, inflammatory bowel diseases, high cholesterol, obesity and cancer. The balance of gut bacteria is also thought to influence behavior and the presence of certain mental disorders, such as depression.
At last month's microbiology meeting in New Orleans many scientists discussed their concerns about the increasingly bacteria-free environment that we are encasing ourselves and our children within. The concern is that we are causing an imbalance in the mixture of bugs in our environment as well as with us. The challenge is in knowing which bugs to champion and which to destroy. Sometimes, we get the mixture wrong and the consequences can take many years to become obvious. For example, Helicobacter pylori can cause ulcers and stomach cancer; its prevalence in Western societies is decreasing and fewer people suffer from ulcers or stomach cancer today because of the intervention of medical science. Unfortunately, H. pylori appears to have been our friend as well. The loss of this bacterium has led to an increased incidence of gastroesophageal reflux disease (GERD) and esophageal cancers. Even worse, the absence of this bacterium leads to more ghrelin being released from the stomach - the consequence is increased eating and more obesity.
Interfering with the balance of bugs in our bodies makes it more likely to develop childhood asthma and food allergies. As a society we bathe too often, we breast feed too seldom, we take too many antibiotics, families are smaller and cesarean sections are too frequent; all of these contribute to an imbalance in the mixture of bugs within ourselves and our children. There are many thoughts on how to approach this problem; however, one should be the choice of last resort - fecal transplants. Yes, transplanting the fecal matter from a healthy person into the colon of a sick person has been shown to cure infection from invading bacteria. If you ever needed a reason to keep your bacteria happy, avoidance of this treatment would be it!
© Gary L. Wenk, Ph.D. Author of Your Brain on Food (oxford, 2010)