This is Part II of my four-part series reviewing the key issues covered in Missing Microbes, How the Overuse of Antibiotics is Fueling our Modern Plagues, by Martin J. Blaser, MD. In Part I, I explained that the human body has co-evolved with slate of resident bacteria, collectively our microbiome, which performs a number of vital biological tasks for us. Our reliance on these bacteria can correctly be inferred from the noteworthy fact that approximately 99% of the DNA within our bodies is theirs, not ours. Over the past several decades, due to both an increasing number of babies delivered via C-section and a dramatic increase in the use of antibiotics, a growing number of people in the United States and around the world are now missing important species of bacteria, a condition I will refer to below as a low-diversity microbiome.
Before proceeding, I should make it clear that neither this column nor Dr. Blaser’s book are intended to reject the use of antibiotics, but rather to advise against their overuse. The invention of antibiotics is perhaps one of human kind’s greatest triumphs. It has been the key contributor to the dramatic reduction in the rate of deaths of children in the United States during their first year of life from one 1 in 4 in 1850 to 1 in 1,000 today. However, since the assumption has long been that there are no downsides to the use of antibiotics, we have gone overboard and caused some very serious problems. I will review Dr. Blaser’s suggestions on how to better use antibiotics in Part III next week.
A low diversity microbiome can result in harm through two modes of action. First, when an expected species of bacteria is missing, your body is deprived of whatever biological services that species provides. In addition, eliminating species of bacteria from your body often allows space and resources to be available either for other resident species to expand their populations beyond what the body is expecting, and/or for pathogenic species to gain a foothold. Below is a discussion of some of the health issues which can arise from these imbalances.
Obesity
The number of obese people – defined as having a body mass index (BMI) of greater than 30 – in the United States grew from 12% in 1990 to 30% in 2010! While an increase in calories in the average diet and a decrease in our average amount of exercise during this time period are contributing factors to this phenomenon, they are not sufficient to explain this dramatic rise in obesity on their own. A growing number of scientists, Dr. Blaser in particular, now believe that disrupted microbiomes are the missing link in explaining this obesity epidemic. While some details of the mechanism are still unclear, a key factor seems to be an over-representation of types of bacteria that can digest short-chain fatty acids, which otherwise would have left the body undigested. Therefore, people with this type of this microbiome will extract more calories and store more energy as fat from the same diet as someone with a high-diversity microbiome. Further, the fact that antibiotic use in humans results in weight gain should not be surprising to us. Farmers have long been “fattening up” their livestock by feeding them antibiotics.
Type I Diabetes
Type I Diabetes occurs in children when their immune systems attack the islet cells which produce insulin in their pancreases. The incidence of Type I Diabetes has been doubling every 20 years and the average age of diagnosis has dropped from 9 to 6 years old. This is a very challenging, life-long condition for these children.
Before proceeding, we need a quick lesson on the role of two classes of white blood cells in your immune system, known has helper T-cells and suppressor T-cells. Helper T-cells circulate through the blood stream “looking” for unfamiliar proteins. These unfamiliar proteins, since they could be from a virus, pathogenic bacteria, or other toxin, are a potential health risk. Your helper T-cells “tag” these unfamiliar proteins, which signals the rest of the immune system to attack. Suppressor T-cells moderate the actions of your immune system such that it does not go overboard and harm you while eliminating the suspected threat. Therefore, maintaining an appropriate balance of helper and suppressor T-cells is an important factor in maintaining good health.
The development of Type I Diabetes occurs in two steps. First, you must have a genetic predisposition such that proteins on the surface of your islet cells look “suspicious” to your helper T-cells. Second, your suppressor T-cells must fail in their job to moderate the response of your immune system after it has been summoned by your helper T-cells. In a person with a diverse microbiome, resident bacteria and suppressor T-cells exist in an equilibrium that has developed over the last couple hundred million years of evolution. When a person’s microbiome contains fewer species of bacteria, their body generates fewer suppressor T-cells. That is not the case with helper T-cells. People who generate fewer suppressor T-cells are unable to “calm” their immune systems and thus are far more susceptible to autoimmune diseases, type I diabetes and otherwise.
Food Allergies
Your likely impression that rates of food allergies in children have been rising dramatically is correct. In the United States, the rate of peanut allergies tripled from 1997 to 2008, and Celiac’s Disease, an allergy to wheat gluten, has quadrupled since 1950. The dynamics behind these increases are nearly identical to those described above for Type I diabetes. An allergic reaction occurs when the body considers a protein from things such as nuts, soy, eggs, milk, bee stings and the like, to be a potential threat and triggers the immune system to respond. Here again, if you have a less diverse microbiome and therefore have fewer suppressor T cells, you are vulnerable to being allergic to more things and more prone to having a an extreme, possibly life-threatening, reaction.
Others
If I were to describe all of the medical issues covered in Missing Microbes that arise from having a low-diversity microbiome, this column would be far too long. I recommend reading Dr. Blaser’s book to learn more about the relationship between your microbiome and asthma, eczema, heartburn/reflux, ulcerative colitis, Crone’s Disease, and early-onset of puberty in girls.
Conclusion
Despite the complexity and thoroughness of Missing Microbes, the key conclusions are not difficult to grasp. In Part I we discussed that humans co-evolved with their resident bacteria and along the way outsourced key biological functions to them. With that background in mind, we’ve now explored how killing off some species of our bacterial partners can harm our health. Next week in Part III, I will review Dr. Blaser’s recommendations for what we can do about these problems. As a bit of foreshadowing, it appears that we can address these problems without giving up the benefits that antibiotics have brought us. See you next week.
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