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A world of bacteria living in our bodies can make or break our health
Called one of the top scientific discoveries of the 21st century, the human microbiome—the collection of microorganisms living in and on our bodies—is as important to human health as the human genome. The microbes that inhabit the human body can determine if we are obese or lean, how often (and severely) we get sick, if we develop gastrointestinal diseases, and if we suffer from mental illness. And while these may seem like very different health conditions, they often occur together and likely share a common denominator—an inner ecosystem out of balance. As it turns out, we are only as healthy as our bacteria.
You may think of the human body as, well, human. Numerically speaking, though, we are only 10 percent human and 90 percent bacterial. 100 trillion bacterial cells call the human body home. 100 trillion! Bacterial cells outnumber human cells 10:1. It’s not a far stretch to say that the physical body is merely a habitat for bacteria—and emerging research is finding that human health largely depends on those bacteria.
Over the past seven years, researchers working for the Human Microbiome Project have identified and categorized thousands of species of bacteria that inhabit the bodies of healthy men and women. This has given them insight into what a healthy microbiome should look like—at least for a healthy Western population—and how changes in the microbiome are related to illness and disease. Among their discoveries, the researchers found that the microbiome may have a bigger influence on our health than our genes. And while our genes are fixed, the microbiota can be shaped.i
Researchers also discovered that every person’s microbiome is as unique as their fingerprints. It appears each one of us carries a unique and varied collection of bacteria that is shaped by genetics, how we were born (vaginal vs. Cesarean), how we were fed as infants, our eating habits, our environment, and medication use.ii iii
The largest concentration of bacteria is found in our guts, mostly in the small intestine and colon. In health, gut bacteria transport nutrients from food into our bodies, protect us from pathogens and toxins, produce neurotransmitters, certain vitamins, and anti-inflammatory compounds, and maintain the health of the epithelium that lines our digestive tracts. The health and diversity of those bacteria are what influences our body weight, how our bodies respond to pathogens, and the state of our mental health. When there is a shift toward an overgrowth of “bad” microbes and a loss of diversity, the health problems begin. An unhealthy gut microbiome has been linked to gastrointestinal diseases, obesity, and immune and brain dysfunction.
Scientists have found that obese people tend to have less microbial diversity in their guts compared to lean people,iv and it appears that gut bacteria play a key role in whether a person will be obese or lean. Researchers from Washington University, St. Louis, took gut bacteria from four sets of human twins that each included a lean twin and an obese twin, and introduced them into the guts of sterile mice. The mice given the bacteria from a lean twin stayed lean, while the mice given the bacteria from an obese twin quickly gained weight, even though all the mice were eating the same amount of food. What’s more, when the mice with the “obese bacteria” were exposed to the gut bacteria from the lean mice before they had a chance to gain weight, their gut bacteria morphed to match that of the lean mice—and they remained lean. However, when the researchers fed the mice a diet that resembled a typical human diet mimicking breakfast cereal and pizza, the transfer of “lean bacteria” did not occur and the mice with the obese bacteria gained weight.v
Earlier this year a study published in the British Journal of Nutrition found that daily supplementation of probiotics containing Lactobacillus rhamnosus helped women (but not men) lose weight. All of the subjects were put on a weight-loss diet, while half took the probiotic supplement and the other half took a placebo. Over the 12-week diet period, the women taking the probiotics lost twice as much weight as the placebo group. They also continued to lose weight during the 12-week maintenance period, in which they stopped the supplements. Researchers noted a lower concentration of bacteria related to obesity in the probiotic group. They speculate that the probiotics strengthen the integrity of the epithelial lining of the gut, preventing pro-inflammatory molecules from entering the bloodstream, preventing the “chain reaction” that leads to glucose intolerance, type-2 diabetes, and obesity.vi Other research has confirmed that increased gut microbe diversity is associated with improved insulin resistance, which is often a driving force of obesity.vii
While probiotics appear to play a critical role in maintaining a healthy weight, researchers are quick to point out that a healthy diet is also crucial. Our long-term dietary patterns are one of the factors that establish our microbiota and the Standard American Diet (SAD) is associated with a loss of microbial diversity in the gut.
When my daughter was born we lived in an old Victorian house that seemed to attract more dirt and dust than normal, with a shaggy dog that shed, a lot. As a new mom, the thought of my baby crawling among the dirt, dust, and dander gave me fits of anxiety. As it turns out, that dirt and dander were doing her immune system a lot of good. As soon as we are born, microbes begin colonizing our guts. How we are born, how we are fed as infants, and the environments we are exposed to determine what kind of bacteria set up house. And it seems that early exposure to dirt and bacteria of all sorts is essential for normal immune development.viii ix x xi It is important to establish a healthy gut microbiota early on, as seventy percent of our immune cells are located in the gut. The microbes that reside there communicate with immune cells throughout the body, ensuring that the immune system neither under-reacts nor overreacts.xii xiii Scientists have discovered that signals from our gut bacteria are necessary for an effective immune response to various viral and bacterial germs, and that “changes in the natural intestinal flora resulting from antibiotics, hygiene [i.e., overuse of antibacterial products], or lifestyle can have substantial consequences for the entire immune system.”xiv Because a healthy gut microbiota plays such an important role in immunity, it is no surprise that probiotics (specifically Lactobacillus rhamnosus, Lactobacillus acidophilus, and Bifidobacterium animalis) have been shown to reduce the duration and severity of cold and flu symptoms in children and adults. The key is to take them long-term to provide protection before you get sick.xv xvi
Have you ever had a “gut feeling” or experienced butterflies in your stomach? Science is proving there’s something to that. Gut bacteria produce neurochemicals (including serotonin, dopamine, norepinephrine, GABA, and melatonin) that the brain uses to regulate both physiological processes and neurological functions. Research is discovering that the gut has a profound effect on the brain, influencing neural development, brain chemistry, behavior, and how the brain responds to stress. The brain also influences the gut, with a number of studies showing that stress can change the microbial makeup in the gut, making us more susceptible to infection.xvii
The research is in its infancy and has mostly relied on animal studies, but some of the outcomes have been intriguing. A study led by gastroenterologist Premysl Bercik, MD of McMaster University, Ontario, swapped the gut bacteria of two groups of mice—one type that typically exhibits anxious, timid behavior and another type that is bold and “exploratory.” The timid mice that received gut bacteria from the “brave” mice began to explore more, while the bold mice that received gut bacteria from the “shy” mice became more hesitant and shy. Additionally, brain-derived neurotrophic factor (BDNF), a neurochemical that plays an important role in memory and mood, increased in the mice that received the “brave” gut bacteria.xviii Studies have also found that beneficial bacteria can reduce anxiety, depression, and stress. In one such study, a group of mice fed the probiotic Lactobacillus rhamnosus showed reduced signs of anxiety, depression, and stress compared to the control group. The group receiving the probiotic also had an increase in brain receptors for GABA, a calming neurotransmitter.xix How does this translate to human subjects? A study published in the British Journal of Nutrition in 2011 found that supplementation with Lactobacillus helveticus and Bifidobacteria longum led to decreased anxiety and depression in healthy human subjects.xx While the research is ongoing, it is clear that a healthy gut microbiome is critical for mental health.
Western society tends to be afraid of bacteria, and while we have succeeded in sterilizing our environments, we have unwittingly disrupted our microbiota. Additionally, the overuse of antibiotics has possibly created an epidemic of unhealthy guts, which may be driving the dramatic increases in obesity, diabetes, allergies and asthma, and gastrointestinal disorders. And it appears that the more courses of antibiotics a person takes, the harder it is for their microbiota to bounce back.xxi At the end of 2012, a study was published showing that the diversity of gut bacteria in infants who received antibiotic treatment remained diminished eight weeks after treatment, and disease-causing Proteobacteria were the dominant bacteria. Researchers speculate that this sets the stage for long-term poor health.xxii The typical American diet, often lacking the fiber-rich fruit and vegetables that our bacteria thrive on, isn’t doing us any good either.
It’s time for us to begin viewing the bacteria that share our bodies as friends, with a vested interest in our health. These bacteria influence our weight, our mental health, our immunity, and overall vitality. Ensure that yours has the right balance. Feed your gut bacteria with fiber-rich foods and maintain a healthy balance with probiotics—the good news is that much of the research has utilized probiotics that are commonly available. As this fascinating area of research continues to grow, be proactive now and give your gut the healthy bacteria it needs to do its job and keep you healthy!
ii Tao Ding, Patrick Schloss. “Dynamics and associations of microbial community types across the human body.” Nature. 15 May 2014:357-360
vi Université Laval. "Certain probiotics could help women lose weight, study finds." ScienceDaily, 28 Jan 2014. www.sciencedaily.com/releases/2014/01/140128103537.htm
viii Scudellari Megan. “Let Them Eat Dirt: Early exposure to microbes shapes the mammalian immune system by subduing inflammatory T cells.” The Scientist; March 22, 2012
x Johns Hopkins Medicine. "Newborns exposed to dirt, dander, germs may have lower allergy, asthma risk." ScienceDaily. ScienceDaily, 6 June 2014. www.sciencedaily.com/releases/2014/06/140606091157.htm
xi University of Gothenburg. "Allergies: Gut flora affects maturation of B cells in infants." ScienceDaily, 7 May 2012. www.sciencedaily.com/releases/2012/05/120507141146.htm
xiii Purchiaroni F, Tortora A, et. al. “The role of intestinal microbiota and the immune system.” Eur Rev Med Pharmacol Sci. 2013 Feb;17(3):323-33
xiv Albert-Ludwigs-Universität Freiburg. "Natural intestinal flora strengthen immune system.". ScienceDaily, 2 July 2012. www.sciencedaily.com/releases/2012/07/120702152940.htm
xv Gregory Leyer, PhD, Shuguang Li, MS, et al. “Probiotic Effects on cold and Influenza-Like Symptom Incidence and Duration in Children.” PEDIATRICS. Vol. 124 No. 2 Aug 1, 2009
xvi University of Medicine and Dentistry of New Jersey (UMDNJ). "Probiotics are secret weapon for fighting symptoms of the common cold in college students, study suggests." ScienceDaily, 22 October 2012. www.sciencedaily.com/releases/2012/10/121022162335.htm
xvii Carpenter, S. “That Gut Feeling.” American Psychological Association. Sep 2012;43(8):50
xviii Premysl Bercik, Emmanuel Denou, Josh Collins, et. al. “The Intestinal Microbiota Affect Central Levels of Brain-Derived Neurotropic Factor and Behavior in Mice.” Gastroenterology. Aug 2011;141(2):599-609
xix John Bienenstock, et. al. “Ingestion of Lactobacillus strain regulates emotional behavior and central GABA receptor expression in a mouse via the vagus nerve.” Proceedings of the National Academy of Sciences of the United States of America. Aug 29, 2011.
xx Michael Messaoudi, Robert Lalonde, Nicolas Violle, et. al. “Assessment of psychotropic-like properties of a probiotic formulation (Lactobacillus helveticus R0052 and Bifidobacterium longum R0175) in rats and human subjects.” British Journal of Nutrition. March 2011;105(5):755-764
xxi Les Dethlefsen and David Relman. “Incomplete recovery and individualized responses to the human distal gut microbiota to repeated antibiotic perturbation.” Proceedings of the National Academy of Sciences of the United States of America. Sept 16, 2010.
xxii Fiona Fouhy, Caitriona M. Guinane, et. al. “High-Throughput Sequencing Reveals the Incomplete, Short-Term Recovery of Infant Gut Microbiota following Parenteral Antibiotic Treatment with Ampicillin and Gentamicin.” Antimicrobial Agents and Chemotherapy. Nov 2012;56(11):5811-5820