Gut Bacteria’s Far-Reaching Effects
Manipulating body fat, mood, and IBS by mixing up your gut bacteria
Non-human cells in and on our bodies outnumber our human cells ten to one. Each person has a unique microbiome, a particular combination and quantity of bacteria, fungi, viruses, and other microorganisms. Each of our gastrointestinal (GI) tracts hosts about 100 trillion microbial cells and 1,000 microbial species, collectively weighing about three to four pounds. More than 8,000,000 genes present in the microbiome also influence our own body’s 23,000 genes that control who we are. Studies increasingly demonstrate a relationship between an individual’s microbiome and such seemingly diverse health issues as obesity, fluctuating mood, and irritable bowel syndrome (IBS). We’re limiting this article to these three conditions, which have common underlying physiological conditions and are affected by gut bacteria.
Obesity and Microbiome Diversity
Based on research with twins, obese individuals appear to have smaller, less diverse bacterial populations in the GI tract compared to those who are not obese.1
The effects of the microbiome on susceptibility to obesity might start at birth. Babies born via the birth canal ingest more of their mother’s bacteria than those delivered through C-section, allowing their bodies to colonize a variety of beneficial bacteria to which C-section babies are not exposed. Early consumption of antibiotics, which often destroys both pathogenic and beneficial bacteria in the gut, could further undermine the developing GI tract’s ability to cultivate a diverse and healthy microbiome.
Some researchers are already developing new treatments. One recent study involved exposing babies delivered via C-section to their mothers’ vaginal fluid at birth in an effort to help them colonize a diverse combination of beneficial bacteria as early as possible.
In C. difficile infection (CDI), these pathogenic bacteria thrive in the GI tract, resulting in ongoing diarrhea and abdominal pain. Research into fecal transplants from healthy donors to individuals with difficult-to-treat CDIs is showing that this might be a breakthrough treatment. Fecal transplants work by re-establishing healthy populations of beneficial bacteria in the GI tract. With a similar goal, scientists are starting to conduct studies involving fecal transplants to help diversify the microbiome in obese individuals.
Not all Beneficial Bacteria are Equal
In a recent animal study, mice injected with bacteria from obese humans developed more body fat and a less diverse microbiome than mice injected with bacteria from lower-weight individuals.1 When researchers injected the higher-weight mice with a combination of 39 types of bacteria from the lower-weight mice, there was no effect; however, when they injected a combination containing 54 different types of bacteria from the lower-weight mice, the higher-weight mice resumed a healthy weight.
This study demonstrated a cause and effect relationship between gut bacteria and weight, and it suggests it might be possible to alter an individual’s microbiome. Scientists speculate that certain types of bacteria serve specific roles and that some are responsible for maintaining a healthy metabolism and body weight. For example, research shows that some beneficial bacteria help to suppress ghrelin, a hormone that makes us feel hungry.2
Future research will focus on further isolating those strains.
The Right Probiotics Plus the Right Diet
Diets high in processed foods are associated with less diverse gut bacteria. In the same animal study described above, when the higher- and lower-weight mice were kept together and ate the same balanced, high-fibre diet, the beneficial bacteria from the leaner mice colonized in the guts of the higher-weight mice, which then lost weight. However, when fed a diet of processed and low-fibre foods, the higher-weight mice were unable to colonize the beneficial bacteria they ingested from contact with the lower-weight mice. This suggests that probiotic supplementation alone might not be effective.
Bacteria in the GI tract metabolize insoluble fibre, which results in small chain fatty acids (SCFAs). SCFAs are associated with appetite regulation, gastrointestinal and cardiovascular health, and cancer prevention.2,3
Some SCFAs that enter the brain can have a psychotropic effect (similar to medications for psychological conditions).4
Fibre and bacteria must both be present in the GI tract in order to yield these beneficial SCFAs. Future treatments for obesity, related conditions such as type II diabetes, and even psychological conditions such as depression might incorporate a balanced, high-fibre diet along with specialized probiotic supplements.1
Psychobiotics for Depression and Anxiety
Decades ago, researchers observed that stressed mice had reduced levels of lactobacilli in their guts, a major group of beneficial bacteria.5 Studies are starting to show that colonizing beneficial bacteria in the gut can have a positive psychological effect. ‘Psychobiotics’ is a new term that describes probiotics used as a treatment for psychological conditions.6
In a recent study involving 55 healthy participants (no depression, etc.), individuals who took a daily probiotic supplement of L. helveticus and B. longum for one month experienced a significant reduction in psychological stress compared to participants taking a placebo.4
Early studies suggest that while most bacterial strains probably do not have an effect on mental health, emerging research will attempt to isolate the specific strains that do. It is important to note that this is all very early research and there are currently no tested psychobiotic formulations available on the market.
Harnessing the Gut-Brain Axis
Depression and anxiety disorders, as well as obesity, are associated with certain biomarkers in the brain. Animal studies show that a less diverse microbiome with fewer beneficial bacteria is associated with these same biomarkers. In studies on mice, altering gut bacteria has shown an effect on depressed and anxious behaviours as well as on the underlying biomarkers.
The endocrine system helps to regulate mood, appetite, and other functions by releasing hormones through glands, including those found in the GI tract. Major depression is associated with elevated levels of the hormone cortisol, released in response to stress. In animal studies, administration of L. helveticus and B. longum reduced cortisol levels.2
The complex immune system has developed ways to benefit from some bacterial strains while fighting pathogenic organisms. A balance between anti- and pro-inflammatory responses is a sign of health and is beneficial to our organs, including our brains. Underlying inflammation in the brain that researchers observe with depression and obesity is related to this immune response.7,8 A rich and balanced microbiome keeps these inflammation processes in check.
Depression is sometimes associated with a deficiency in the neurotransmitter GABA.6,7 Some bacteria in the gut secrete GABA and some produce other neurotransmitters related to mental health, such as norepinephrine, serotonin, and dopamine.
The next steps in this area are human studies that look at the specific microbiome of individuals who suffer from depression and anxiety conditions.8
Irritable Bowel Syndrome
Studies using probiotics as treatments for irritable bowel syndrome (IBS) – a functional gastrointestinal disorder involving recurring abdominal discomfort and bowel movement irregularities –are promising but inconsistent.6 Researchers are now looking deeper into this complex condition.
In a recent study, 16 patients with diarrhea-predominant IBS (IBS-D) had reduced microbiome diversity, decreased beneficial bacteria, and increased levels of pathogenic bacteria compared to 21 healthy control participants.9 Research has demonstrated a link between early life stress with both depression and IBS. Early life stress, at least in animal studies, is also associated with the development of a less diverse microbiome.
About half of IBS patents suffer from a psychological condition.5 Current research is looking at how probiotics could improve IBS symptoms by altering underlying physiological conditions related to depression and anxiety. In one small eight-week study involving 77 participants with IBS, B. infantis improved IBS symptoms over placebo, but L. salivarius did not.10
Treatment with B. infantis, in addition to improving physical symptoms, seemed to normalize these patients’ inflammatory immune response biomarkers.
Future Research
A continuing challenge to research in this area is the level of difference between microbiomes, even among individuals who grow up in the same community or family. Future studies will likely involve in-depth DNA sequencing of the microbiome in order to better understand these differences and develop more targeted treatments.