Intro to Gut Health: The Psychobiotic Revolution by John F. Cryan, Scott C. Anderson, and Ted Dinan
This week we will discuss the intro to the book “The Psychobiotic Revolution”. This book really dives into the gut flora and fauna and how to keep them happy and working properly to avoid disease, improve mental health, and to aid in better digestion.
Let’s talk about your gut microbiota. A single bacterium, given enough to eat, could multiply until its brethren reached the mass of Earth in just two days. That’s a big clue to their superpower: They are excellent at reproduction. They are also profligate interbreeders and think nothing of swapping genes with whoever is nearby. In your gut today, you are host to trillions of bacteria. They are online 24/7, duking it out with rogue microbes and even helping you nutritionally by producing vitamins and eking out the last few calories from every speck of fiber. The community of microbes living in your gut—your so-called microbiota—is not just made of bacteria. Your microbiota is also home to ancient life-forms related to the colorful creatures that tint hot springs, called Archaea. It includes the kings of fermentation, the yeasts. It hosts swimming single celled protozoans, constantly on the prowl. It also includes an even more insane number of viruses. Your gut microbiota is spectacularly cosmopolitan, making it a challenging beast to study. Your microbiota communicates directly with the second brain. Your second brain is the network of nerves surrounding your gut. A good set of microbes encourages this second brain to keep the feast moving. For good health, including mental health, the food you eat needs to be good for you and for your microbiota. In 2013, we defined a psychobiotic as a live organism that, when ingested in adequate amounts, produces a health benefit in patients suffering from psychiatric illness. As a class of probiotic, these bacteria are capable of producing and delivering neuroactive substances such as gamma-aminobutyric acid and serotonin, which act on the brain-gut axis. Preclinical evaluation in rodents suggests that certain psychobiotics possess antidepressant or anxiety-reducing activity. Your microbiota can make you feel better if you feed it what it wants, and it can make you feel miserable if you don’t. One way is through cravings. You may feel that you just have a personal penchant for certain candies, but it might not be up to you at all. Your cravings, it seems, might belong more to the second brain in your gut than the one in your head. Your microbiota can affect your mood as well. For example let’s look at food poisoning: your microbiota recognizes the pathogenic intruders and starts to attack them. It tries starving and poisoning them and —importantly—it alerts your immune system. Now kicked into high gear, your second brain prepares to purge your system. Your mood at that point is acute anxiety. Now imagine that happening day after day. That’s what occurs when you have chronic inflammation, often caused by a breach in your microbiotic defenses. Anxiety and depression can become a constant companion. The microbiota is renewed every hour or so. The turnover is huge, and you may be able to divert it with some surprisingly easy tricks. But the gut-brain connection is a two-way street. Despair, anxiety, and depression can lead to negative changes to your microbiota, called dysbiosis. That disruption can channel anxiety and depression right back into your brain. It creates what most of life tries its best to avoid: a positive feedback loop, otherwise known as a vicious cycle. Research continues to reveal connections between gut health and other diseases, both mental and physical. Depression accompanies many of these diseases, including Parkinson’s, Alzheimer’s, irritable bowel syndrome (IBS), inflammatory bowel disease (IBD), obesity, psoriasis, arthritis, multiple sclerosis (MS), autism, and many more. These diseases sometimes start with depression or anxiety—and sometimes they end with them. Your immune system can detect these bacterial molecules at very low levels. This is part of your innate immune system, and it doesn’t require any training. It just reacts, and it does so with alacrity. But you have another immune system that is more subtle and that requires training: Your adaptive immune system is deliriously complex. This adaptive system—working closely with your microbiota—can protect you against pathogens that it has never seen before. A microbiota that is unbalanced and that provokes an immune response is called dysbiotic. It can lead to inflammation, which is a significant contributor to depression and anxiety. Worse yet, it is a major predictor of mental decline, making dysbiosis important to everyone, regardless of mood. Depression is associated with brain atrophy. So your depression is not only setting you back today, but could have even worse long-term effects. The gut has its own nervous system, independent of the central nervous system. It isn’t organized in a lump like the brain but rather as an intricate double-layered lacework surrounding your entire gut like a tube sock. You have your own homegrown microbes that are friendly. These are your commensals, from Latin, meaning “together at the table.” If these friendly microbes also keep your mood on an even keel, they are called psychobiotics. Astonishingly, your microbes themselves can talk to both brains using chemicals similar to neurotransmitters (the communication molecules of your brain) and other molecules like hormones, fatty acids, metabolites, and cytokines. This is because the gut is a highly specialized tissue whose challenging job is to suck nutrition out of food without also sucking in pathogens. If pathogens leak through the lining and get out of your gut, your immune cells will follow them into your blood system and spark a systemic inflammation—a condition often called leaky gut. When your gut is out of whack you may get body signals like we explained in the food poisoning example which is called sickness behavior, and it has a lot in common with depression. When it comes to your digestive tract, most of it we cannot feel. You can move your tongue and swallow at the near end, and you can control your sphincter at the far end. Everything in between is pretty much out of your control. That’s one less thing you need to worry about, which is always welcome, but it eliminates a lot of important inside information. Without definitive cues from your gut, you often have to guess what’s wrong with you. If you just feel a general malaise, you may not place the source of your worries where it often belongs: in your gut. The microbiota, they found, plays an important role in the formation of stress circuits. But most antibiotics are broad-spectrum, killing a wide variety of microbes. As long as you think all bacteria are bad, that scorched-earth treatment sounds great. But it turns out that most bacteria are not bad. In fact, your good bacteria rival your own immune system when it comes to killing pathogens. Indiscriminate dosing can damage friendly microbes—and we’re only now realizing how great that damage can be. Yeasts are notorious for stepping in when commensal bacteria are knocked down. Candida is a yeast that loves the intestinal environment. Under siege by Candida, your gut may become poked with holes and start to leak food bits into your bloodstream. Your immune system may then attack the out-of-place food particles, in the process setting up food allergies, which are often linked to anxiety and depression. It is not uncommon to have allergies disappear when yeast infections are cleared up. In the lab, the researchers collected fecal samples from 34 depressed human patients and 33 healthy controls. They found that the microbiota of the patients with depression was less diverse than the controls. They then transferred these samples to rats. The rats that received fecal matter from depressed patients showed symptoms of depression and anxiety, while the controls did not. This suggested to us that the microbiota may play a causal role in the development of depression, and might present a target for treatment and prevention of this disorder. If your immune system is continuously on alert, you’ll develop chronic inflammation, which can lead to depression and anxiety.
