At one time people believed that illness was caused by an imbalance in humors, but now it’s clear that a balanced immune system is a key to good health. It’s your immune system that vanquishes invaders, including bacteria, viruses, and other pathogens, and creates an unwelcome environment where they can’t thrive or survive; yet, it’s important that the immune response to pathogens be balanced. If the immune system goes overboard, it can attack healthy tissues and lead to tissue-damaging inflammation. You don’t want that since chronic inflammation is linked with almost every chronic health problem.
What you might not realize is 70% of your immune system lies in your gut. According to the Human Microbiome Project, the human gut contains trillions of bacteria. The species and their ratio vary from person to person. Some liken the composition of an individual’s gut microbiome to a fingerprint; no two microbiomes are exactly alike. Most of these bacteria are friendly, meaning they don’t cause health problems. In fact, healthy gut bacteria known as probiotics may have beneficial effects for human health. The totality of a person’s gut bacteria is called the gut microbiome, an ecosystem teeming with bacteria of various types.
Scientists now know that the gut microbiome can change in as little as a day if you alter your diet. Yet some factors cause long term damage. For example, if you disrupt your gut microbiome by taking antibiotics, it can take as long as a year for it to recover. Some experts even question whether complete recovery from long term antibiotic use ever occurs. Unfortunately, antibiotics don’t discriminate between the good and the bad guys as well as they should. They destroy healthy gut bacteria along with the ones that cause illness.
You might wonder how gut bacteria affect immune function. Probiotic bacteria digest components like fermentable fiber that humans can’t. When they do, they release short-chain fatty acids that are beneficial for the lining of the colon and for immune health. They also release neurotransmitters and biologically active compounds that influence immune function. Plus, probiotic bacteria interact directly with immune cells that line the outer layer of the intestines and activate immune cells called regulatory T-cells. They also help reinforce the intestinal barrier and slow the growth of harmful bacteria in the gut by competing for their resources.
It’s clear that the gut bacteria that make up the microbiome have a close relationship with the immune players that make up the gut immune system. One way to maintain a diverse gut microbiome is to eat a diet high in fiber. Although fiber is non-digestible by the human digestive tract, gut bacteria thrive on it. In response to a high fiber diet, the bacterial players that make up the gut reproduce freely and seed your gut with probiotic bacteria. Plus, fermented foods, such as yogurt, supply probiotic bacteria in food form.
Unfortunately, research shows people who eat a Western diet consume only half the recommended amount of fiber daily. Therefore, most people have a fiber shortfall! Fortunately, you can seed your gut directly with probiotic bacteria by eating fermented foods or taking a probiotic supplement. Supplements supply the gut with bacterial species scientists believe support optimal health. So, supplements provide a way to get probiotic bacteria if you don’t eat a lot of fermented foods and fall short on your fiber intake.
The Bottom Line
Now you know why healthy immunity starts in your gut and why the composition of your gut microbiome matters. Unlike bacteria that cause infection, these bacteria help support immune health and that’s important for all aspects of health.
“Gut Microbiome: The Peacekeepers” in Scientific American 312, 3, 88-96 (March 2015)
Curr Opin Immunol. 2011 Jun;23(3):353-60. doi: 10.1016/j.coi.2011.03.001. Epub 2011 Apr 3.
Front Cell Infect Microbiol. 2019; 9: 454.Published online 2020 Jan 15. doi:10.3389/fcimb.2019.00454.