Americans are obsessed with their health. Whether it’s working out, eating a more balanced diet, taking supplements or trying the latest health craze, we spend a lot of time working towards creating healthier versions of ourselves. Fad diets, get-thin-quick exercise schemes and health and fitness programs are very popular. From Blue Apron to CrossFit, we’ll try anything that we hear about on TV to live a healthier lifestyle. However, we often forget to check if these fads are scientifically proven to provide health benefits, or if they are even healthy at all.

One of these fads I’ve been hearing a lot about lately is “probiotics.” But, do people even know what probiotics really are? Probiotics are supposed to regulate your digestive system and even cause long term health benefits. But, the questions are, how and why? I have been seeing a lot of commercials lately about probiotic supplements and yogurt that is supposed to regulate the digestive system. To be honest, I wasn’t even entirely sure what probiotics worked until I was required to do a presentation on them for a course I took entitled Applied Microbiology. I hope to impart my amateur wisdom on this topic with my readers as I relay my understanding of how probiotics are supposed to work, the benefits they may provide, and the scientific facts of their efficacy.

To begin, I first must answer the question: What are probiotics? Probiotics are living microorganisms that exert health benefits beyond basic nutrition upon ingestion. Probiotics are typically Bacteria, specifically lactic acid bacteria, or LAB for short. For all my microbiologist friends out there, LAB are Gram positive, aerotolerant anaerobes that ferment glucose, primarily into lactic acid. For all my non-microbiologist friends, this means that LAB have a thick cell wall, they do not use oxygen to produce energy (ATP), but can tolerate the presence of oxygen, and they use glucose to as an energy source, but convert it into lactic acid. LAB are the same type of Bacteria that are used to make yogurt. The production of acid prevents the growth of harmful microbes, which cannot tolerate the acidity. Don’t worry about LAB living in your food though; LAB are not known to cause illness in humans. (Don’t forget that not all Bacteria are pathogenic!)

LAB can be found living inside the human gut as part of the natural microbiota. As you may know, the human GI tract is home to many species of Bacteria that thrive inside this microbiome. There are over 400 species and concentrations range from 10⁴ Bacteria/mL of intestinal secretion in the stomach to 10¹¹ Bacteria/g of stool in the large intestine. Many of these Bacteria are LAB. Within the gut, Bacteria play a major role in human health. They can displace enteric pathogens, meaning they can occupy surface area of the human GI tract, allowing little to no space for pathogens to attach to the walls of the GI tract and grow. This means they can inhibit growth of common disease-causing bacteria such as Clostridium difficile, Campylobacter jejuni and Helicobacter pylori, just to name a few. Although the Bacteria living in your gut naturally inhibit the growth of enteric pathogens, it is thought additional Bacteria acquired through the intake of probiotics can enhance these effects, particularly in people who have a diminished microbiota due to various reasons (which I will discuss later).

As probiotics, LAB are thought to impart some other health benefits other than just displacing pathogens. First, they are thought to help with allergic reactions. The T_h2 pathway, which causes inflammatory reactions, is dependent on a cell-signaling protein called TNF-α. Inflammation of the intestine by pathogenic Bacteria can increase the concentration of TNF-α. However, as LAB compete with pathogenic Bacteria, inflammation can be reduced.

In addition, LAB are known to be capable of scavenging free radicals. Free radicals are atoms or molecules that have one or more unpaired electrons, which makes them very reactive, due to the principle that electrons “want” to be paired up. The most relevant free radicals to biological systems are reactive oxygen species (ROS), which are generated by normal aerobic reactions. The problem with ROS is that they are toxic to cells, since they are reactive enough to damage biological macromolecules such as lipids, proteins, and nucleic acids, which could lead to cancer and other chronic diseases. LAB are known to inhibit peroxidation of certain molecules, which could produce ROS. Certain species of LAB were found to enhance phagocytosis (a process where certain cells can ingest harmful or unnecessary cellular materials and break them down) of ROS. Some LAB can secrete enzymes important for the breakdown of ROS.

Not only can LAB probiotics provide potential benefits to healthy people, but they may be able to aid those with certain gastrointestinal disorders. In some instances, LAB were found to reduce the effects of lactose intolerance, a condition in which sufferers are unable to completely break down lactose, a sugar found in dairy products. Some probiotic LAB produced β-galactosidase, an enzyme which can break down lactose. Ulcerative Colitis, or UC, causes inflammation and ulcers in the colon. One possible cause of UC is an abnormal immune response that attacks not only the Bacterial invader, but the normal cells of the gut microbiome. However, the direct cause is unknown. Clinical studies have shown that lactic acid-producing probiotic Bacteria can decrease the pH of the colon. It is thought that this can create a more acidic environment unlivable for certain Bacteria thought to contribute to the effects of UC.

However, there is one major problem with LAB probiotics: can they survive and grow within the gut? It is very difficult for LAB from probiotic sources such as yogurt, kefir, or probiotic supplements to actually adhere to the mucous membrane of the digestive tract. Although some can survive in the GI tract, they don’t all survive, and those that do are unlikely to stick around. Instead, they provide all of these beneficial effects only as they are passing through the digestive system, but they pass completely through and exit the body in feces. Therefore, probiotics must be ingested continually to exert any type of long-term beneficial effects.