Probiotics: Beyond Digestive Health

Once regarded mainly as a key to digestive balance, the gut microbiome is now recognized as a powerful influencer of overall human health. Mounting evidence suggests that its reach extends well beyond the digestive tract—affecting everything from mood and metabolism to heart, immune, and skin health. This growing understanding has prompted scientists and product developers alike to explore how probiotics—live microorganisms such as beneficial bacteria and yeasts—can be used to shape the gut microbiome for better health outcomes. Though research is still in its early stages, studies are revealing intriguing connections between probiotic supplementation and improvements across multiple systems of the body, signaling a new frontier in the science of wellness.

Gut–Brain Axis Potential

One of the most widely recognized ways the microbiome influences areas of human health outside of digestion comes in the form of the gut–brain axis. This is due to the gut and brain being linked directly via the vagus nerve, as well as through the enteric nervous system. They are also connected through common products, as the microbiome functions to synthesize neurotransmitters such as GABA and serotonin. Specifically, serotonin is of particular interest, as it plays a major role in mood regulation, among other functions. However, it is important to note that these molecules cannot directly cross the blood–brain barrier. Instead, they indirectly affect brain function via the enteric nervous system, which is reached by passing through the intestinal mucosal layer.

In comprehensively reviewed clinical trials over the past decade that explored the effect probiotics have on anxiety and depression, probiotics act on serotonin pathways, producing a notable change in levels produced (Merkouris et al. 2024). Despite inconsistencies among studies, two-thirds of relevant studies produced statistically significant improvement in depressive and/or anxiety symptoms, supporting the theory of probiotics as a potential option to alleviate depressive and anxiety symptoms. Notably, studies showed higher rates of relief from symptoms in patients with milder forms of anxiety or depression. These findings are promising, especially given the current landscape of mental health and the constant search for alternative options. With this, the study touches on what is a common theme regarding probiotic research: although evidence is promising, the wide range of studies provides no standardization in terms of which specific probiotic strains were used, and evidence remains insufficient to produce conclusive findings or guidelines for use.

Cardiovascular Impact

Cardiovascular disease (CVD) is a major public health concern, continuing to be the leading cause of death in the United States. While public health efforts combined with the rapidly advancing environment of medicine have made significant progress in recent years, novel therapies to reduce CVD are being explored, including the use of probiotics. In a review of human studies linking probiotic supplementation with positive cardiac outcomes, researchers found that probiotics positively impacted many cardiovascular functions such as blood pressure (Lopez-Yerena et al.). Hypertension is considered the strongest causality for CVD, and probiotics are known to enhance and protect the production of nitric oxide, a naturally occurring vasodilator that functions to reduce hypertension. Two studies found significant reductions in both systolic and diastolic blood pressures, while another three found no statistical significance. Still, it’s crucial to note these all utilize different probiotic strains, delivery methods, and time frames, highlighting potential gaps in research along with the many strains that were not used.

Much like hypertension, hyperlipidemia is another major risk factor for CVD, with low-density lipoprotein (LDL) cholesterol levels specifically being a useful indicator of a patient’s risk of CVD, along with high-density lipoprotein (HDL) cholesterol serving as an inverse marker for CVD. The same 2024 publication examined studies under the same parameters as they did for hypertension, and while some studies did not produce statistically significant changes in these levels, it’s also important to consider that each of the six studies utilized different strains. Even with this, the researchers noted a trend across the studies reviewed showing LDL levels decreased significantly with probiotic supplementation, along with HDL levels increasing. Although many more variables were measured and showed response, these two levels alone point to the potential use of probiotic supplementation as an intervention useful in preventing CVD.

Metabolism Potential

Positive health outcomes were associated with live microbe intake from foods, including fermented foods, assessed using the National Health and Nutrition Examination Survey database (Hill et al. 2023). These included blood pressure, blood lipid markers, and anthropometric markers. Methods to estimate intake of live microbes, including probiotics, were previously published (Marco et al. 2022). Intake of live microbes, including probiotics, is low across the United States population, and limited specific information on live microbe intake restricts the ability to generalize relationships from cross-sectional studies. It is hoped that having a database to estimate intake of live microbes will encourage additional studies on probiotic intake and health and disease outcomes.

Similarly to the topic of the gut microbiome, our understanding of human metabolism is rapidly evolving, partially due to the complex relationships and pathways that function in harmony to produce what we label as our metabolism. Metabolic disorders continue to be an ever-present issue, and public interest has increased as individuals search for ways to “control” their metabolism.

In a 2023 meta-analysis, researchers analyzed 26 previous studies that examined the effect probiotics have on several key markers of human metabolism (Noormuhammadi et al. 2023). These primarily included leptin and adiponectin, which are adipokines functioning as a hormone and cytokine produced by white adipose tissue. Leptin is produced in direct relation to the amount of adipocyte storage, working to balance long-term satiety but can be disrupted with obesity, leading to further obesity and related complications. Adiponectin, on the other hand, is directly related to insulin sensitization and anti-inflammation, and levels are inversely related to metabolism-related conditions such as obesity, type 2 diabetes mellitus (T2DM), and CVD.

The 2023 study found that probiotic supplementation across reviewed human trials produced a decrease in leptin, which was interestingly amplified in patients with nonalcoholic fatty liver disease, even more than in patients with obesity or T2DM. It also observed a trend of increasing levels of adiponectin with probiotic supplementation, which was strongest in patients with T2DM, prediabetes, and metabolic syndrome. These shifts in adipokine levels could prove to be crucial in combatting complex metabolic disorders and show the potential of probiotics for patients with metabolic disorders.

Immune System Health

Cancer rates continue to climb, and with this, the demand for effective cancer treatments. Although classic treatments such as chemotherapy or radiation have proved effective and will continue to be main treatments for cancers in the foreseeable future, adverse effects combined with their inefficacy to treat certain cancers have led researchers to explore other avenues of treatment. One such avenue is immunotherapy, which has transformed cancer treatment. Immunotherapy works not by directly attacking the cancer but by activating the patient’s own immune system and harnessing it to fight the cancer. Due to its effectiveness in treating various cancers, along with producing fewer adverse effects than other treatments, immunotherapy has rapidly become a favorite of oncologists in their arsenal of treatments to combat cancer.

With this success, attention has turned to finding ways to make this already effective treatment even more potent. In a 2024 study, scientists looked at microbiome-targeting approaches, including probiotic supplementation in relation to the gut microbiome and immunotherapy (Kang et al. 2024). They reviewed a retrospective study in Japan showing that the use of probiotics containing Bifidobacterium and C. butyricum was associated with better outcomes in patients with advanced or recurrent non-small cell lung cancer (NSCLC) undergoing immunotherapy. They also reviewed other Japanese studies focused on NSCLC patients, which showed that patients with a higher microbiome abundance of strains such as Lachnospiraceae, Alistipes, Agathobacter, Lactobacillus, and Clostridium had increased overall response, took longer to reach treatment failure, and produced longer progression-free periods and overall survival rates.

These studies examined the existing gut microbiomes of patients and indicate that probiotic supplementation potentially can be used to amplify beneficial bacteria in the microbiome and, therefore, responses to immunotherapy. However, it is crucial to consider that patients receiving chemotherapy treatment are labeled as immunocompromised, meaning they are more vulnerable to infections. While patients on immunotherapy are generally not considered immunocompromised, many patients have received chemotherapy in conjunction with immunotherapy. The most concerning potential adverse effect of immunotherapy alone is the risk of increased inflammation, particularly in the lungs or gastrointestinal tract, which could, in theory, be triggered by probiotic supplementation. These potential adverse effects pose a serious obstacle regarding trials using probiotic supplementation, and careful consideration is required to ensure the safety of patients during trials.

Skin Health

Research also has linked microbiomes to skin health. In 2024, a randomized clinical trial explored the effect of probiotic supplementation on acne vulgaris (Eguren et al. 2024). This trial was a double-blind, placebo-controlled study where, for 12 weeks, patients with acne vulgaris were given a capsule containing Lacticaseibacillus rhamnosus and the cyanobacterium Arthrospira platensis. After 12 weeks, there was a statistically significant reduction in noninflammatory acne vulgaris lesions in the probiotic group compared to the control group, along with a nearly statistically significant (0.06) reduction in total acne vulgaris lesions between the groups. There was also a reduction, though not statistically significant, in inflammatory acne vulgaris lesions. The trial was also well tolerated by all patients, with this in combination with results pointing to probiotics as a potential treatment for acne vulgaris, along with other skin conditions.

Probiotic Discovery

These studies highlight the exciting yet still-unfolding world of probiotics and their potential to beneficially modify the gut microbiome and, in turn, improve human health. A clear theme emerges across this body of research: while results are promising and offer a glimpse into the broad potential of probiotics, much remains to be discovered. Extensive research is still needed to determine which strains, delivery methods, dosages, and treatment durations are most effective and safe. Only through such work can standardized guidelines for practical application be developed. Even so, current evidence paints an encouraging picture of the many opportunities probiotics may offer to enhance health and well-being.ft