Your gut is home to large populations of good bacteria, which are live microorganisms that support your digestive health, immune system, mood, and much more. Imbalances in these microflora, whether caused by dietary choices, medication side effects, or infections can cause a wide range of troublesome symptoms. Probiotics can help put your gut, and your overall health, back on track.
Probiotics Support the Immune System
Your gut bacteria directly influence your immune system and your overall health, and probiotics help. 
Your digestive system contains the highest density of immune cells in the entire body. Probiotics can balance your microbiome  and encourage the growth of healthy bacteria in your gut , which leads to a healthy immune system response in your digestive system. [4, 5, 6]
Maintaining a balance of beneficial bacteria helps keep you healthy by supporting immune function. Rebalancing your good bacteria can help prevent infections, such as urinary tract infections , vaginal infections , oral infections , and symptoms like bloating . They have also been shown to decrease the incidence of viral illnesses  which may be useful during the current COVID-19 (coronavirus) pandemic.
Generally, probiotics are a simple, health-supportive strategy to maintain your immune health, with very few side effects.
Probiotics for Digestive Disorders
There is good evidence that probiotic supplements help treat and resolve many digestive disorders.
As you might imagine, probiotics have a significant effect on digestive health conditions. Many digestive health conditions are influenced or caused by imbalances between your good and bad bacteria, including SIBO, Irritable Bowel Syndrome (IBS), and Inflammatory Bowel Disease (IBD), including Crohn’s disease, and ulcerative colitis. This also applies to acute digestive ailments like infectious diarrhea from food poisoning or traveler’s illness.
Promote more rapid recovery from imbalances in your gut flora 
Fight pathogens (harmful bugs) and their toxins [13, 14, 15]
Reduce the gut inflammation that accompanies digestive disorders 
Probiotics may be worth trying to improve your depression and anxiety.
According to the Anxiety and Depression Association of America, millions of Americans experience either depression or anxiety. If this is you, you’re not alone. The good news is, if you’re depressed, probiotic supplements may help support you.
Two meta-analyses (the highest quality scientific data) found that probiotics improved depression, with no negative side effects. [22, 23]
Probiotics Improve Allergies and Food Sensitivities
Probiotics may help resolve or improve these food sensitivities and seasonal allergies.
Many people with digestive problems also report food allergies or food sensitivities to things like dairy products or gluten. There is some evidence that probiotics positively influence food sensitivities and allergies:
A systematic review of 15 clinical trials showed that probiotics improved dairy product tolerance. 
A meta-analysis of 22 randomized control trials concluded that probiotics can improve seasonal allergies. 
Other high-quality studies support this same finding. 
It’s thought by some that some food sensitivities are triggered by partially-digested food particles entering the bloodstream through a “leaky gut”. Because probiotics help repair and treat the underlying causes of leaky gut , this may explain their benefit.
If you are experiencing food sensitivities or seasonal allergies, probiotics may be worth a trial.
Probiotics Balance Sex Hormones and Thyroid
Probiotic supplements may help you with men’s and women’s sex hormone symptoms and thyroid conditions.
Women commonly experience symptoms related to their monthly hormone cycles, or when they go through menopause. Men also experience hormonal changes as they age, and may experience prostate trouble or sexual dysfunction. There is some evidence to suggest that probiotic supplements may help improve these symptoms.
Your gut bacteria play an important role in the breakdown of hormones and their removal from your blood. This is especially important for women because problems in this area can easily cause imbalances in estrogen, which can contribute to conditions such as PCOS (Polycystic Ovarian Syndrome), endometriosis, reproductive cancer, or cardiovascular disease. 
Women aren’t the only ones with hormone trouble. Men with unchecked inflammation and an unhealthy gut may experience prostate problems, low libido, low testosterone, and erectile difficulties. [30, 31, 32] Improving gut health may lead to improvement for these men.
Because there is a close relationship between your thyroid and gut health disorders like SIBO (Small Intestinal Bacterial Overgrowth), improving your gut health with probiotics may help improve thyroid function. [33, 34, 35]
Probiotics Improve Sleep
It’s been shown that digestive tract inflammation decreases sleep quality. 
According to the National Sleep Foundation, about 30% of Americans experience insomnia regularly, while another roughly 20% experience insomnia a few nights a week. Poor sleep is frustrating and can negatively affect your immune system and brain function. What’s maybe less intuitive is that sleep and gut health are a two-way street.
Consider these studies:
A randomized controlled trial showed that 4th year medical students who supplemented with a Lactobacillus probiotic showed improved sleep scores and sleep satisfaction. 
Another randomized controlled study showed sleep and fatigue improvements from a 6-week course of Lactobacillus and Bifidobacteria probiotics. 
Another randomized controlled study showed sleep improvements for IBS-D subjects who used a probiotic. 
One more study showed a small improvement in sleep quality for subjects using a probiotics compared to the placebo group. 
Though more studies need to be done, the data indicate that using probiotics may help improve your sleep quality and duration.
Probiotics May Improve Metabolic Health
Probiotics may help improve markers of cardiovascular disease, metabolic disease.
Evidence suggests that probiotics have modest benefits for type 2 diabetes, cardiovascular disease markers such as cholesterol and blood pressure. For example:
Two meta-analyses suggest that probiotic supplementation helps control lipids and blood sugar in type 2 diabetics. [41, 42]
Two meta-analyses suggest that probiotic supplementation significantly impacts total cholesterol levels. [43, 44]
One more meta-analysis suggests that probiotic supplements and foods may be valuable therapies for metabolic syndrome, but concluded they didn’t produce a great enough effect to be a stand-alone treatment. 
However, despite lots of market to the contrary, there isn’t very good evidence to suggest probiotics are useful for weight loss.
While probiotics can be helpful for metabolic health, it’s also important to modify your diet and lifestyle for best improvements.
Probiotic foods, while healthy, may not have enough probiotics to create a clinical effect. Many sources recommend using probiotic foods, such as kefir, miso, kimchi, and sauerkraut as a source of probiotics. Probiotic foods are healthy foods, but usually don’t have enough colony-forming units (CFUs) to have a clinical benefit for you and your digestive system. If you enjoy and tolerate them, feel free to include them in your diet. But if you want to use probiotics to support a health concern, I recommend using probiotic supplements with CFUs in the billions.
What About Prebiotics?
Use prebiotics with caution.
Prebiotics are fibrous starches that feed your probiotics. It’s important to feed your probiotics, but many people experience a worsening of their digestive symptoms, like bloating and gas, when they use them. Use prebiotics, like FOS (fructo-oligosaccharides), and GOS (galacto-oligosaccharides) with caution.
Whether you are a woman or man, young or old, healthy or have chronic conditions, using probiotics can help you feel your best and promote balanced health. Including a high-quality probiotic from each of the three categories can help you balance your digestion, hormones, sleep, food sensitivities, and even your metabolic health.
Hendijani F, Akbari V. Probiotic supplementation for management of cardiovascular risk factors in adults with type II diabetes: A systematic review and meta-analysis. Clin Nutr. 2018;37(2):532‐541. doi:10.1016/j.clnu.2017.02.015
He J, Zhang F, Han Y. Effect of probiotics on lipid profiles and blood pressure in patients with type 2 diabetes: A meta-analysis of RCTs. Medicine (Baltimore). 2017;96(51):e9166. doi:10.1097/MD.0000000000009166
Wang L, Guo MJ, Gao Q, et al. The effects of probiotics on total cholesterol: A meta-analysis of randomized controlled trials. Medicine (Baltimore). 2018;97(5):e9679. doi:10.1097/MD.0000000000009679
PLoS One. 2017 Jun 8;12(6):e0178868. doi: 10.1371/journal.pone.0178868. eCollection 2017.
Ann Nutr Metab. 2019;74(3):224-241. doi: 10.1159/000499028. Epub 2019 Mar 19.
Ansari JM, Colasacco C, Emmanouil E, Kohlhepp S, Harriott O. Strain-level diversity of commercial probiotic isolates of Bacillus, Lactobacillus, and Saccharomyces species illustrated by molecular identification and phenotypic profiling. PLoS One. 2019;14(3):e0213841. Published 2019 Mar 22. doi:10.1371/journal.pone.0213841
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