It seems like probiotics are all the rage right now – they're taking over health food stores, social media, and even restaurant menus. But what exactly are probiotics, and what can they do for your health? Here's a round-up of the latest research.
What are probiotics?
We have millions of bacteria – some good, some bad – living in our colon, which is the very last portion of our gut (aka the large intestine). The combination of all these microorganisms is often referred to as our microbiome. But because they have so many beneficial health effects, probiotics are often referred to as "the good bacteria."Since most of our digestion happens in our small intestine (before the colon) much of what reaches probiotics are things like fiber and other compounds that we as humans are unable to digest. Bacteria have different equipment than we do, however, and are able to break down much of what we can't, making them an integral part of obtaining nutrients from the food we eat that we would have missed otherwise.
Why are probiotics helpful?
Ready for the full battery of health benefits currently known from consuming live active cultures? Buckle up.
Ease GI discomfort
It probably makes sense that, because probiotics live in the gut, they can heavily influence our digestive health. So if you have persistent belly discomfort, it might be a sign that your microbiome is top-loaded with unhealthy bacteria. Taking a daily probiotic supplement can help to relieve a number of symptoms, including gas, constipation, diarrhea, and bloating.1,2,3If you suffer from leaky gut syndrome, probiotics can help to regulate any issues in your intestinal lining, which is linked to a number of different ailments, like diabetes, chronic fatigue, arthritis, acne, and obesity. Some people are genetically predisposed to this, but it can also be catalyzed from lifestyle factors like diets high in sugars, processed foods, and alcohol.
Get your lipid biomarkers in check
The benefits from probiotics aren’t just limited to your belly, though. Fermented foods like kimchi, kochujang, and kefir can improved lipid biomarkers like LDL, HDL, and total cholesterol, as well as triglycerides.4,5,6,7,8,9 As a result of these changes, probiotics and fermented foods are associated with a lower risk of cardiovascular disease.The mechanisms for these changes aren’t clearly known yet, but research suggests healthy microorganisms in our gut may help to keep cholesterol from being absorbed in our intestines.7
Reduce glucose levels
Probiotic supplements have a clear connection to improved blood glucose control, both by helping to lower fasting blood glucose level and by improving insulin sensitivity.10 This is even the case in pre-diabetics and type 2 diabetics.11,12
Support immune system health and fight inflammation
Ever heard that ‘most of your immune system lives in your gut?’ That’s because the bacteria in your colon are an important line of defense against their pathogenic counterparts. So giving your microbiome a boost with probiotics or fermented foods is like sending extra troops to the battlefront. In fact, research shows having daily servings of fermented milk like kefir can lower your risk for the common cold.13,14,15Chronically-elevated inflammation levels can also weaken the immune system and leave you vulnerable to potential invaders. Luckily, probiotics can also help to lower inflammation markers like hsCRP.16
Help with weight loss and maintenance
There’s pretty well-established evidence that probiotics can help influence weight loss.8 But relatively new research shows that a healthy gut microbiome might also play a key role in keeping lost weight off.As we mentioned above, your diet can heavily influence the types of bacteria that colonize your gut. So if you eat lots of high-sugar foods, bacteria which thrive on sugar will begin to take over – it seems like this shift, if not reversed, might drive weight regain in some individuals.17 So if you feel like you crave sugar, or consider yourself someone with a sweet tooth, your gut microbiome might be to blame (or thank, depending on your outlook).
Reduce muscle soreness
One study found that drinking fermented milk both before and after resistance training reduced muscle soreness, seemingly due to an improvement in the body’s ability to efficiently use glucose for muscle repair.18No, the words “fermented milk” might not sound particularly appetizing, but kefir has a tang similar to Greek yogurt, and often comes in flavors like strawberry and blueberry. Try swapping out your typical pre- or post-workout supplements for a week or so and see if there are any noticeable changes in your recovery times.
Improve markers of liver function
Looking to lower your liver enzyme levels? Probiotic therapies have been connected to reduced levels of AST and ALT. This can be especially relevant in people with current or family history of liver disease.19
Modulate mood and cognition
The study of the connection between your gut and your brain is relatively new, but there appears to be a connection between your belly bacteria and things like mood and cognition. Certain strains of probiotics may also have anti-Alzheimer's effects.20 There still needs to be more research on the topic, but it’s a very promising field.
The bottom line
Like many health and nutrition topics, probiotics isn't one that can be easily and completely summarized. But countless studies have examined and confirmed their widely varying health benefits. 
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References
[1] Kim SE, Choi SC, Park KS, Park MI, Shin JE, Lee TH, Jung KW, Koo HS, Myung SJ.”Change of Fecal Flora and Effectiveness of the Short-term VSL#3 Probiotic Treatment in Patients With Functional Constipation.” Journal of Neurogastroenterology and Motility. 2015 Jan 1;21(1):111-20.
[2] Nobaek S, Johansson ML, Molin G, Ahrné S, Jeppsson B. “Alteration of intestinal microflora is associated with reduction in abdominal bloating and pain in patients with irritable bowel syndrome.” The American Journal of Gastroenterology. 2000 May;95(5):1231-8.
[3] Dimidi E, Christodoulides S, Fragkos KC, Scott SM, Whelan K. “The effect of probiotics on functional constipation in adults: a systematic review and meta-analysis of randomized controlled trials.” The American Journal of Clinical Nutrition. 2014 Oct;100(4):1075-84.
[4] Choi, I. H., Noh, J. S., Han, J. S., Kim, H. J., Han, E. S., & Song, Y. O. (2013). Kimchi, a fermented vegetable, improves serum lipid profiles in healthy young adults: randomized clinical trial. Journal of medicinal food, 16(3), 223-229.
[5] Fathi, Y., Ghodrati, N., Zibaeenezhad, M. J., & Faghih, S. (2017). Kefir drink causes a significant yet similar improvement in serum lipid profile, compared with low-fat milk, in a dairy-rich diet in overweight or obese premenopausal women: A randomized controlled trial. Journal of clinical lipidology, 11(1), 136-146.
[6] Lim, J. H., Jung, E. S., Choi, E. K., Jeong, D. Y., Jo, S. W., Jin, J. H., ... & Chae, S. W. (2015). Supplementation with Aspergillus oryzae-fermented kochujang lowers serum cholesterol in subjects with hyperlipidemia. Clinical nutrition, 34(3), 383-387.
[7] Andrade, S., & Borges, N. (2009). Effect of fermented milk containing Lactobacillus acidophilus and Bifidobacterium longum on plasma lipids of women with normal or moderately elevated cholesterol. Journal of dairy research, 76(4), 469-474.
[8] Mohammadi-Sartang, M., Bellissimo, N., de Zepetnek, J. T., Brett, N. R., Mazloomi, S. M., Fararouie, M., ... & Mazloom, Z. (2018). The effect of daily fortified yogurt consumption on weight loss in adults with metabolic syndrome: A 10-week randomized controlled trial. Nutrition, Metabolism and Cardiovascular Diseases, 28(6), 565-574.
[9] Kiessling, G., Schneider, J., & Jahreis, G. (2002). Long-term consumption of fermented dairy products over 6 months increases HDL cholesterol. European journal of clinical nutrition, 56(9), 843.
[10] Ruan, Y., Sun, J., He, J., Chen, F., Chen, R., & Chen, H. (2015). Effect of probiotics on glycemic control: a systematic review and meta-analysis of randomized, controlled trials. PloS one, 10(7), e0132121.
[11] Ejtahed, H. S., Mohtadi-Nia, J., Homayouni-Rad, A., Niafar, M., Asghari-Jafarabadi, M., & Mofid, V. (2012). Probiotic yogurt improves antioxidant status in type 2 diabetic patients. Nutrition, 28(5), 539-543.
[12] An, S. Y., Lee, M. S., Jeon, J. Y., Ha, E. S., Kim, T. H., Yoon, J. Y., ... & Han, S. J. (2013). Beneficial effects of fresh and fermented kimchi in prediabetic individuals. Annals of Nutrition and Metabolism, 63(1-2), 111-119.
[13] Shida, Kan, et al. "Daily intake of fermented milk with Lactobacillus casei strain Shirota reduces the incidence and duration of upper respiratory tract infections in healthy middle-aged office workers." European journal of nutrition 56.1 (2017): 45-53.
[14] Hao, Qiukui, Bi Rong Dong, and Taixiang Wu. "Probiotics for preventing acute upper respiratory tract infections." Cochrane Database of Systematic Reviews 2 (2015).
[15] Berggren, A., Ahrén, I. L., Larsson, N., & Önning, G. (2011). Randomised, double-blind and placebo-controlled study using new probiotic lactobacilli for strengthening the body immune defence against viral infections. European journal of nutrition, 50(3), 203-210.
[16] Mazidi, M., Rezaie, P., Ferns, G., & Vatanparast, H. (2017). Impact of probiotic administration on serum c-reactive protein concentrations: systematic review and meta-analysis of randomized control trials. Nutrients, 9(1), 20.
[17] Thaiss, C. A., Itav, S., Rothschild, D., Meijer, M. T., Levy, M., Moresi, C., ... & Dori-Bachash, M. (2016). Persistent microbiome alterations modulate the rate of post-dieting weight regain. Nature, 540(7634), 544.
[18] Iwasa, M., Aoi, W., Mune, K., Yamauchi, H., Furuta, K., Sasaki, S., ... & Sato, K. (2013). Fermented milk improves glucose metabolism in exercise-induced muscle damage in young healthy men. Nutrition journal, 12(1), 83.
[19] Ma, Y. Y., Li, L., Yu, C. H., Shen, Z., Chen, L. H., & Li, Y. M. (2013). Effects of probiotics on nonalcoholic fatty liver disease: a meta-analysis. World journal of gastroenterology: WJG, 19(40), 6911.
[20] Nimgampalle, M., & Kuna, Y. (2017). Anti-Alzheimer properties of probiotic, Lactobacillus plantarum MTCC 1325 in Alzheimer’s disease induced albino rats. Journal of clinical and diagnostic research: JCDR, 11(8), KC01.
