Inside the human intestinal tract live a vast collection of microorganisms, termed gut microbiota, comprising bacteria, archaea, and eukarya that have been estimated to number in the quadrillions. Hundreds of distinct bacterial species have been identified in the human gut alone, and together they play a critical role in keeping our immune and metabolic systems functioning properly.1

WHAT ARE PROBIOTICS?

The term probiotic refers to beneficial microorganisms (e.g., certain types of bacteria and yeast) that naturally occur in the human gut. Probiotics help combat harmful bacteria or pathogens in the body and offset any disruptions to system functioning bad bacteria may have caused. The Cleveland Clinic refers to this process as simply “keeping your body in neutral;”2 the scientific community refers to it as homeostasis.

Probiotics aid in digestion; demolish harmful, disease-causing cells; moderate inflammation; assist in the absorption of nutrients and medications; and help the body create certain vitamins.2

If there are not enough good bacteria in the body, the microbiome becomes unbalanced, which has negative consequences for the body—this is called dysbiosis. Dysbiosis can be categorized into three types: “1) loss of beneficial organisms, 2) excessive growth of potentially harmful organisms, and 3) loss of overall microbial diversity.”3 Gut dysbiosis can lead to unpleasant conditions, such as constipation or diarrhea (from either infection or as a side effect of medication), or more serious complications, such as inflammatory bowel disease, yeast infections, urinary tract infections, upper respiratory infections, gum disease, eczema, and even systemic diseases, such as obesity and Type 2 diabetes.4,5

Unhealthy dietary choices,6 overconsumption of alcohol,7 use of antibiotics,8 inactivity,9 smoking,10 lack of sleep,11 and stress12 all can negatively affect the growth and activity of our gut microbiome and cause dysbiosis. Probiotics can be obtained and increased through the ingestion of healthful foods and, possibly, supplements containing probiotics.

WHAT ARE PREBIOTICS?

The term prebiotic has been defined by the scientific community as “a selectively fermented ingredient that results in specific changes in the composition and/or activity of the gastrointestinal microbiota, thus conferring benefit(s) upon host health.”13 A broader, more simple definition provided by WebMD is “special plant fibers that help healthy bacteria grow in your gut.”14

When you consume foods high in fiber, some of the fiber, called soluble fiber, dissolves and forms a gel that aids the digestive process and helps regulate blood sugar and cholesterol; and some of the fiber, called insoluble fiber, passes through the gastrointestinal tract undigested, reaching the colon where it is fermented and utilized by the gut microbiota.15

Fiber is the basis of all prebiotics. Research has indicated that, “Although all prebiotics are fiber, not all fiber is prebiotic….”To be considered a prebiotic, a food ingredient must demonstrate the following: “1) Resist gastric acidity, hydrolysis by mammalian enzymes, and absorption in the upper gastrointestinal tract; 2) be fermented by the intestinal microflora; and selectively stimulate the growth and/or activity of intestinal bacteria potentially associated with health and well-being.”15 The prebiotics that come from fiber consumption can sustain and fuel the growth of probiotics, assisting these good bacteria in keeping the body balanced and healthy.15 This group effort is not only beneficial for the gastrointestinal tract but also for supporting optimal blood sugar, triglycerides, high-density lipoprotein (HDL/”good”) cholesterol, and blood pressure levels.16

SUPPORTING PROBIOTIC HEALTH

Diet. Generally, a diet rich in whole grains, fruits, legumes, nuts, and vegetables  preserve and increase probiotic activity in the body. (e.g., pickles, miso, tempeh, kombucha, sauerkraut) are also thought to encourage growth and beneficial activity of the gut microbiota. Because all prebiotics are fiber, most adults should try to consume around 40g18 of fiber daily.

The following foods are the highest ranked sources of fiber in the 2015–2020  Dietary Guidelines for Americans (though there are countless other sources of dietary 18 fiber):

• High-fiber bran (ready-to-eat) cereal, 1⁄2 to 3/4 cup, 9.1 to 14.3g fiber
• Navy beans, cooked, 1⁄2 cup, 9.6g fiber
• Small white beans, cooked, 1⁄2 cup, 9.3g fiber
• Yellow beans, cooked, 1⁄2 cup, 9.2g fiber • Shredded wheat ready-to-eat cereal (various) 1 to 11⁄4 cup, 5.0–9.0g fiber
• Cranberry (Roman) beans, Cooked 1⁄2 cup, 8.9g fiber
• Adzuki beans, cooked, 1⁄2 cup, 8.4g fiber
• French beans, cooked, 1⁄2 cup, 8.3g fiber
• Split peas, cooked, 1⁄2 cup, 8.1g fiber
• Chickpeas, canned, 1⁄2 cup, 8.1g fiber

Supplements. Probiotic supplements are generally thought to be safe for most people. A healthcare provider may recommend probiotic supplements to someone who simply wants to improve his or her overall health. Probiotic supplements might also be recommended to someone who is taking antibiotics to counteract the antibiotic’s ability to kill both good and bad bacteria in the body.2

In some cases, probiotic supplements might cause mild side effects, such as upset stomach, diarrhea, gas, or bloating the first couple of days after you start taking them. They may also trigger allergic reactions. Additionally, individuals with compromised immune systems or severe illnesses, those who have recently undergone surgery, and/ or those who are very young or very old are should not take probiotic supplements unless under direct supervision of a physician due to higher risk of developing infections. Before taking a probiotic supplement, consult with your healthcare provider to determine what will work best for your health needs and overall well-being.

SOURCES

1. Thursby E, Juge N. Introduction to the human gut microbiota. Biochem J. 2017; 474(11):1823–1836.
2. Cleveland Clinic site. Probiotics. Last reviewed 3 Mar 2020. https://my.clevelandclinic.org/health/articles/14598-probiotics. Accessed 30 Dec 2020.
3. DeGruttola AK, Low D, Mizoguchi A, Mizoguchi E. Current understanding of dysbiosis in disease in human and animal models. Inflamm Bowel Dis. 2016;22(5):1137–1150.
4. Bull MJ, Plummer NT. Part 1: the human gut microbiome in health and disease. Integr Med (Encinitas). 2014;13(6):17–22.
5. Jandhyala SM, Talukdar R, Subramanyam C, Vuyyuru H, Sasikala M, Nageshwar Reddy D. Role of the normal gut microbiota. World J Gastroenterol. 2015;21(29):8787–8803. 
6. Mills S, Stanton C, Lane JA, et al. Precision nutrition and the microbiome, part I: current state of the science. Nutrients. 2019;11(4): 923.
7. Bajaj JS. Alcohol, liver disease and the gut microbiota. Nat Rev Gastroenterol Hepatol. 2019 Apr;16(4):235–246. 
8. Langdon A, Crook N, Dantas G. The effects of antibiotics on the microbiome throughout development and alternative approaches for therapeutic modulation. Genome Med. 2016;8(1):39.
9. Bermon S, Petriz B, Kajėnienė A, Prestes J, Castell L, Franco OL. The microbiota: an exercise immunology perspective. Exerc Immunol Rev. 2015;21:70–79. 
10. Nos P, Domènech E. Management of Crohn’s disease in smokers: Is an alternative approach necessary?. World J Gastroenterol. 2011;17(31):3567–3574.
11. Smith RP, Easson C, Lyle SM, et al. Gut microbiome diversity is associated with sleep physiology in humans. PLoS One. 2019; 14(10): e0222394.
12. Madison A, Kiecolt-Glaser JK. Stress, depression, diet, and the gut microbiota: human–bacteria interactions at the core of psychoneuroimmunology and nutrition. Curr Opin Behav Sci. 2019; 28:105–110.
13. Gibson GR, Scott KP, Rastall RA, et al. Dietary prebiotics: current status and new definition. Food Sci Technol Bull Funct Foods. 2010;7:1–19.
14. WebMD site. Prebiotics. Last reviewed 14 Sep 2020. https://www.webmd.com/digestive-disorders/prebiotics-overview. Accessed 30 Dec 2020.
15. Slavin J. Fiber and prebiotics: mechanisms and health benefits. Nutrients. 2013;5(4):1417 –1435.  
16. Kocsis T, Molnár B, Németh D, et al. Probiotics have beneficial metabolic effects in patients with type 2 diabetes mellitus: a meta-analysis of randomized clinical trials. Sci Rep. 2020:10;11787.
17. Heiman ML, Greenway FL. A healthy gastrointestinal microbiome is dependent on dietary diversity. Mol Metab. 2016;5(5):317–320. 
18. US Department of Health and Human Services and US Department of Agriculture. 2015–2020 Dietary Guidelines for Americans. https://health.gov/our-work/food-nutrition/previous-dietary-guidelines/2015. Accessed 30 Dec 2020.