Home » Probiotics Part 2: Effectiveness and Unknowns

Probiotics Part 2: Effectiveness and Unknowns

Submitted by Amy Sercel MS RD CD

Edited by Marcia Bristow MS RDN CSSD CD

The idea that eating bacteria could actually improve your health might not seem logical, but that’s exactly the thought behind probiotics.  Probiotics are live bacteria that you get from food or take in pill form to gain some health benefit.1,2 The long list of diseases that probiotics might impact includes obesity, heart disease, irritable bowel syndrome, and cancer.3

Researchers believe probiotics work by changing your microbiome, or the 39 trillion bacterial cells that live in your body, to help “good” bacteria grow and prevent “bad” bacteria from suriving.4 The bacteria living in your large intestine break down un-digested food components, like fiber, to form products that will be used as food for other bacteria or will be absorbed by the cells of your large intestine.  It’s thought that different types of bacteria produce compounds that affect your health in different ways.  Some might be beneficial, such as improving your immune system or supporting healthy digestion, while others may negatively impact health by promoting inflammation or causing harmful bacteria to grow.1,5 It’s thought that consuming “good” bacteria in the form of probiotics will help more “good” bacteria grow and produce compounds that will benefit your overall health.

Unfortunately, the research on probiotics is still in its early stages. Your microbiome is unique; no one else has the same amounts or types of bacteria that you do.  Scientists have only classified about 14% of the bacteria they’ve identified so far, meaning there’s a lot more to learn about which bacteria are helpful, which bacteria are harmful, how much of each type of bacteria is best to include in a probiotic, and how different types of bacteria might interact to impact health.1

At the same time probiotics are only sold as supplements, which do not have to be approved for safety or proven effective.6 Additionally, the ingredients in supplements are not regulated; a supplement may contain ingredients that are completely different from those listed on its label.  One study found that a probiotic supplement contained smaller amounts and different strains of bacteria than listed on the label, which could lead to a potentially dangerous situation if the bacteria in the probiotic turned out to be harmful.7

Luckily, you can impact your microbiome through your food choices.  People who eat a variety of fiber-rich fruits, vegetables, whole grains, and nuts and have diets lower in cholesterol, saturated fat, and simple carbohydrates tend to have more beneficial bacteria living in their microbiomes.2 Changing your eating habits to include more fiber-rich foods can alter your microbiome within a few days.8,9 You can also choose fermented foods that already contain beneficial bacteria, such as yogurt, kefir, sauerkraut, kimchi, tempeh, or kombucha tea.  This way, you’ll get the nutritional benefits of these whole foods and won’t have to wonder whether you are getting the right amounts or types of bacteria to keep you healthy!


  1.  Hemarajata P, Versalovic J. Effects of probiotics on gut microbiota: mechanisms of intestinal immunomodulation and neuromodulation. Ther Adv Gastroenterol. 2013;6(1):39-51. doi:10.1177/1756283X12459294.
  2.  Ettinger G, MacDonald K, Reid G, Burton JP. The influence of the human microbiome and probiotics on cardiovascular health. Gut Microbes. 2014;5(6):719-728. doi:10.4161/19490976.2014.983775.
  3.  The Role of Prebiotics and Probiotics in Human Health. J Probiotics Health. 2013;1(2):1-8. doi:10.4172/2329-8901.1000108.
  4.  Sender R, Fuchs S, Milo R. Revised estimates for the number of human and bacteria cells in the body. bioRxiv. January 2016:36103. doi:10.1101/036103.
  5.  Shreiner AB, Kao JY, Young VB. The gut microbiome in health and in disease. Curr Opin Gastroenterol. 2015;31(1):69-75. doi:10.1097/MOG.0000000000000139.
  6.  Venugopalan V, Shriner KA, Wong-Beringer A. Regulatory Oversight and Safety of Probiotic Use. Emerg Infect Dis. 2010;16(11):1661-1665. doi:10.3201/eid1611.100574.
  7.  Probiotics: In Depth. NCCIH. https://nccih.nih.gov/health/probiotics/introduction.htm. Published November 21, 2011. Accessed May 30, 2017.
  8.  Turnbaugh PJ, Ridaura VK, Faith JJ, Rey FE, Knight R, Gordon JI. The Effect of Diet on the Human Gut Microbiome: A Metagenomic Analysis in Humanized Gnotobiotic Mice. Sci Transl Med. 2009;1(6):6ra14. doi:10.1126/scitranslmed.3000322.
  9.  David LA, Maurice CF, Carmody RN, et al. Diet rapidly and reproducibly alters the human gut microbiome. Nature. 2014;505(7484):559-563. doi:10.1038/nature12820.

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