Kombucha tea, a fermented beverage, has recently become popular in the United States as part of the functional food movement. This popularity is likely driven by its touted health benefits, coupled with the recent scientific movement investigating the role of the microbiome on human health. The purpose of this systematic review is to describe the literature related to empirical health benefits of kombucha as identified from human subjects research.
In July 2018, we searched the term “kombucha” for all document types in the following databases across all available years: PubMed, Scopus, and Ovid. To identify federal research grants related to kombucha, we searched the National Institutes of Health Research Portfolio Online Reporting Tools. Finally, to identify ongoing human subjects research, we searched clinicaltrials.gov and clinicaltrialsregister.eu. We reviewed a total of 310 articles.
We found one study reporting the results of empirical research on kombucha in human subjects. We found no results for kombucha in Research Portfolio Online Reporting Tools, clinicaltrials.gov, or clinicaltrialsregister.eu.
The nonhuman subjects literature claims numerous health benefits of kombucha; it is critical that these assertions are tested in human clinical trials. Research opportunities are discussed.
Although fermented foods have been a staple of cultures internationally for thousands of years, kombucha has only recently become popular in the United States. Kombucha is reported to have originated in northeast China about 220 B.C., disseminated to Japan in 414 A.D. as a medicine, and spread through trade routes to Russia and eastern Europe , . Kombucha's worldwide popularity has fluctuated since World War II . Recognizing the growing market, in 2016, PepsiCo purchased KeVita, a popular functional probiotic and kombucha beverage maker. In 2017, retail sales of kombucha and other fermented beverages increased 37.4% , and in 2018, kombucha showed a +49% dollar growth over the past 52 weeks . Kombucha is reportedly the fastest growing product in the functional beverage market  and one of the most popular low-alcoholic fermented beverages in the world .
“Kombucha” is a beverage made by fermenting tea (generally black tea or sometimes green and oolong tea) and sugar, with a Symbiotic Culture Of Bacteria and Yeast (SCOBY) , generally for 7–10 days. The SCOBY is a biofilm of microorganisms resembling a mushroom cap, which becomes a starter for subsequent brews. The SCOBY comprises various acetic acid bacteria (e.g. Acetobacter xylinum , , Acetobacter aceti , , Acetobacter pasteurianus , , and Gluconobacter oxydans , ) and yeasts (e.g. Saccharomyces sp. , Zygosaccharomyces kombuchaensis , Torulopsis sp. , Pichia spp., , Brettanomyces sp. , and Zygosaccharomyces bailii , ). Several lactic acid bacteria have also been isolated . After fermentation, kombucha is a cocktail of chemical components , , including sugars; tea polyphenols; organic food acids; fiber; ethanol; amino acids including lysine; essential elements such as Cu, Fe, Mn, Ni, and Zn; water-soluble vitamins such as vitamin C, and several B vitamins; carbon dioxide; antibiotic substances; and hydrolytic enzymes .
Kombucha's popularity as a functional food ,  is driven by its purported health benefits, which include “multiple functional properties such as anti-inflammatory potential and antioxidant activity,”  and “the reduction of cholesterol levels and blood pressure, reduction of cancer propagation, the improvement of liver, the immune system, and gastrointestinal functions .” This popularity developed in parallel with the scientific movement investigating the role of the microbiome on health , , .