60 second summary
- Kefir is a drink made by fermenting milk
- Lactobacillus is one bacteria thought to decrease cholesterol levels
- Gut microbes can ever alter cholesterol production by altering our DNA through the release of molecules called short chain fatty acids
What is kefir?
Kefir is a type of fermented cow’s milk drink with a slightly acidic flavor and thick consistency. Its unique texture is caused by kefiran which is an exopolysaccharide.
Exopolysaccharide- a large molecule made of sugar which are secreted by microbes into the extracellular environment.
Its health beneficial functions are driven by its ability to balance out communities of gut bacteria, a feature shared by other probiotic foods such as yoghurt and saukraut.
How is kefir made?
Commercially, kefir is made in bulk by fermenting milk using starter cultures of microbes which allows the products to be consistent but traditional kefir making requires a kefir grain. A kefir grain is made up of long sugar chains such as kefiran mentioned above. It provides a perfect environment for the proliferation of yeast cultures and bacteria which live in a symbiotic relationship in other words, a close physical association.
The main bacterial strains include Lactobacillus , Lactococcus ,Streptococcus and Leuconostoc, whilst the main yeast species are Saccharomyces, Kluyveromyces, and Candida.
One of kefir’s most studied properties is its effects on cardiovascular health in particular cholesterol levels.
Cholesterol – type of lipid and despite its often negative connotations, is an essential component of cell membranes and a precursor in production of molecules such as vitamin D.
Kefir and cholesterol synthesis modification
The science: Cholesterol can of course come from diet but 80% of it is produced in the liver and intestines. High cholesterol, also called hypercholesterolemia, is dangerous because it may form fatty deposits in arteries leading to them narrowing and potentially becoming obstructed. In experiments with animals, fermented milk products have shown to decrease serum cholesterol levels by 41-84% , decrease its production by the liver, as well as increase its excretion as bile, a dark yellow fluid.
Serum cholesterol- measurement of blood cholesterol levels
Which bacteria in kefir is thought to affect cholesterol levels?
On of the main bacterial strain thought to be majorly responsible for the altered biological mechanisms is Lactobacillus.
Lactobacillus is a type of probiotic which resides in areas of the digestive tract like the small intestine where it affects the metabolism of bile acids and cholesterol.
The science– In the intestine Lactobacillus is able to produce enzymes which deconjugate bile acids, increasing the rate by which they are excreted from the body.
Deconjugation- a reaction disrupting a system of conjugated double bonds
Cholesterol itself is made of bile acids and to replace the bile which was excreted, it converts itself to the acids thereby reducing its overall concentration.
Other excreted products also include propionic acid which decrease cholesterol synthesis by inhibiting one of the steps in the biosynthesis pathway. 
How do bacteria affect cholesterol metabolism on a genetic level?
Molecules released by bacteria in kefir are able to modify the production and degradation of chemicals our body produces naturally.
The science– In one study, the Niemann-Pick C1-like 1 (NPC1L1) gene, a crucial gene in absorption of cholesterol was shown to be downregulated where rats were fed bacteria present in kefir. Gut bacteria can modify production of proteins in our body through release of molecules called short chain fatty acids.
Overall, kefir is a simple and effective dietary component which acts as a probiotic enhancing your gut microbial flora. Interestingly, it is even suitable for those who are sensitive to dairy products because despite being based on milk, the lactose in it is consumed by the culture releasing acids and a pleasant tangy flavour.  It should be available to buy in regular supermarkets or in European food stores. Enjoy!
 PMC. (2016). The Microbiota and Health Promoting Characteristics of the Fermented Beverage Kefir. [online] Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4854945/ [Accessed 9 Jul. 2019].
 PLOS ONE. (2018). Cholesterol-lowering effect of Lactobacillus rhamnosus BFE5264 and its influence on the gut microbiome and propionate level in a murine model. Available at: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0203150 [Accessed 9 Jul. 2019].
 Frontiers in Microbiology. (2016). The Microbiota and Health Promoting Characteristics of the Fermented Beverage Kefir. Available at: https://www.frontiersin.org/articles/10.3389/fmicb.2016.00647/full [Accessed 9 Jul. 2019].
 PMC. (2012). Cholesterol-Lowering Probiotics as Potential Biotherapeutics for Metabolic Diseases. [online] Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3352670/ [Accessed 9 Jul. 2019].
Cultures for health. (2019). Reducing The Lactose Content Of Kefir. [online] Available at: https://www.culturesforhealth.com/learn/milk-kefir/reducing-lactose-content-kefir/ [Accessed 9 Jul. 2019].