Kefir is a fermented drink traditionally made using cow, sheep or goat milk. It is made by adding so-called Kefir grains into milk, with a fermentation time of about 24 hours. Kefir grains look similar to cauliflower. They are are cultures of bacteria and yeast that ferment the sugar in milk (lactose) and thereby, turning the milk into kefir.
Traditional kefir was made in goatskin bags that were hung near a doorway; the bags would be knocked by anyone passing through to keep the milk and kefir grains well mixed.
There are many health benefits to fermented food. Early 20th centuries Nobel Prize winner Ilya Metchnikoff wrote in his work Prolongation of Life: Optimistic Studies that fermented food products have the potential to promote health and increase the lifespan. His theory was based on the observation that people living in areas where it was very common to consume fermented milk products were healthier and became older.
Nutrient Composition 500ml Kefir
Protein: 16 grams
Carbs: 20 grams
Fat: 18 grams
Calcium: 50% of RDI
Phosphorus: 50% of RDI
Vitamin B12: 40% of RDI
Vitamin B2: 50% of RDI
Additionally, a variety of bioactive compounds, including organic acids and peptides [1, 2]
The biggest health benefits Kefir has to offer come from the high number of probiotics. Kefir can contain up to 61 different strains of bacteria and yeast, depending on the origin of the kefir grains. 
As a side note, the high diversity of microorganisms makes kefir a more potent source of probiotics than yogurt.
Probiotics can influence our health in numerous ways including weight management, inflammation, mental health, and digestion. (Health Benefits of Probiotics)
Probably most scientific health benefits of probiotics also apply to kefir, as it is so densely packed with beneficial microbes. However, for the sake of accuracy, let’s focus on what science has shown us about kefir, and kefir only.
Scientific Health Benefits of Kefir
Researchers found that kefir has anti-inflammatory properties and that drinking Kefir can protect the gut cell wall through immunomodulation. [4, 5]
Another study found that supplementing an unhealthy diet with a Lactobacillus bacterium isolated from Kefir could significantly lower low-density lipoprotein, and triglycerides levels in rats, compared to those rats that consumed the unhealthy diet without supplementation. 
A recent review published in the journal Medical Oncology describes the anticancer properties of Kefir. However, most studies the researchers refer to are done on isolated cancer cells and therefore, more studies on animals or humans are needed to really draw conclusions here. 
If you are one of the million people with lactose intolerance, chances are that you are still able to consume Kefir, as the lactic acid bacteria in Kefir turn the lactose into lactic acid. Fermented milk products are lower in lactose compared to milk and they can also contain enzymes that support the breakdown of lactose. A study actually found that “Kefir improves lactose digestion and tolerance in adults with lactose maldigestion.” 
In animal studies, Kefir seems to have strong anti-allergic effects. A study compared the efficiency of Kefir with Zileuton, a commercially available drug used for the treatment of asthma, and found that Kefir possesses similar strong immunosuppressant capabilities. 
A research group from Brazil also found that Kefir has positive effects on blood sugar levels and reduces insulin resistance in rats. 
Osteoporosis is a disease where increased bone weakness increases the risk of a broken bone. Kefir made from full-fat dairy is not only a great source of calcium but also of vitamin K and a meta-analysis, which is a study that looks at many studies done about a particular topic, found that vitamin K supplementation decreases the risk of bone fractures. 
In fact, a recent animal study found that Kefir increases calcium uptake and decreases the turnover of bone. In addition to that, a randomized controlled trial on humans found that Kefir-fermented milk therapy was associated with short-term changes in bone turnover and greater 6-month among osteoporotic patients. [12, 13]
Take Home Message:
Kefir has the potential to improve health. Likely by the high variety of probiotics it provides.
Ahmed et al., Kefir and Health: A Contemporary Perspective, Crit Rev Food Sci Nutr. 2013
Machado de Oliveira Leite Microbiological, technological and therapeutic properties of kefir: a natural probiotic beverage
Bourrie et al., The Microbiota and Health Promoting Characteristics of the Fermented Beverage Kefir, Front Microbiol, 2016
Rodrigues et al., Anti-inflammatory properties of kefir and its polysaccharide extract, Inflammopharmacology, 2005
Vinderola et al., Effects of the oral administration of the exopolysaccharide produced by Lactobacillus kefiranofaciens on the gut mucosal immunity, Cytokine, 2006
Wang et al., Effects of Lactobacillus plantarum MA2 isolated from Tibet kefir on lipid metabolism and intestinal microflora of rats fed on high-cholesterol diet, Appl Microbiol Biotechnol, 2009
Sharifi et al., Kefir: a powerful probiotics with anticancer properties, Med Oncol. 2017
Hertzler and Clancy, Kefir improves lactose digestion and tolerance in adults with lactose maldigestion., J Am Diet Assoc, 2003
Lee et al., Anti-inflammatory and anti-allergic effects of kefir in a mouse asthma model, Immunobiology, 2007
Rosa et al., Kefir reduces insulin resistance and inflammatory cytokine expression in animal model of the metabolic syndrome, Food & Function, 2016
Cockayne et al., Vitamin K and the prevention of fractures: systematic review and meta-analysis of randomized controlled trials., Arch Intern Med, 2006
Chen et al., Kefir improves bone mass and microarchitecture in an ovariectomized rat model of postmenopausal osteoporosis, Osteoporos Int, 2015
Tu et al., Short-Term Effects of Kefir-Fermented Milk Consumption on Bone Mineral Density and Bone Metabolism in a Randomized Clinical Trial of Osteoporotic Patients, PLOS ONE, 2015