Kefir: Health Benefits, Nutrition, and How to Buy or Make It
Kefir is a fermented milk drink produced by inoculating milk with kefir grains (a symbiotic culture of bacteria and yeasts embedded in a polysaccharide matrix). The fermentation produces a slightly effervescent, tangy drink with a more complex microbial profile than yogurt: while most commercial yogurts contain 2 to 5 bacterial strains, traditionally made kefir contains 30 to 56 distinct microbial species. This microbial diversity has attracted significant research interest in the context of gut microbiome health, and the evidence base for kefir's probiotic effects is stronger than for most commercial probiotic products. The most robustly supported health effects of kefir are improved lactose digestion (the fermentation bacteria partially break down lactose), reduced gastrointestinal symptoms, and modest improvement in markers of gut microbiome diversity.
Nutritional Profile
Per 100ml of full-fat cow's milk kefir (approximately):
- Energy: 52 to 65 kcal
- Protein: 3.0 to 3.5g (complete protein, high in casein and whey)
- Fat: 3.0 to 3.5g (similar to whole milk; lower in low-fat versions)
- Carbohydrate: 3.5 to 4.5g (lower than milk due to partial fermentation of lactose)
- Calcium: approximately 120 to 130mg (approximately 12% of the RDA per 100ml)
- Vitamin K2 (menaquinone MK-4): present in varying amounts depending on fermentation duration; higher than regular milk due to bacterial synthesis during fermentation
- B vitamins: B12, riboflavin (B2), and folate are present; some strains synthesise additional B vitamins during fermentation
The Research on Kefir and Health
Gut Health and Microbiome
A 2021 study in Cell (Wastyk et al., Stanford University) directly compared a high-fermented-food diet (including kefir, kombucha, kimchi, and fermented vegetables) against a high-fibre diet in healthy adults. The fermented food group showed significant increases in gut microbiome diversity and reductions in 19 inflammatory proteins (including interleukin-17A, a marker of inflammatory conditions) compared to the high-fibre group, which did not increase microbiome diversity. Kefir was one of the primary fermented foods in the intervention group.
Earlier research: a 2017 systematic review in Nutrients (Rosa et al.) reviewed 13 randomised controlled trials on kefir consumption and found consistent evidence for improved gastrointestinal symptoms (bloating, bowel regularity) and reduced H. pylori infection markers in several trials. The evidence for specific clinical outcomes (IBS symptom reduction, Crohn's disease management) is limited by small trial sizes but directionally consistent.
Lactose Intolerance
People with lactose intolerance typically tolerate kefir better than equivalent quantities of regular milk. The fermentation process reduces the lactose content of milk by approximately 30% to 50% (bacteria consume lactose as a fermentation substrate), and the live bacteria in kefir continue to produce lactase in the gut, further assisting digestion. A 2003 study in the Journal of the American Dietetic Association (Hertzler and Clancy) found that milk kefir was significantly better tolerated than milk by lactose-intolerant subjects, with reduced hydrogen production (a marker of lactose malabsorption) and fewer symptoms.
Blood Glucose and Metabolic Effects
Several small trials have found modest improvements in fasting blood glucose and insulin sensitivity in participants consuming kefir regularly. A 2015 RCT in IUBMB Life (Ostadrahimi et al.) found that 600ml of low-fat kefir per day for 8 weeks reduced fasting blood glucose and HbA1c in type 2 diabetes patients compared to fermented conventional milk. The effect size was modest and the trial was small (n=60); the mechanism is thought to involve gut microbiome effects on glucose metabolism.
Kefir vs Yogurt: Key Differences
| Property | Kefir | Yogurt |
|---|---|---|
| Microbial diversity | 30 to 56 species (traditional grain-based) | 2 to 5 strains (commercial starter) |
| Consistency | Drinkable, pourable | Thick, spoonable |
| Lactose content | Lower (30 to 50% of milk) | Moderate (lower than milk but higher than kefir) |
| Yeast content | Yes (mild carbonation, trace alcohol) | No |
| Flavour | Tangy, slightly effervescent | Mildly sour to neutral |
| Protein | 3.0 to 3.5g/100ml | 3.5 to 10g/100g (depending on type) |
Buying and Making Kefir
Commercial kefir: Available in most UK supermarkets. Biotiful (available in Sainsbury's, Tesco, Waitrose) is the most widely distributed UK kefir brand, made from organic Jersey milk. Yeo Valley and Nourish Kefir are also commonly available. Commercial kefir contains live cultures but fewer distinct species than traditional grain-made kefir. Price: approximately £1.50 to £2.50 for 500ml.
Making kefir at home: Kefir grains can be purchased from online suppliers (Cultured Food Life, Kefir Grains UK) for approximately £5 to £15 per tablespoon (a sufficient starter). The process: add 1 tablespoon of grains to 250ml of whole milk in a glass jar. Cover with a cloth (not an airtight lid, as the fermentation produces CO2). Leave at room temperature (18 to 22°C) for 24 to 48 hours, stirring occasionally. Strain through a plastic sieve (not metal, which can damage the grains over time), bottle the kefir, and return the grains to fresh milk to continue fermenting. Cost per litre of homemade kefir: approximately 50 to 70 pence (cost of milk only, after the initial grain purchase).
Related: Fermented Foods Guide: Gut Health Benefits of Sauerkraut, Kimchi, and Miso | Greek Yogurt vs Regular Yogurt: Probiotic Content and Nutrition Compared