Yogurt and Kefir

What it is

Yogurt is milk fermented by live bacteria; commercial yogurts typically contain Lactobacillus bulgaricus and Streptococcus thermophilus [1]. Kefir is a fermented milk drink made with kefir grains [2], and its cultures differ from those in yogurt [3]. In a survey of child-oriented cereals and yoghurts in the UK and Latin America, 33% of the yoghurts were classed as 'less healthy' under the UK nutrient profiling system [4]. Trials used up to 300 g a day of a synbiotic yogurt [5] and 600 ml a day of kefir [6].

How it might work

Proposed: the live bacteria in yogurt may digest lactose before it reaches the colon, because the bacterial lactase survives stomach acid and is active in the small intestine [1]. Proposed: the protective link may depend on the fermented product itself rather than on dairy in general, since one cohort found yogurt linked to lower diabetes risk while other dairy foods were not [7]. Proposed: fermented dairy may calm inflammation; a 9-week trial in 60 premenopausal women found lower inflammatory markers after low-fat yogurt compared with a non-dairy control [8].

What the research says

Yogurt and type 2 diabetes: three US cohorts found each daily serving of yogurt linked to a hazard ratio of 0.83 for type 2 diabetes, while total dairy was not associated [7]. A dose-response meta-analysis of 22 cohorts (579,832 people) found a relative risk of 0.86 at 80 g of yogurt a day, with no added benefit at higher intakes [9]. A Chinese cohort of 22,843 adults found a similar pattern, with a hazard ratio of 0.81 for the highest yogurt intake, although the confidence interval reached 1.00 [10]. Not every analysis agrees: in the EPIC-NL cohort, replacing one serving of whole milk with whole-fat yogurt was not linked to diabetes risk [11].

Lactose: in 15 adults with lactose malabsorption, breath hydrogen after a 20 g lactose load was 224 ppm x h after milk, compared with 76 after plain yogurt and 87 after plain kefir; all the fermented products reduced perceived flatulence by 54% to 71% [3]. In 24 lactose malabsorbers, live-culture yogurt produced less hydrogen and fewer complaints than milk, and yogurt with fewer live bacteria still did better than milk [12]. In 14 children, yogurt with active cultures caused fewer symptoms than milk containing the same lactose [13]. In a test of 850 healthy adults in Galicia, Spain, 13.7% had lactose malabsorption after 250 ml of milk and 3.8% after 250 ml of yogurt [14].

Kefir and blood sugar: across six small trials (323 people), kefir lowered fasting blood sugar and insulin compared with control [2]. The main finding is the fall in fasting blood sugar and insulin; the change in HbA1c (-0.64) was not statistically significant [2]. In a trial of 60 people with type 2 diabetes, probiotic kefir lowered HbA1c more than conventional fermented milk over 8 weeks [6].

Yogurt and gestational diabetes: a meta-analysis of four studies (533 participants) found probiotic yogurt linked to lower gestational diabetes (pooled OR 0.35, 95% CI 0.19 to 0.62) [15].

Bone: a 6-month pilot trial in 40 patients with osteoporosis found greater increases in hip bone density with kefir plus calcium than with calcium alone [16]. The kefir group also received calcium, so this trial does not isolate the effect of kefir. For fractures, three cohorts (102,819 people) pooled to a relative risk of 0.76 for hip fracture with the highest yogurt intake [17]. A 2025 review of six cohorts found no link with hip fracture (HR 1.01) and only a minimal bone density effect [18].

Blood pressure: in 44 adults with metabolic syndrome, 12 weeks of a synbiotic yogurt (probiotic bacteria plus yeast) lowered systolic blood pressure more than regular yogurt [5]. In a meta-analysis of seven trials (385 adults), kefir did not significantly change systolic (WMD -1.76 mmHg) or diastolic blood pressure (WMD -1.19 mmHg) [19].

Weight: ten prospective cohorts found yogurt eaters had lower weight gain and smaller waist changes in point estimates, though not all were statistically significant [20]. In a 10-week trial of 87 adults with metabolic syndrome on a calorie-restricted diet, fortified yogurt (with whey protein, calcium, vitamin D, prebiotic fibre and probiotics) led to a greater fall in body fat mass than plain low-fat yogurt [21]. In a 12-week trial of 89 women, probiotic yogurt and ordinary low-fat yogurt gave similar weight loss, although the probiotic group had larger drops in total cholesterol and insulin resistance [22].

Pregnancy and infant eczema: in a cohort of 2,114 infants followed to 6 months, mothers who ate yogurt in late pregnancy had infants with a lower risk of eczema between 3 and 6 months of age (adjusted RR 0.54) [23].

Doses used in studies

Yogurt trials used 300 g a day of a synbiotic yogurt for 12 weeks [5], 2 servings a day of fortified yogurt for 10 weeks [21], and 400 g a day for 16 weeks in a four-arm trial of milk and yogurt products [24]. Kefir trials used 600 ml a day for 8 weeks [6], and a 1,600 mg kefir dose alongside calcium for 6 months [16]. Lactose tests used 20 g of lactose in a meal with yogurt or kefir [3], and 250 ml servings of milk or yogurt [14]. These are study doses, not recommendations.

Safety and interactions

Side effects: few are recorded. The kefir review says safety was assessed in only 5 of its 18 publications, and kefir appears safe for generally healthy people [25]. In the lactose trials, flatulence was the symptom that changed; abdominal pain and diarrhoea were negligible [3].

Upper limits and toxic doses: no toxic dose was reported in the studies read, and the largest daily amounts tested were 400 g of yogurt [24] and 600 ml of kefir [6].

Medicine interactions: calcium supplements cut levothyroxine absorption by about 20 to 25% in a study of eight adults [26]. A review of levothyroxine and food found limited evidence on milk and calcium, and advises spacing thyroid medicine from food and calcium [27]. A review notes that dairy products can affect how some antiarrhythmic drugs are absorbed [28]. In one trial, a high-fat meal containing dairy taken 2 hours before the antibiotic omadacycline cut its absorption by 59% to 63% [29]. No study of yogurt or kefir with other antibiotics was found.

Cow's milk allergy: yogurt is a milk product. In a study of 132 children with cow's milk allergy who had a yogurt oral food challenge, 68.2% tolerated yogurt [30]. One 5-month-old infant developed vomiting and diarrhoea about 2 hours after eating yoghurt and was diagnosed with food protein-induced enterocolitis syndrome (FPIES) triggered by milk protein [31].

Weakened immune systems and raw milk: Listeria is a common contaminant of raw milk and can cause serious illness in pregnant women, newborns, older adults and people with weakened immune systems [32]. Mycobacterium bovis can spread through raw milk products [33]. In a lab study, fermenting milk with kefir grains reduced the survival of an M. bovis vaccine strain, and the authors say fermentation should be combined with boiling or pasteurisation [33]. Probiotic strains have caused bloodstream infections in intensive-care patients in case reports; the case report describes a commercial probiotic product, not yogurt, and the authors call for checking probiotic safety in immunocompromised people [34].

Added sugar: sweetened yogurts were the third-largest source of added sugar among 1,775 Spanish children aged 7 to 12, at 9.9% of intake [35]. In a UK and Latin America survey, a third of yoghurts were classed as 'less healthy' [4].

Pregnancy and breastfeeding: no safety trial of yogurt or kefir in pregnancy was found. One cohort linked yogurt in late pregnancy to less infant eczema [23]. A Mexican study of 965 women found no overall link between yogurt in pregnancy and preterm delivery [36].

Kidney, liver and bleeding disorders: none of the trials cited here was designed to test safety in people with kidney disease or bleeding disorders. A 16-week trial in 100 men with abdominal obesity found no difference in liver fat between yogurt and milk [24].

Caveats and open questions

Most of the diabetes and bone findings come from observational studies, which show association but not cause and effect [9] [18]. Well-designed randomised trials are still needed, especially in people with or at risk of osteoporosis [18].

Results conflict in places. The diabetes meta-analysis reports high heterogeneity [9], and the hip fracture findings point in opposite directions [17] [18]. In a prospective analysis of 4,545 high-risk older adults in PREDIMED, total yogurt was not significantly associated with a smaller waist, though whole-fat yogurt was associated with a small change [37]. A spirulina-enriched yogurt-like dessert did not lower blood pressure in an 8-week trial; it is not plain yogurt [38].

The trials are small. The kefir blood pressure meta-analysis included 385 adults [19], and the lactose trials included 14 to 24 people each [13] [12]. Many trials used probiotic or fortified yogurts, not plain yogurt, which makes them indirect for plain yogurt [21] [15].

Fermented dairy more broadly was linked to a 4% lower risk of stroke, ischaemic heart disease and cardiovascular death in cohorts, but this is fermented milk in general, not yogurt alone [39]. Fermented dairy was also linked to slightly lower hypertension risk, but the authors rated the certainty of that evidence low or very low [40].

A kefir review found very limited evidence for dyslipidaemia and hypertension and rated 12 of its 16 studies at high risk of bias [25]. Kefir may have potential against oral bacteria and H. pylori, but the review calls for better trials [25].

Funding: a systematic review of yoghurt and weight was funded by the Danone Institute, and one of its authors is employed by Danone [41]. A dairy and cardiometabolic review's authors have received grants from dairy industry bodies [42].