Greek yogurt and kefir: live cultures help with lactose, the rest is thin

The strongest number on this page belongs to kefir. A meta-analysis of 6 randomized trials with 323 people found it lowered fasting blood sugar by about 10 mg/dL (95% CI 4.0 to 16.5 lower), while HbA1c, the longer-term measure, did not change significantly. The trials were few, with different doses and lengths, and the abstract does not say who took part. The clearest everyday finding is about lactose. The EU authorizes the claim that live cultures in yogurt or fermented milk improve lactose digestion in people who digest lactose poorly, and kefir uses different cultures from yogurt. In a small crossover trial, 12 men with lactose malabsorption ate 18 oz (500 g) of yogurt a day for 15 days, and live yogurt gave less breath hydrogen than heated yogurt. UK tables list plain Greek style yogurt at 133 calories, 10.2 g fat and 3.5 g lactose per 3.5 oz (100 g). That British product is not necessarily strained like American Greek yogurt.

How much is actually in it

• UK tables list plain Greek style yogurt at 133 calories, 5.7 g protein, 10.2 g fat, 3.5 g lactose and 126 mg calcium per 3.5 oz (100 g). — food composition record, UK retail Evidence E source
  Per 100 g: energy 133 calories; protein 5.7 g; fat 10.2 g; lactose 3.5 g; calcium 126 mg

What your body absorbs

• In a randomized comparison, lactase-deficient adults absorbed calcium from 5.2 oz (147 g) of plain yogurt as well as from milk and at least as well as controls. — lactase-deficient subjects and controls; 270 mg Ca in 250 g milk or 147 g yogurt Evidence B source
  Equal calcium absorption from yogurt and milk; no advantage of yogurt
• The EU authorizes the claim that live cultures in yogurt or fermented milk improve lactose digestion of the product in people who digest lactose poorly. — EU health claim register Evidence E source
  Status Authorised

What cooking and storage change

• In a randomized crossover trial, 500 g/day of yogurt with live cultures lowered breath hydrogen in men with lactose malabsorption compared with heated yogurt, over 15 days. — two groups of 12 healthy men with or without lactose malabsorption; 15-day periods (n = 24) Evidence B source
  Breath hydrogen lower after fresh than heated yogurt (P < 0.01) in lactose malabsorbers

What it does to your health

• A meta-analysis of 6 randomized trials with 323 people found kefir lowered fasting blood sugar by about 10 mg/dL, without a significant change in HbA1c. — 6 RCTs, 323 subjects (n = 323) Evidence A source
  FBS WMD -10.28 (95% CI -16.53 to -4.02); insulin WMD -2.87; HbA1c -0.64 (95% CI -1.36 to 0.08), not significant

Safety, limits and interactions

• In an open-label phase I study, none of 54 critically ill adults given kefir up to 8.1 fl oz (240 mL) daily showed signs of kefir-related bacteremia. — 54 critically ill adults with intact gastrointestinal tract (n = 54) Evidence C source
  No bacteremia; diarrhea in 2 patients on laxatives; 91% of doses given
• A case report linked a Lactobacillus central-line bloodstream infection in a toddler with intestinal failure to the Greek yogurt he ate daily. — one toddler with intestinal failure and a central venous catheter (n = 1) Evidence D source
  Isolate about 99% similar to Lactobacillus from the yogurt

What people believe that the data does not support

• In an 8-week randomized trial in 75 overweight women, adding kefir to a non-restricted diet gave the same weight loss as adding the same servings of low-fat milk. — 75 overweight or obese premenopausal women aged 25-45; 8 weeks (n = 75) Evidence B source
  Kefir and milk both reduced weight, BMI and waist vs control (p < 0.01); no difference kefir vs milk

Who this works differently for

• In a randomized placebo-controlled trial of 125 children aged 1 to 5 on antibiotics, kefir did not prevent antibiotic-associated diarrhea. — 125 children aged 1-5 years in primary care (n = 125) Evidence B source
  Diarrhea 18% vs 21.9%; relative risk 0.82 (95% CI 0.54-1.43)
• In adolescent athletes over 16 weeks, two daily 6.2 oz (175 g) servings of Greek yogurt produced only modest, often sex-specific changes in bone and inflammation markers, similar to whey protein. — adolescent athletes, GY n = 24, whey n = 23; 16-week intervention (n = 47) Evidence B source
  No consistent difference between Greek yogurt and whey on bone turnover or inflammation markers

What is still unknown

• A systematic review of 68 studies judged the evidence that fermented dairy such as yogurt and kefir lowers blood lipids as neither convincing nor sufficient by EFSA criteria. — healthy adults; 14 controlled, 37 uncontrolled interventions, 17 observational studies (n = 68 studies) Evidence C source
  Mostly no significant change in lipid markers

The bottom line

One old trial speaks to calcium for people short on lactase. In a 1985 trial with a tracer and a single dose, lactase-deficient adults absorbed calcium from 5.2 oz (147 g) of plain yogurt as well as from milk, and at least as well as controls did. Kefir did no better than milk for weight. Over 8 weeks in 75 overweight women aged 25 to 45, of whom 58 finished, adding kefir gave the same drop in weight, BMI and waist as adding low-fat milk. A 2025 review of 68 studies, many uncontrolled, judged the evidence that fermented dairy lowers blood lipids neither convincing nor sufficient. Most trials in it showed no significant change. In 125 children aged 1 to 5 taking antibiotics, one commercial kefir did not prevent antibiotic-associated diarrhea (18% against 21.9% on placebo, relative risk 0.82, 95% CI 0.54 to 1.43). In 47 adolescent athletes over 16 weeks, two 6.2 oz (175 g) servings of Greek yogurt a day went with modest, often sex-specific shifts in bone and inflammation markers, and the protein source made no consistent difference against whey. The trial measured markers and did not count fractures. The caution we can document concerns people with a central line or severe illness. A case report tied a Lactobacillus bloodstream infection in a toddler with intestinal failure and a central venous catheter to the Greek yogurt he ate every day, with the bacteria about 99% alike. It is one child. In an uncontrolled safety study, 54 critically ill adults took up to 8.1 fl oz (240 mL) of kefir a day and none developed kefir-related bacteremia, while two patients on laxatives had diarrhea. That study was small and had no comparison group. We have no card on milk allergy, pregnancy or medicines, and there is no US survey record for either food, so this page gives no portion sizes.

Sources

1. gyk-r6-01 · food composition dataset
   CoFID 12-555 Yogurt, Greek style, plain (group BN): Energy 133 kcal; Protein 5.7 g; Fat 10.2 g; Lactose 3.5 g; Calcium 126 mg
   CoFID 2021 (Public Health England), code 12-555 Yogurt, Greek style, plain · checked 2026-09-29
2. gyk-r6-03 · randomized trial
   Based on the areas under the blood radioactivity curves, lactase-deficient subjects and controls absorbed calcium equally well from yogurt and milk. Lactase-deficient subjects absorbed 45Ca from both sources at least as well as did the controls.
   Am J Clin Nutr, 1985 · checked 2026-09-29
3. gyk-r6-04 · regulatory decision
   POL-HC-6400 Live yoghurt cultures Live cultures in yoghurt or fermented milk improve lactose digestion of the product in individuals who have difficulty digesting lactose Authorised
   EU Register on nutrition and health claims, claim POL-HC-6400, snapshot 2026-09-21 · checked 2026-09-29
4. gyk-r6-02 · randomized crossover trial
   Subjects were randomly assigned in a crossover design to 500 g/d of either fresh or heated yogurt for 2 periods of 15 d each, separated by a 15-d washout interval. ... In the same group, the production of breath hydrogen was lower after fresh yogurt consumption than after heated yogurt consumption (P: < 0.01).
   Am J Clin Nutr, 2000 · checked 2026-09-29
5. gyk-r6-05 · meta-analysis of RCTs
   In total, six RCTs on a total sample size of 323 subjects were selected for the meta-analysis. ... Findings indicated a more significant reduction in the FBS (WMD = -10.28; 95% CI: -16.53-4.02; P = 0.001) and insulin (WMD = -2.87; 95% CI: -3.96-1.78; P < 0.00001) in the kefir treatment group.
   Complement Ther Clin Pract, 2021 · checked 2026-09-29
6. gyk-r6-08 · phase I safety study
   After administering kefir, none of the 54 critically ill patients exhibited signs of kefir-related bacteremia. No side effects like bloating, vomiting, or aspiration were noted, except for diarrhea in two patients concurrently on laxatives.
   BMC Med, 2024 · checked 2026-09-29
7. gyk-r6-09 · case report
   Comparative genetic analysis predicted ~99% similarity between the CLABSI isolate and Lactobacillus isolates from the Greek yogurt the patient regularly consumed. Our patient's Lactobacillus CLABSI was linked to the consumption of Greek yogurt.
   Nutr Clin Pract, 2026 · checked 2026-09-29
8. gyk-r6-10 · randomized controlled trial
   we found that at 8 weeks, subjects in the kefir and milk groups had significantly greater reductions in weight, BMI, and WC compared to those in the control group (all p < 0.01). However, no such significant differences were found between the kefir and milk groups.
   Eur J Nutr, 2016 · checked 2026-09-29
9. gyk-r6-06 · randomized controlled trial
   There were no differences in the rates of diarrhea per group, with 18% in the active group and 21.9% in the placebo group (relative risk, 0.82; 95% confidence interval, 0.54-1.43).
   Arch Pediatr Adolesc Med, 2009 · checked 2026-09-29
10. gyk-r6-07 · randomized controlled trial
    GY consumed 2 servings per day of 175 g GY (17 g protein); WP received an isonitrogenous WP supplement. ... Increased protein intake, independent of source, was associated with modest, often sex-specific fluctuations in bone and inflammatory markers in adolescent athletes, potentially influenced by growth and training-related factors across the competitive season.
    J Nutr, 2026 · checked 2026-09-29
11. gyk-r6-11 · systematic review
    Findings from intervention studies were largely inconsistent, with most trials reporting no significant changes in lipid markers following fermented dairy intake. ... Overall, study quality, result consistency, and mechanistic evidence were deemed "neither convincing nor sufficient" per EFSA criteria.
    Front Nutr, 2025 · checked 2026-09-29