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How much vitamin K changes warfarin effect
Effect of vitamin K intake on the stability of oral anticoagulant treatment: dose-response relationships in healthy subjects.
Overview
Schurgers LJ et al.. Blood 2004 · PMID 15231565 · DOI 10.1182/blood-2004-04-1525
In anticoagulated volunteers, INR fell significantly only from 150 micrograms of daily vitamin K1 upwards; spinach and broccoli meals had short-lived effects.
Does the study support the specific claim? Partly · Methodological quality: moderate
Plain-language summary
Study explained simply
- What did the researchers want to find out?
Find the vitamin K dose that disturbs anticoagulation.
- How was the study done?
Step-wise supplement doses and test meals.
- How many people, animals or samples were studied?
Healthy anticoagulated volunteers.
- What was actually found?
Effect began at 150 ug/day; vegetable meals had brief effects.
- How large was the observed effect?
Threshold 150 ug/day; clinically relevant in 25%.
- What do the results mean in everyday life?
Small, irregular servings of greens matter less than often feared.
- What are the study's limitations?
Not patients; supplements; size not in abstract.
- What does it say about Elawady's specific claim?
Partly supports and partly limits the claim.
- Do the results apply to Germany and Europe?
Universal.
- What can explicitly not be concluded from this study?
Cannot quantify risk of bleeding or clotting.
Scientific in-depth review
1. Full reference
- Authors
- Schurgers LJ et al.
- Journal
- Blood
- Year
- 2004
- Identifier
- PMID 15231565 DOI 10.1182/blood-2004-04-1525
- Verification
- verified – PubMed efetch 2026-10-26; Crossref 2026-10-26
- Retracted
- no
- Corrections
- none
- Extent checked
- abstract only
- Conflicts of interest and funding
not stated in abstract; first author affiliation includes VitaK BV (a vitamin K research company) per PubMed record
2. Research question
What vitamin K intake threshold changes INR during oral anticoagulation?
3. Study design and population
- Study design
- Other – Dose-response study in stably anticoagulated healthy volunteers (weekly incremental vitamin K1 supplements 50-500 ug/day) plus test meals of spinach and broccoli
- Population
Healthy volunteers anticoagulated for 13 weeks on individualized doses
- Country
Netherlands (Maastricht; from author affiliation)
- Period
not stated in abstract
- Sample size
not stated in abstract
- Inclusion and exclusion criteria
not stated in abstract
4. Methods and endpoints
- Exposure / intervention
Vitamin K1 supplements 50-500 ug/day for 7 days each; vitamin K-rich meals
- Comparator
Lower doses / baseline
- Endpoints
INR; undercarboxylated FII and osteocalcin
- Statistical methods
Dose-response comparison
5. Actual results
- Results
Threshold K1 dose causing a significant INR decrease: 150 ug/day; clinically relevant INR change in 25% of participants at 150 ug/day. Response to spinach and broccoli meals was short-lived, suggesting inefficient bioavailability.
- Absolute risks
not reported
- Authors' conclusion
Short-term variability in vitamin K1 intake is less important for INR fluctuations than commonly assumed; supplements providing 100 ug/day do not significantly interfere with oral anticoagulant therapy.
6. Strengths
- Controlled dose-response design
- Tests real foods as well as supplements
7. Weaknesses
- Healthy volunteers, not patients
- Sample size not given in abstract
- Supplements differ from food matrix
8. Risk of bias
- Orienting tool
- none applicable
- Type of assessment
- simplified assessment (abstract only)
- Full formal assessment
- no
- Overall risk of bias
- some concerns
- Rationale
Non-randomized dose escalation.
9. Assessment of the statistical interpretation
Threshold effect at 150 ug/day; Violi 2016 translates 150 ug/day to ~30 g cooked spinach.
10. Relevance to the specific claim
Mixed: shows an INR effect above ~150 ug/day but suggests food vitamin K is less bioavailable and short-term variability matters less than assumed.
11. Independent contrary findings
Franco 2004 found larger diet effects in patients.
12. Transferability to Germany and Europe
Universal.
13. Overall methodological judgement
Limits the claim: only sustained, substantial increases shift INR.
| Multidimensional assessment | |
|---|---|
| Authenticity of the source | verified |
| Methodological quality | moderate |
| Risk of bias | some concerns |
| Precision of results | low |
| Directness | indirect |
| Reproducibility | partly replicated |
| Consistency with the overall evidence | mixed |
| Regional transferability | high |
| Adequacy of the interpretation (by the archive) | not applicable |
14. Open questions
Effects of habitual high-greens diets in real patients.
15. Linked fact checks
- Vitamin K in leafy greens destabilises warfarin therapy (archive citing a Medicine (Baltimore) review). – Studies with mixed or unclear results