Cohort studyindependent cross-checkabstract checked★ key study I-VEG-102 in-depth review

Week-to-week vitamin K swings move INR

Dietary vitamin K variability affects International Normalized Ratio (INR) coagulation indices.

Overview

Couris R et al.. Int J Vitam Nutr Res 2006 · PMID 16941417 · DOI 10.1024/0300-9831.76.2.65

A weekly change of about 714 micrograms of vitamin K changed INR by about one unit in 43 new warfarin users.

Does the study support the specific claim? Partly · Methodological quality: moderate

Study explained simply

What did the researchers want to find out?: Quantify the effect of vitamin K variability on INR.
How was the study done?: Daily food records and weekly INR for 5 weeks.
How many people, animals or samples were studied?: 43 completers.
What was actually found?: Bigger swings in vitamin K went with bigger swings in INR.
How large was the observed effect?: 714 ug/week per 1 INR unit; r = -0.60.
What do the results mean in everyday life?: Eating very different amounts of greens from week to week can make INR unstable.
What are the study's limitations?: Small, short, early-treatment phase.
What does it say about Elawady's specific claim?: Supports "changes in greens intake affect warfarin", not "greens must be avoided".
Do the results apply to Germany and Europe?: Applies to all VKA users.
What can explicitly not be concluded from this study?: Cannot show that consistent greens intake is harmful.

How much does week-to-week variability in dietary vitamin K change INR in patients starting warfarin?

Study design: Cohort study – Prospective dietary-assessment cohort with weekly INR, 5 weeks
Population: Outpatients who began warfarin within 14 days, Massachusetts General Hospital
Country: USA
Period: not stated in abstract
Sample size: 60 enrolled, 43 completed
Inclusion and exclusion criteria: Excluded >2 alcoholic drinks/day, non-English speakers, liver disease, terminal illness

Exposure / intervention: Week-to-week change in dietary vitamin K1 (validated self-monitoring tool)
Comparator: Within-person variation
Endpoints: Weekly INR
Statistical methods: Pearson correlation; multiple regression adjusted for age, sex, weight, height, interacting medications

Results: Variability in INR inversely correlated with change in vitamin K intake (r = -0.600, p < 0.01). A weekly change of 714 ug dietary vitamin K altered weekly INR by 1 unit (p < 0.01); 14.5 mg warfarin per week altered INR by 1 unit.
Absolute risks: not reported
Authors' conclusion: Patients taking warfarin and consuming markedly changing amounts of vitamin K may have a variable weekly INR with potentially unstable anticoagulant outcomes.

714 ug/week corresponds to ~102 ug/day sustained change per INR unit (stats/C-VEG-12_warfarin.py); regression model R 0.848 includes warfarin dose.

Direct: quantifies the effect of CHANGING vitamin K intake on INR.

Dentali 2016 and Sconce 2007: low and unstable intake rather than high intake per se linked to instability.

Universal pharmacology.

Supports the conditional claim (variability in intake affects INR).

Multidimensional assessment
Multidimensional assessment
Authenticity of the source	verified
Methodological quality	moderate
Risk of bias	some concerns
Precision of results	low
Directness	direct
Reproducibility	partly replicated
Consistency with the overall evidence	mixed
Regional transferability	high
Adequacy of the interpretation (by the archive)	not applicable

Replication in larger cohorts with stable maintenance doses.