Supplement analysis backed by scientific evidence · PT

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Vitamin K1

Vitamin K1 (phylloquinone) is essential for blood clotting, and dietary deficiency is rare outside newborns and fat malabsorption. As a bone supplement it has been directly tested and failed: 5 mg/day for up to four years produced no change in bone mineral density.

What the science says

  • No effect on bone mineral density in postmenopausal women Limited evidence

    The ECKO trial randomised 440 postmenopausal women with osteopenia to 5 mg/day K1 or placebo for 2-4 years. There was no difference in lumbar spine BMD change (difference -0.06%, 95% CI -0.67 to 0.54) or at any other site or time point. A secondary, exploratory observation of fewer clinical fractures (9 versus 20, P=0.04) involved small numbers, was not a prespecified primary outcome, and has not been replicated.

  • The famous fracture meta-analysis is about K2, not K1 Limited evidence

    Cockayne's review is routinely cited as proof that vitamin K prevents fractures (OR 0.23 for hip, 0.40 for vertebral, 0.19 for all non-vertebral fractures). But all seven trials contributing fracture data were Japanese and used menaquinone-4 at 45 mg/day — a pharmaceutical dose of a different vitamer. No K1 trial has ever shown a fracture reduction.

  • Essential for coagulation and the direct antidote to warfarin Strong evidence

    K1 is the cofactor for gamma-carboxylation of clotting factors II, VII, IX and X, and is the standard reversal agent for vitamin K antagonist over-anticoagulation. Swings in dietary or supplemental K1 destabilise INR; consistent low-dose K1 (roughly 100-200 mcg/day) has been used clinically to reduce INR variability in unstable warfarin patients. This is a medical intervention, not a self-care use.

Dosage & safety

Studied doseAdequate intake is 90 mcg/day for women and 120 mcg/day for men; typical Western diets supply 70-150 mcg/day, almost all from leafy greens and vegetable oils. Supplement products range from 100 mcg to 5 mg — the failed ECKO bone trial used 5 mg/day. Newborn prophylaxis is a single 1 mg intramuscular dose at birth. K1 is fat-soluble and absorbs poorly on an empty stomach; take with a meal containing fat.
SafetyVery low intrinsic toxicity — no tolerable upper limit has been set for phylloquinone and no adverse effect level has been identified from food or oral supplements. The clinically important issue is interaction: doses above roughly 100-150 mcg/day antagonise warfarin, acenocoumarol and phenprocoumon and lower INR. It does not interact with DOACs such as apixaban and rivaroxaban. Intravenous phytonadione can cause anaphylactoid reactions and is hospital-only. People on vitamin K antagonists should keep intake steady rather than avoid vitamin K, and should not start or stop a supplement without telling their anticoagulation clinic.

Scientific references

Where to buy

Swanson Vitamins, Vitamin K1, 100 mcg, 100 Tablets

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