Vitamins · 4 min read ·

Vitamin K: vegetables, K2 and blood thinners

Vitamin K is needed for blood clotting and for bones. You get most of it from green vegetables; supplements mostly offer K2. What we know about them — and why they are not for everyone.

K

In a nutshell

1What it does and how much you need

The body needs vitamin K to activate proteins for blood clotting and bone mineralisation. With a deficiency the blood clots less well and you bleed more easily; in healthy adults, however, this only shows after 2–3 weeks of very low intake.1

EFSA recommendation 1 µg/kg EFSA set the adequate intake of vitamin K1 (phylloquinone) at 1 µg per kilogram of body weight a day for all age groups. It set no value for vitamin K2 (menaquinones), because there are few data on its absorption and effects in the body.1

Newborns are given vitamin K right after birth, because breast milk contains little of it and breastfed babies are prone to bleeding.1

2Where to find it

Data 483 µg Vitamin K1 is mainly in dark green leafy vegetables and brassicas. A hundred grams of parsley have 1,640 µg, spinach 483 µg, kale 390 µg and cooked broccoli 141 µg. Rapeseed and olive oil also have a fair amount.1,2

Vitamin K2 is made by bacteria. It is in meat, cheese and eggs, and fermented foods have a lot of it, for example Japanese natto made from soya. Gut bacteria make it too, but it is not clear how much of it is absorbed from there.1,5

3Absorption and forms

Weak evidence — a small study 7.5× / 24× Five volunteers were given 1 mg of vitamin K1 as a medicine or in spinach. After spinach with butter they had 7.5 times less vitamin K in their blood than after the medicine, and after spinach without fat as much as 24 times less.3
Weak evidence — a small study 6× In another study, about six times more vitamin K1 was absorbed from a 500 µg tablet than from 150 g of raw spinach with the same content. There was no difference between spinach, broccoli and lettuce, nor between raw and cooked broccoli.4
Weak evidence — a small study 7–8× Synthetic K1 and K2 as MK-7 from natto were both absorbed well. But MK-7 stays in the blood much longer, so with regular use it builds up to levels 7–8 times higher.5

In the EU, supplements may contain vitamin K as phylloquinone (K1) or menaquinone (K2, mainly MK-7).6

4Bones, arteries and medicines

Weak evidence 36 studies A summary of 36 studies in post-menopausal women and people with osteoporosis: vitamin K did not affect bone density or vertebral fractures. It suggested fewer other fractures, but in the better studies the difference was no longer significant.7
Weak evidence −9% A summary of 13 studies with 2,162 people: vitamin K slowed calcium build-up in the arteries by 9% but did not improve arterial stiffness. These are surrogate markers of vascular health, not heart attacks and strokes.8
Weak evidence — a small study 10 µg Healthy volunteers took a blood thinner similar to warfarin (acenocoumarol) together with K2 (MK-7). As little as 10 µg a day noticeably weakened the drug’s effect in some of them, and 45 µg — a usual dose in supplements — weakened it by about 40% on average.9

What to take from this

Sources

Written on , based on studies available at that date. Newer research may change the conclusions.

  1. EFSA NDA Panel (2017). Dietary reference values for vitamin K. EFSA Journal 15:4780. doi.org/10.2903/j.efsa.2017.4780
  2. USDA ARS (2018). FoodData Central — SR Legacy, vitamin K1 (phylloquinone) per 100 g: parsley leaves 1,640 µg (FDC 170416), spinach 482.9 µg (FDC 168462), kale 389.6 µg (FDC 168421), broccoli, cooked 141.1 µg (FDC 169967), canola (rapeseed) oil 71.3 µg (FDC 172336), olive oil 60.2 µg (FDC 171413). fdc.nal.usda.gov
  3. Gijsbers B. L. et al. (1996). Effect of food composition on vitamin K absorption in human volunteers. Br J Nutr 76:223–229. doi.org/10.1079/bjn19960027
  4. Garber A. K. et al. (1999). Comparison of phylloquinone bioavailability from food sources or a supplement in human subjects. J Nutr 129:1201–1203. doi.org/10.1093/jn/129.6.1201
  5. Schurgers L. J. et al. (2007). Vitamin K-containing dietary supplements: comparison of synthetic vitamin K1 and natto-derived menaquinone-7. Blood 109:3279–3283. doi.org/10.1182/blood-2006-08-040709
  6. Directive 2002/46/EC on food supplements, Annex II — forms of vitamins and minerals permitted in supplements (consolidated version 26 Nov 2025). eur-lex.europa.eu
  7. Mott A. et al. (2019). Effect of vitamin K on bone mineral density and fractures in adults: an updated systematic review and meta-analysis of randomised controlled trials. Osteoporos Int 30:1543–1559. doi.org/10.1007/s00198-019-04949-0
  8. Lees J. S. et al. (2019). Vitamin K status, supplementation and vascular disease: a systematic review and meta-analysis. Heart 105:938–945. doi.org/10.1136/heartjnl-2018-313955
  9. Theuwissen E. et al. (2013). Effect of low-dose supplements of menaquinone-7 (vitamin K2) on the stability of oral anticoagulant treatment: dose-response relationship in healthy volunteers. J Thromb Haemost 11:1085–1092. doi.org/10.1111/jth.12203

This is general information and does not replace a doctor. If you take warfarin or other blood thinners, talk to a doctor about vitamin K.