Vitamin K is probably the least understood of the fat-soluble vitamins. It’s role in blood clotting is well recognized, but vitamin K’s other functions are less well known and include supporting bone and cardiovascular health.

Why is vitamin K important?

Vitamin K1 assists in blood clotting. Vitamin K2 promotes bone health – reducing the risk of cavities and fractures; it also helps prevent calcification of blood vessels which may reduce the risk of hypertension (high blood pressure). Vitamin K may also promote healthy blood sugar regulation, help prevent kidney stones, and offer protection against certain types of cancer (e.g. liver and prostate). More research is needed to confirm the potential benefits afforded by vitamin K.

Different forms of vitamin K

There are three main types of vitamin K.

  • Vitamin K1 (phylloquinone) – naturally produced by plants and most abundant in green vegetables.
  • Vitamin K2 (menaquinone) – found in grass-fed dairy products and some fermented foods. Also, synthesized by bacteria in the intestinal tract.
  • Vitamin K3 (menadione) – a synthetic form of vitamin K; no longer used therapeutically due to potential toxicity.

What are signs of deficiency?

Severe deficiency of vitamin K will lead to blood clotting problems and increased risk of hemorrhage. This is rare in healthy adults as vitamin K1 is commonly found in many foods and the body is efficient at recycling vitamin K1 to ensure blood clotting capabilities. In adults, bleeding from vitamin K deficiency is most common in chronic liver disease, obstructive jaundice, and patients receiving anticoagulants or prolonged antibiotic therapy.

Newborn babies who are exclusively breast-fed are at increased risk for vitamin K deficiency, because human milk is relatively low in vitamin K compared to infant formula.[1] It is for this reason that a vitamin K shot is recommended to newborn babies to prevent bleeding due to vitamin K deficiency.

If you are uncomfortable with your newborn baby getting a vitamin K shot, oral vitamin K drops are an alternative option that can provide a similar degree of protection if properly administered (e.g. 2 mg at birth, followed by 1 mg weekly of vitamin K1).[2][3][4] Consult a trusted healthcare provider to decide what feels best given your situation.

What is the daily recommended intake?

There is currently no established Recommended Dietary Allowance (RDA) for vitamin K. Instead, the Adequate Intake (AI) – amount estimated to be adequate for a healthy group of people – is 90 micrograms for adult females and 120 micrograms for adult males. Chris Masterjohn, who has done an extensive write up on vitamin K, recommends 100-200 micrograms per day of vitamin K2 for healthy adults.[5]

How much can I take?

There is no established tolerable upper intake limit (UL) for vitamin K, and there is currently no known toxicity associated with high doses of the phylloquinone (vitamin K1) or menaquinone (vitamin K2) forms of vitamin K. As mentioned earlier, synthetic menadione (vitamin K3) is associated with toxicity.

Doses of 45 mg/day of vitamin K2 have been used in randomized controlled trials of postmenopausal women for a period of 2–3 years with no reports of severe side effects. For the average healthy adult, it is probably reasonable to limit daily vitamin K2 supplementation to 1 mg/day, unless you are using vitamin K for therapeutic reasons under the care of a healthcare professional.

Drug interactions

Patients taking anticoagulant (blood thinner) medications like warfarin (Coumadin) should not take vitamin K supplements (nor should they eat natto) without the strict supervision of their prescribing physician.

What is the best source of vitamin K?

Green leafy vegetables are said to be the best food source of vitamin K1.[6] Vitamin K1 can be converted to vitamin K2 by bacteria that normally inhabit a healthy intestinal tract.[7] However, antibiotics can kill off the bacteria that make this conversion happen, leaving us at greater risk for vitamin K2 deficiency. Animals that eat green grass similarly convert vitamin K1 to vitamin K2 in their digestive tracts, making grass-fed dairy products a potentially good source of vitamin K2.

Natto – fermented soybeans found in Japanese cuisine – is the highest food source of vitamin K2 on the planet containing about 1 mg of vitamin K2 (as MK-7) per 100 grams.[8] It is, however, an acquired taste (I think I spit it out the first time I tried it; but after another half dozen or so attempts, I can now eat it with gusto).

How to increase absorption of fat-soluble vitamins

Vitamin K, like all fat-soluble vitamins (A, D, E, K), is better absorbed when eaten together with fat at the same meal. So, eating your leafy green vegetables with extra virgin olive oil, or natto with grass-fed butter (over rice with a green onion garnish) will help increase vitamin K absorption.

The fat-soluble trifecta

I think of vitamins A, D, and K as forming a nutritional trinity whose synergy improves our health far more than when any one of these vitamins is taken alone. That said, a proper balance between these three fat-soluble vitamins is important. High-dose vitamin A (retinol) can antagonize vitamin D, and some studies have shown increased risk of bone fracture associated with high intakes of preformed vitamin A in the presence of vitamin D deficiency.[9]

Vitamin D increases calcium absorption, but vitamin K2 is needed to transport this calcium into the bones and teeth where it belongs. Without vitamin K2, calcium supplementation could increase the risk of cardiovascular disease as calcium could get lodged into soft tissues like blood vessels, instead of being deposited into bones and teeth. In short, vitamins A, D, and K are like three legs of a stool; too much or too little of one can make us nutritionally lopsided.

What vitamin K supplements do you recommend?

Exclusively breastfed newborns, who have not received a vitamin K shot at birth, might benefit from taking oral vitamin K1 drops to prevent vitamin K deficiency bleeding. The rest of us can get all the vitamin K1 we need from food sources, like leafy green veggies.

Vitamin K2 is harder to obtain from dietary sources. If you can’t stand the taste of natto and don’t eat much in the way of grass-fed dairy products, then taking a vitamin K2 supplement may be a good idea. If you regularly supplement with vitamin D, then a combination vitamin D3/K2 supplement can be a convenient option. Professional-grade vitamin K1 and K2 supplements can be found in our online supplement dispensary.

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