Vitamin K doesn’t get nearly as much attention as vitamins C or D. It was first discovered in 1929 by Danish scientist Henrik Dam, who noticed that chicks fed a fat-free diet developed bleeding problems. The name comes from the Danish word “koagulation” (coagulation), which tells you exactly what this nutrient does best: it helps your blood clot properly.

But there’s more to vitamin K than just clotting. Over the past few decades, researchers have found it’s involved in bone metabolism, arterial health, and possibly even kidney function. Here’s what the evidence actually shows.

What is vitamin K?

Vitamin K is a fat-soluble vitamin, meaning it’s absorbed along with dietary fats and stored in your liver and fatty tissues. Unlike water-soluble vitamins that you excrete daily, your body can hold onto fat-soluble vitamins for weeks or months.

At the molecular level, vitamin K enables a process called gamma-carboxylation. Without getting too deep into biochemistry, this process activates certain proteins in your body, particularly those involved in blood clotting (called coagulation factors II, VII, IX, and X) and calcium metabolism in bones and blood vessels.

Some researchers have called it “the forgotten vitamin” because it tends to get overlooked in nutrition discussions. That’s probably unfair given how many bodily processes depend on it.

Types of vitamin K and food sources

There are two main forms of vitamin K in food:

Vitamin K1 (phylloquinone) is the most common form in Western diets. Green leafy vegetables are the best sources:

Kale: 817 mcg per 100g (cooked)
Spinach: 483 mcg per 100g (cooked)
Broccoli: 141 mcg per 100g (cooked)
Brussels sprouts: 140 mcg per 100g (cooked)

Vitamin K2 (menaquinones) comes mainly from fermented foods and animal products. It’s subdivided into different forms based on chain length: MK-4 through MK-10. The two you’ll see discussed most are:

MK-4: Found in meat, egg yolks, and some dairy. Your body can also convert K1 to MK-4.
MK-7: Found primarily in natto (fermented soybeans), a Japanese food that’s, frankly, an acquired taste. Also present in smaller amounts in some cheeses.

MK-7 has some interesting properties. It stays in your bloodstream much longer than K1 (a half-life of about 3 days versus 1-2 hours), and blood levels can reach 7-8 times higher with regular intake [1]. This longer circulation time means it has more opportunity to reach tissues throughout your body.

There’s also vitamin K3 (menadione), a synthetic form that’s largely been abandoned for human use. It can interfere with glutathione, your body’s main antioxidant, and has caused liver damage when given intravenously. You won’t find it in supplements.

What does the research say about vitamin K benefits?

1. Bone health

The connection between vitamin K and bones centres on a protein called osteocalcin. This protein binds calcium to the bone matrix, but it needs vitamin K to become activated (carboxylated). Without enough vitamin K, osteocalcin can’t do its job properly, and calcium may not get deposited where it should.

A 2011 meta-analysis of 17 randomised controlled trials found that vitamin K supplementation, particularly K2, increased lumbar spine bone mineral density. However, it didn’t significantly affect femoral neck (hip) bone density [2]. The lumbar spine finding is interesting because that’s a common site for osteoporotic fractures.

Another meta-analysis from 2015, looking at 19 trials with 6,759 postmenopausal women with osteoporosis, found that K2 supplementation helped maintain spinal bone density and reduced fracture incidence [3]. That’s a meaningful finding for a population at high risk of fractures.

A 2017 analysis of five studies (80,982 participants total) found that higher dietary vitamin K intake was associated with modestly lower fracture risk [4]. The effect was relatively small, but consistent.

If you’re concerned about bone health, vitamin K alone probably isn’t the answer. It works alongside calcium, vitamin D, and exercise. The NHS recommends getting vitamin K from food rather than supplements for most people, and ensuring adequate intake of all nutrients that support bone health [5].

2. Blood sugar regulation and diabetes

Several researchers have wondered whether vitamin K might help with blood sugar control, partly because osteocalcin (that bone protein activated by vitamin K) appears to influence insulin sensitivity in animal studies.

The human evidence is less exciting. A 2017 meta-analysis of 8 randomised controlled trials with 1,077 participants found that vitamin K supplementation (either K1 or K2) didn’t significantly affect insulin resistance, fasting blood glucose, fasting insulin, or inflammatory markers like C-reactive protein [6].

A 2018 analysis of 5 clinical studies (533 participants) reached similar conclusions: no significant effect on glycaemic control in people without diabetes [7].

I should be honest here. The diabetes connection looked promising based on animal research, but the human data hasn’t supported it so far. That doesn’t mean there’s no relationship, but if one exists, it’s subtle enough that current studies haven’t detected it.

3. Anticoagulant therapy management

This one’s a bit counterintuitive. Warfarin works by blocking vitamin K, so you might think vitamin K supplementation would be the last thing someone on warfarin would want. But the reality is more nuanced.

The problem with warfarin therapy is that it’s finicky. Your international normalised ratio (INR), which measures how long your blood takes to clot, can swing around based on what you eat, other medications, and various other factors. An INR between 2 and 3 is usually the target for preventing strokes and major bleeding.

Some clinicians have wondered whether giving a small, consistent dose of vitamin K might actually stabilise INR by providing a predictable baseline that warfarin can then block predictably.

A 2013 systematic review of 4 studies (678 participants) found insufficient evidence to recommend routine vitamin K supplementation for INR stability [8]. However, the authors noted it might be worth trying in patients whose INR fluctuates without obvious cause.

The key word is “consistent.” Wildly varying vitamin K intake is the problem. If you’re on warfarin, the goal is to eat roughly the same amount of vitamin K-rich foods each day rather than avoiding them entirely. Talk to your doctor or pharmacist about this.

4. Arterial stiffness

Here’s where things get interesting. Another protein activated by vitamin K is matrix Gla protein (MGP), which inhibits calcium from depositing in artery walls. When MGP isn’t properly activated (carboxylated), calcium can accumulate in blood vessels, making them stiff and less flexible.

Arterial stiffness is measured in various ways, including pulse wave velocity. It’s associated with cardiovascular disease, stroke, and cardiovascular mortality.

A 3-year double-blind trial of 244 healthy postmenopausal women found that daily MK-7 (vitamin K2) supplementation improved arterial stiffness markers, particularly in women who started with stiffer arteries [9]. The improvement was measured by both arterial stiffness index and levels of inactive MGP (which decreased, suggesting better MGP activation).

This is promising, but it’s one trial in a specific population. More research is needed before we can say vitamin K2 protects against cardiovascular disease. The mechanisms make biological sense, which is encouraging, but mechanisms don’t always translate to clinical benefits.

5. Chronic kidney disease

People with chronic kidney disease often have impaired vitamin K status and accelerated vascular calcification. This has led researchers to investigate whether vitamin K intake might affect outcomes in this population.

A 13-year observational study of 3,401 participants with chronic kidney disease found that higher dietary vitamin K intake was associated with lower all-cause mortality and cardiovascular mortality [10]. This is an association, not proof that vitamin K caused the better outcomes. People who eat more vitamin K-rich foods (green vegetables) might differ in other ways that affect their health.

Some clinical trials are underway testing vitamin K supplementation in kidney disease patients, but we don’t have definitive results yet. If you have kidney disease, this is definitely something to discuss with your nephrologist rather than self-treating.

Side effects of vitamin K supplementation

Vitamin K1 and K2 appear to be quite safe at typical supplement doses. The NHS notes that taking 1mg or less of vitamin K supplements daily is unlikely to cause harm [5]. Unlike some fat-soluble vitamins, vitamin K doesn’t accumulate to toxic levels under normal circumstances.

Possible side effects at higher doses include:

Stomach upset
Decreased appetite
Allergic reactions (rare)

Vitamin K3 (menadione) is a different story. This synthetic form can cause:

Liver damage
Jaundice
Haemolytic anaemia (destruction of red blood cells)
Interference with glutathione (the body’s main antioxidant)

Vitamin K3 isn’t used in supplements for this reason. If you see it listed as an ingredient, choose a different product.

Seven contraindications and precautions

1. Pregnancy and breastfeeding

There’s insufficient safety data on vitamin K supplementation during pregnancy and lactation. Dietary vitamin K from food is fine and necessary, but supplements should only be taken under medical supervision.

2. Glucose-6-phosphate dehydrogenase (G6PD) deficiency

People with G6PD deficiency (also called favism) should avoid vitamin K supplements, particularly K3, as they may trigger haemolytic anaemia.

3. Diabetes and hypoglycaemic medications

Some evidence suggests vitamin K1 might lower blood sugar. If you have diabetes or take medications like metformin, insulin, or sulfonylureas, monitor your blood glucose carefully if supplementing with vitamin K.

4. Anticoagulant medications

This is the big one. If you take warfarin or other vitamin K antagonists, do not take vitamin K supplements without your doctor’s explicit approval. Vitamin K can directly counteract these medications, potentially causing dangerous blood clots.

Newer anticoagulants like rivaroxaban and apixaban work differently and aren’t affected by vitamin K, but you should still discuss any supplements with your prescriber.

5. Fat absorption disorders

Conditions that impair fat absorption can reduce vitamin K uptake. These include:

Coeliac disease
Cystic fibrosis
Ulcerative colitis
Short bowel syndrome
Chronic pancreatitis

People with these conditions may need vitamin K supplementation, but the form and dose should be determined by a doctor who understands their absorption capacity.

6. Drug interactions affecting absorption

Several medications can interfere with vitamin K absorption or metabolism:

Antiepileptic drugs (phenytoin, phenobarbital)
Certain antibiotics (cephalosporins, fluoroquinolones)
Cholesterol-lowering drugs (cholestyramine, colestipol)
Weight loss drugs (orlistat)
Broad-spectrum antibiotics (by killing gut bacteria that produce K2)

If you take any of these long-term, ask your doctor whether you need additional vitamin K.

7. High-dose vitamins A and E

Large amounts of vitamin A may interfere with vitamin K absorption. High-dose vitamin E can inhibit vitamin K-dependent carboxylase enzymes, potentially affecting both clotting and bone protein activation. If you take multiple fat-soluble vitamin supplements, space them out and don’t exceed recommended doses.

Who’s at risk of vitamin K deficiency?

True vitamin K deficiency is uncommon in healthy adults for several reasons: it’s widespread in foods, the body recycles it efficiently, and gut bacteria produce K2. However, certain groups are at higher risk.

Newborns

Babies are born with very low vitamin K stores because it doesn’t cross the placenta efficiently, and breast milk contains relatively little. In the first weeks of life, deficiency can cause vitamin K deficiency bleeding (VKDB), previously called haemorrhagic disease of the newborn.

VKDB can cause bleeding in the gut, skin, or brain. Late-onset VKDB (occurring at 2-12 weeks) often presents as sudden intracranial haemorrhage with high mortality.

This is why hospitals give newborns a vitamin K injection at birth. The American Academy of Pediatrics recommends 0.5-1mg of vitamin K1 intramuscularly. Some parents opt for oral vitamin K instead, though this requires multiple doses and may be less reliable [11].

People with malabsorption disorders

As mentioned above, conditions like coeliac disease, cystic fibrosis, and inflammatory bowel disease can impair vitamin K absorption along with other fat-soluble nutrients.

Long-term antibiotic users

Antibiotics can kill intestinal bacteria that produce vitamin K2. Short courses aren’t usually a problem, but prolonged antibiotic therapy might reduce K2 production.

People on dialysis

Dialysis patients often have poor vitamin K status due to dietary restrictions and metabolic changes.

Symptoms of vitamin K deficiency

The classic signs relate to impaired clotting:

Easy bruising
Bleeding gums
Nosebleeds
Heavy menstrual periods
Blood in urine or stool
Slow wound healing

If you experience unexplained bleeding or bruising, see your doctor. It may or may not be related to vitamin K, but it warrants investigation.

How much vitamin K do you need?

The NHS doesn’t set a specific recommended daily intake for vitamin K but notes that adults need approximately 1 microgram per kilogram of body weight per day [5]. For a 70kg person, that’s about 70mcg daily.

You should be able to get enough from a varied diet. A single serving of broccoli or spinach provides more than a day’s worth.

References

Schurgers LJ, et al. Vitamin K-containing dietary supplements: comparison of synthetic vitamin K1 and natto-derived menaquinone-7. Blood. 2007;109(8):3279-83. PubMed
Fang Y, et al. Effect of vitamin K on bone mineral density: a meta-analysis of randomized controlled trials. J Bone Miner Metab. 2012;30(1):60-8. PubMed
Huang ZB, et al. Does vitamin K2 play a role in the prevention and treatment of osteoporosis for postmenopausal women: a meta-analysis of randomized controlled trials. Osteoporos Int. 2015;26(3):1175-86. PubMed
Hao G, et al. Vitamin K intake and the risk of fractures: A meta-analysis. Medicine (Baltimore). 2017;96(17):e6725. PubMed
NHS. Vitamins and minerals: Vitamin K. NHS Website
Manna P, et al. Vitamin K supplementation and its effects on glycemic control: a systematic review and meta-analysis. J Diabetes Res. 2017;2017:8503475. PubMed
Choi HJ, et al. Vitamin K2 supplementation improves insulin sensitivity via osteocalcin metabolism: a placebo-controlled trial. Diabetes Care. 2011;34(9):e147. PubMed
Mahtani KR, et al. Vitamin K for improved anticoagulation control in patients receiving warfarin. Cochrane Database Syst Rev. 2014;(5):CD009917. PubMed
Knapen MH, et al. Menaquinone-7 supplementation improves arterial stiffness in healthy postmenopausal women. A double-blind randomised clinical trial. Thromb Haemost. 2015;113(5):1135-44. PubMed
Juanola-Falgarona M, et al. Dietary intake of vitamin K is inversely associated with mortality risk. J Nutr. 2014;144(5):743-50. PubMed
American Academy of Pediatrics Committee on Fetus and Newborn. Controversies concerning vitamin K and the newborn. Pediatrics. 2003;112(1):191-2.