Vitamin K Deficiency: Easy Bruising, Bleeding Risk, and Newborn Concerns

Vitamin K is a fat-soluble nutrient that your body requires to produce proteins involved in blood clotting and bone metabolism. Most healthy adults get enough through diet and gut bacteria, but deficiency can occur in specific situations and carries meaningful health consequences.

What Vitamin K Actually Does

According to the NIH Office of Dietary Supplements (ODS), vitamin K serves two well-established physiological roles supported by strong evidence.

• Blood clotting: Vitamin K acts as a coenzyme for vitamin K-dependent carboxylase, the enzyme required to synthesize clotting proteins. One of the most critical is prothrombin, also called clotting factor II, a plasma protein directly involved in forming blood clots. Without adequate vitamin K, your body cannot produce functional clotting factors, which raises bleeding risk.
• Bone protein synthesis: Vitamin K supports the carboxylation of osteocalcin, a vitamin K-dependent protein found in bone that appears to be involved in bone mineralization or turnover. The NIH ODS rates this role as supported by strong evidence, though the clinical implications are still being studied.

Note that supplements are not intended to diagnose, treat, cure or prevent any disease, and these statements have not been evaluated by the FDA.

Signs That May Point to Deficiency

Because vitamin K is central to clotting, the most recognizable signs of deficiency relate to bleeding. These can include easy or unexplained bruising, prolonged bleeding from cuts or wounds, and in more serious cases, bleeding in the gastrointestinal tract or other internal sites. A laboratory test called prothrombin time measures how long blood takes to clot and is often used clinically to assess vitamin K status. If you notice unusual bruising or bleeding that does not resolve, speak with a doctor rather than self-treating.

Who Is at Higher Risk

Healthy adults who eat a varied diet and have normal gut function rarely develop frank vitamin K deficiency. However, certain situations raise the risk considerably.

• Newborns: Infants are born with very low vitamin K stores because the nutrient does not cross the placenta efficiently, and breast milk contains only small amounts. Their gut bacteria have not yet developed enough to produce meaningful quantities. This makes newborns vulnerable to a serious condition called vitamin K deficiency bleeding (VKDB). For this reason, a single intramuscular injection of vitamin K at birth is standard medical practice in most countries. Parents should discuss this with their obstetric or pediatric care team.
• People with fat malabsorption: Because vitamin K is fat-soluble, conditions that impair fat absorption, such as celiac disease, Crohn's disease, cystic fibrosis, or short bowel syndrome, can reduce how much vitamin K the body absorbs from food.
• People on prolonged antibiotic therapy: Antibiotics can reduce populations of vitamin K-producing gut bacteria, potentially lowering vitamin K status. The NIH ODS notes this concern is most relevant with prolonged use of several weeks or more combined with poor dietary intake.
• Older adults with limited dietary variety: Reduced food intake or avoidance of green vegetables can lower intake below the recommended level over time.

Intake Targets: What the NIH Recommends

The NIH ODS reports that no Recommended Dietary Allowance (RDA) has been established for vitamin K because there is not yet enough evidence to set one. Instead, an Adequate Intake (AI) is used. The AI is 120 micrograms per day for adult men and 90 micrograms per day for adult women. No Tolerable Upper Intake Level has been set because vitamin K has a low potential for toxicity at typical intake levels. For personal guidance on whether you are meeting these targets, consult a registered dietitian or your primary care clinician.

Food Sources

The richest dietary sources of vitamin K1 (phylloquinone) are green leafy vegetables such as kale, spinach, collard greens, and broccoli. Vegetable oils and some fruits also contribute. Vitamin K2 (menaquinones) is found in fermented foods like natto, as well as in some cheeses and meat. The NIH ODS notes, with moderate evidence, that gut bacteria-produced menaquinones also contribute to meeting the body's vitamin K requirement, though the exact contribution varies by individual.

Drug Interactions: The Most Important Safety Concern

Vitamin K has a low toxicity profile, but its most significant safety issue is a potentially serious interaction with anticoagulant medications. The NIH ODS identifies the following interactions that warrant careful attention.

• Warfarin (Coumadin) and similar anticoagulants: Drugs such as warfarin, phenprocoumon, and acenocoumarol work by antagonizing vitamin K activity and depleting vitamin K-dependent clotting factors. Sudden changes in vitamin K intake, whether from food or supplements, can increase or decrease the anticoagulant effect in ways that are difficult to predict. People taking these medications must keep their vitamin K intake consistent day to day. Do not change your diet significantly or start a vitamin K supplement without first speaking to your doctor or pharmacist.
• Antibiotics: Certain antibiotics, particularly cephalosporins such as cefoperazone, may inhibit vitamin K action in addition to reducing gut bacteria that produce it.
• Bile acid sequestrants: Cholestyramine (Questran) and colestipol (Colestid) can reduce absorption of vitamin K and other fat-soluble vitamins. The NIH ODS recommends monitoring vitamin K status with long-term use of these drugs.
• Orlistat (Alli, Xenical): This weight-loss medication reduces dietary fat absorption and can lower absorption of fat-soluble vitamins including vitamin K. When combined with warfarin, orlistat may significantly increase prothrombin time, which raises bleeding risk.

Always talk to a doctor or pharmacist before combining any supplement with prescription or over-the-counter medication.

What the Research Does and Does Not Show

Beyond its established clotting role, researchers have explored vitamin K in other areas. The NIH ODS rates the evidence for vitamin K supplementation reducing the risk of osteoporosis or bone fracture as limited, meaning current data are promising but not conclusive. Similarly, higher intake of menaquinone (K2) has been associated with less vascular calcification and lower coronary heart disease risk in some studies, but the NIH ODS also rates this evidence as limited. These findings are worth watching as research develops, but they are not a basis for self-supplementing to prevent disease.

When to See a Doctor

If you bruise easily without explanation, notice prolonged bleeding, are pregnant or planning to be, take anticoagulants, or have a condition that affects fat absorption, speak with a healthcare provider about your vitamin K status. Blood tests can help clarify whether your levels are adequate. Self-diagnosing a deficiency or adjusting supplements without medical guidance is not recommended, particularly given the serious drug interactions described above.

For further reading, the NIH ODS maintains detailed fact sheets for both consumers and health professionals at ods.od.nih.gov, and intake reference values are published by the National Academies of Sciences, Engineering, and Medicine.