Vitamin K deficiency happens when your body can’t get or use enough of this nutrient to produce the proteins responsible for blood clotting and bone strength. The most common causes are fat malabsorption disorders, certain medications, very low dietary intake, and the natural vulnerability of newborns. Adults eating a varied diet rarely become deficient from food alone, but several medical conditions and drugs can tip the balance quickly.
How Your Body Absorbs Vitamin K
Vitamin K is fat-soluble, which means it doesn’t dissolve in water and can’t simply pass through your intestinal wall on its own. Instead, it requires bile salts, the digestive compounds your liver produces and your gallbladder releases into the small intestine, to be absorbed. When bile salts mix with dietary fat, they form tiny clusters that carry vitamin K across the intestinal lining and into your bloodstream.
Any condition that disrupts fat digestion or bile flow can block this process. That’s why the most medically significant causes of vitamin K deficiency aren’t about what you eat. They’re about what your body can actually absorb.
Fat Malabsorption Disorders
Diseases that damage the small intestine or reduce bile production are the leading medical causes of vitamin K deficiency in adults. Inflammatory bowel disease (Crohn’s disease and ulcerative colitis), celiac disease, and cystic fibrosis all interfere with fat absorption, and vitamin K gets lost along with the unabsorbed fat.
Liver disease and bile duct obstruction create a different version of the same problem. If your liver can’t produce enough bile, or if a gallstone or tumor blocks the duct that carries bile to the intestine, fat-soluble vitamins including K pass through unabsorbed. Chronic pancreatitis has a similar effect because the pancreas produces enzymes needed to break down dietary fat in the first place.
People who have had portions of their small intestine surgically removed, such as after bariatric surgery or treatment for Crohn’s disease, lose absorptive surface area and face a higher ongoing risk.
Medications That Interfere With Vitamin K
Several widely prescribed drugs can cause a functional vitamin K deficiency, meaning your body has the vitamin but can’t use it properly.
Blood thinners like warfarin work by deliberately blocking an enzyme called vitamin K epoxide reductase. This enzyme normally recycles vitamin K so it can be reused to activate clotting proteins. When warfarin shuts down that recycling process, the result is a controlled, intentional reduction in clotting ability. That’s the whole point of the drug, but it also means anyone on warfarin is living in a state of managed vitamin K suppression. If the dose is too high or interacts with other medications, the suppression can become dangerous.
Broad-spectrum antibiotics taken over long periods can also contribute. Your gut bacteria produce a form of vitamin K (called K2), and antibiotics that wipe out those bacteria reduce this internal supply. On its own, losing gut-produced vitamin K usually isn’t enough to cause deficiency, but combined with poor dietary intake or malabsorption, it can push levels below what your body needs.
Certain anti-seizure medications and cholesterol-lowering drugs that bind bile acids can also reduce vitamin K absorption over time.
Why Newborns Are Especially Vulnerable
Babies are born with very little vitamin K. Only minimal amounts cross the placenta during pregnancy, so even healthy, full-term infants start life with low stores. Breast milk, while ideal in nearly every other nutritional respect, provides relatively low concentrations of vitamin K. This combination means exclusively breastfed infants’ blood levels of vitamin K often fall below adult norms from about 6 weeks to 6 months of age.
Without supplementation, this gap can lead to a condition called vitamin K deficiency bleeding (VKDB), which ranges from mild bruising to life-threatening bleeding in the brain. VKDB is the reason hospitals routinely give newborns a vitamin K injection shortly after birth. The shot is a single dose that provides enough of the vitamin to bridge the gap until the infant’s diet and gut bacteria can maintain adequate levels on their own.
Dietary Causes
True dietary vitamin K deficiency is uncommon in adults because the vitamin is abundant in everyday foods. The recommended adequate intake is 120 micrograms per day for adult men and 90 micrograms for adult women, including during pregnancy. A single cup of cooked spinach, broccoli, or kale provides several times that amount. Other good sources include Brussels sprouts, lettuce, soybean oil, and canola oil. Fermented foods like certain cheeses and natto (a Japanese soybean dish) supply the K2 form.
That said, people who eat very restricted diets, those with severe appetite loss from illness or treatment, and individuals on long-term intravenous nutrition without adequate supplementation can fall short. Older adults living alone, people with eating disorders, and those with chronic alcoholism are at higher practical risk because their diets tend to lack green vegetables and other vitamin K sources consistently.
Signs of Vitamin K Deficiency
The hallmark symptom is abnormal bleeding. This can show up as easy bruising, nosebleeds that are hard to stop, or prolonged bleeding from cuts and wounds. More serious signs include blood in the urine or stool, black tarry stools (indicating bleeding in the digestive tract), and vomiting blood. In some cases, bleeding occurs internally without an obvious external trigger.
Over time, vitamin K deficiency may also weaken bones. Vitamin K plays a role in activating proteins that help regulate calcium in bone tissue, so prolonged deficiency has been linked to reduced bone density.
Doctors typically suspect vitamin K deficiency when a blood clotting test called the prothrombin time (PT) comes back significantly prolonged while other clotting markers remain normal. An INR above 3.5, combined with a normal platelet count and normal levels of other clotting proteins, is highly suggestive of the deficiency.
Who Faces the Highest Risk
- People with chronic digestive diseases such as Crohn’s disease, celiac disease, or cystic fibrosis, because fat malabsorption reduces vitamin K uptake.
- People with liver or gallbladder disease that limits bile production or flow.
- Newborns who did not receive a vitamin K injection at birth, particularly those who are exclusively breastfed.
- People on long-term antibiotics combined with poor dietary intake.
- People taking warfarin or similar anticoagulants, who are in a state of intentional but carefully managed suppression.
- People with severely restricted diets or chronic alcohol use disorder.
In most of these groups, the deficiency develops gradually and can be corrected with supplementation or dietary changes once identified. The exception is newborn VKDB, which can progress rapidly and is best prevented with the routine injection at birth rather than treated after symptoms appear.