Vitamin K deficiency happens when your body can’t get or absorb enough vitamin K to support normal blood clotting. In healthy adults, outright deficiency from diet alone is uncommon because the vitamin is widespread in leafy greens, vegetable oils, and certain fermented foods, and gut bacteria produce a form of it as well. The more typical path to deficiency involves something interfering with absorption: a digestive condition, medications, liver or gallbladder problems, or surgery that alters the gut. Newborns are a special case, born with almost no vitamin K stores at all.
Why Diet Alone Rarely Causes It
Vitamin K1 is found in green vegetables like kale, spinach, broccoli, and lettuce, as well as in soybean and canola oil. A second form, vitamin K2, is produced by bacteria living in your large intestine and is also present in fermented foods and some animal products. Between these two sources, most people eating a reasonably varied diet get enough. Pure dietary deficiency typically only shows up in people who are severely malnourished or on extremely restricted diets for a prolonged period.
Fat Malabsorption Disorders
Vitamin K is fat-soluble, meaning your intestines need dietary fat and bile to absorb it properly. Any condition that disrupts fat absorption can starve your body of vitamin K even if your diet contains plenty of it. Celiac disease, Crohn’s disease, ulcerative colitis, and cystic fibrosis all fall into this category. In these conditions, the intestinal lining is inflamed or damaged, and fat-soluble nutrients pass through without being taken up.
Short bowel syndrome, where a significant portion of the small intestine has been surgically removed, creates the same problem by reducing the surface area available for absorption.
Liver and Gallbladder Problems
Your liver produces bile, which gets stored in the gallbladder and released into the small intestine during meals. Bile acts as a detergent that breaks dietary fats into small enough droplets for your gut to absorb. When bile flow is blocked or reduced, a condition called cholestasis, fat-soluble vitamins including vitamin K can’t be absorbed properly. Gallstones, bile duct strictures, and certain liver diseases all reduce bile availability.
What makes cholestasis particularly stubborn is that even taking vitamin K by mouth may not fix the problem, because intestinal absorption itself is compromised without adequate bile. In these cases, the vitamin often needs to be delivered by injection to bypass the gut entirely.
Medications That Deplete Vitamin K
Several drug classes interfere with vitamin K levels through different mechanisms:
- Antibiotics such as tetracycline and trimethoprim-sulfamethoxazole kill off gut bacteria that produce vitamin K2. A long course of broad-spectrum antibiotics can significantly reduce the intestinal supply.
- Cholestyramine, a cholesterol-lowering drug, binds bile acids in the gut. Since bile is essential for absorbing fat-soluble vitamins, this medication reduces vitamin K uptake as a side effect.
- Blood thinners like warfarin work by deliberately blocking the vitamin K cycle. They don’t lower vitamin K levels directly, but they prevent the body from using it, creating a functional deficiency that’s the whole point of the drug.
If you’re on any of these medications long-term, your doctor may monitor your clotting function periodically.
How Gut Bacteria Factor In
Bacteria in your large intestine produce a family of vitamin K2 compounds called menaquinones, which they use for their own energy metabolism. These menaquinones also serve as a secondary vitamin K source for your body. Disruptions to the gut microbiome, whether from antibiotics, illness, or major dietary changes, can reduce this bacterial production.
Interestingly, the relationship runs both ways. Some gut bacterial species have lost the ability to make their own menaquinones and instead depend on neighboring bacteria to supply them. When overall vitamin K availability in the gut drops, these dependent species may get outcompeted, which can shift the composition of the entire microbial community. This means that a period of low vitamin K intake could reshape your gut flora in ways that further reduce internal production, creating a feedback loop.
Bariatric Surgery
Weight-loss surgeries that reroute or reduce the digestive tract create lasting risk for fat-soluble vitamin deficiencies. A study of pregnant women who had previously undergone bariatric surgery found their average vitamin K1 levels were less than half those of women who hadn’t had surgery (0.29 versus 0.61 ng/mL). The risk was fivefold higher overall in post-bariatric pregnancies, and gastric bypass carried a dramatically higher risk than sleeve gastrectomy, with a 17-fold increase in the odds of deficiency compared to the sleeve procedure.
These deficiencies can persist years after surgery because the anatomical changes are permanent. People who’ve had bariatric procedures generally need lifelong monitoring and supplementation of fat-soluble vitamins.
Why Newborns Are Especially Vulnerable
Babies are born with very little vitamin K. Only small amounts cross the placenta during pregnancy, and the gut bacteria that produce vitamin K2 haven’t yet colonized the newborn’s intestines. Breast milk, while excellent nutrition in other respects, contains low levels of vitamin K. This combination leaves exclusively breastfed infants at particular risk for a condition called vitamin K deficiency bleeding (VKDB).
VKDB can occur in the first days of life or as late as six months. Late-onset VKDB is especially dangerous because it frequently involves bleeding in the brain. In roughly one-third of late cases, bruising on the skin appears weeks before more serious bleeding starts, which can serve as an early warning sign. This is why newborns routinely receive a vitamin K injection shortly after birth: a single 1.0 mg dose for babies over 1,500 grams, or 0.5 mg for smaller infants.
Recognizing the Signs
Vitamin K deficiency shows up as a bleeding problem because the vitamin is essential for making several clotting proteins. In adults, the earliest sign is often easy bruising or bruises that seem disproportionate to the bump that caused them. Nosebleeds, bleeding gums, and blood in the stool or urine can follow. Wounds may ooze longer than expected, and in severe cases, internal bleeding can occur without obvious injury.
The only clinically reliable way to confirm vitamin K deficiency is through a test called prothrombin time, which measures how long your blood takes to clot. When clotting time is significantly prolonged and other common causes (like low platelets) are ruled out, vitamin K deficiency becomes the leading suspect. Blood levels of vitamin K itself are less useful because people with levels slightly below the normal range (which spans roughly 0.29 to 2.64 nmol/L in healthy adults) often have no clotting problems at all. The functional test, how well your blood actually clots, matters more than the raw number.