Vitamin K is essential for blood clotting, bone strength, and cardiovascular health. Without it, your liver cannot produce the proteins that stop bleeding, your bones lose their ability to absorb calcium properly, and your blood vessels become vulnerable to mineral buildup. Adults need between 90 and 120 micrograms daily, an amount most people can get from a single serving of leafy greens.
Blood Clotting: Vitamin K’s Primary Job
Vitamin K’s most critical function is enabling your blood to clot. Your liver needs it to manufacture four specific clotting factors. Without these proteins, even a minor cut or bruise could lead to prolonged, uncontrolled bleeding. This is why the vitamin was named “K” in the first place, from the German word “Koagulation.”
The process works like this: vitamin K acts as a helper molecule for an enzyme that “switches on” clotting proteins by modifying their structure so they can bind calcium. Calcium binding is what allows these proteins to stick to the site of an injury and form a clot. When vitamin K is missing, the clotting proteins remain in an inactive form, and bleeding continues unchecked.
How It Strengthens Bones
Vitamin K plays a less well-known but equally important role in bone health. Your bone-building cells produce a protein called osteocalcin, which is responsible for pulling calcium out of the bloodstream and locking it into your bone structure. But osteocalcin starts out in an inactive form. It needs vitamin K to be chemically modified before it can bind calcium and the mineral framework of bone effectively.
Without enough vitamin K, inactive osteocalcin accumulates in the blood. In this state, it has limited ability to support bone mineralization. Researchers have identified elevated levels of inactive osteocalcin as a marker of poor vitamin K status, and this incomplete activation may contribute to weaker bones over time.
Protecting Your Blood Vessels
Vitamin K activates another protein that works in the walls of your arteries to prevent calcium from depositing where it shouldn’t. This protein is secreted by cells in the artery walls and acts as a natural calcification inhibitor. Just like osteocalcin in bone, it requires vitamin K to become fully functional.
The consequences of losing this protection are dramatic. In animal studies, organisms genetically unable to produce this protein developed massive arterial calcification within weeks and died from ruptured blood vessels within two months. In humans, higher blood levels of the inactive (vitamin K-deprived) form of this protein are associated with worse cardiovascular outcomes. Vitamin K supplementation has been shown to improve arterial stiffness in healthy postmenopausal women, suggesting this protective mechanism can be supported through diet.
In short, vitamin K helps direct calcium to where your body needs it (bones) and away from where it causes harm (arteries).
K1 vs. K2: Two Forms, Different Sources
Vitamin K comes in two main forms. Vitamin K1 (phylloquinone) is made by plants and is the dominant form in most diets. You’ll find it in high concentrations in green leafy vegetables like kale, spinach, broccoli, and collard greens, as well as certain plant oils like soybean and canola oil. A single cup of cooked kale can provide several times your daily needs.
Vitamin K2 (menaquinones) is a family of related compounds mostly produced by bacteria. You’ll find K2 in fermented foods like natto (a Japanese fermented soybean dish, which is by far the richest source), certain cheeses, egg yolks, and other animal products. Bacteria in your large intestine also synthesize K2, though it’s unclear how much of that your body actually absorbs and uses.
Both forms support clotting, but some research suggests K2 may be more relevant for bone and cardiovascular health because of its distribution in the body and longer circulation time. K1 is rapidly cleared by the liver for clotting factor production, while certain K2 subtypes remain available in the bloodstream longer, reaching tissues like bone and blood vessel walls more effectively.
How Much You Need
The recommended adequate intake for vitamin K is 120 mcg per day for adult men and 90 mcg per day for adult women. Children need less, ranging from 30 mcg at ages 1 to 3 up to 75 mcg for teenagers. Newborns have the lowest requirement at 2 mcg per day, though they face unique challenges meeting even that small amount.
These targets are relatively easy to hit with a diet that includes vegetables. Half a cup of cooked broccoli or a small salad with spinach gets most people close to or above their daily need for K1. People who eat very few vegetables, those with fat absorption disorders (vitamin K is fat-soluble and requires dietary fat for absorption), and those on long-term antibiotics that wipe out gut bacteria are at greatest risk for falling short.
Why Newborns Get a Vitamin K Shot
Babies are born with very little vitamin K in their bodies. The vitamin doesn’t cross the placenta easily during pregnancy, and a newborn’s gut hasn’t yet developed the bacterial population needed to produce K2. This combination leaves infants vulnerable to a condition called vitamin K deficiency bleeding, which can cause uncontrolled bleeding into the brain and other organs during the first weeks of life.
A single injection of vitamin K into the thigh muscle shortly after birth effectively prevents this. The CDC identifies this shot as the most reliable protection against a condition that, while rare with prophylaxis, can cause brain damage or death when it occurs. Infants who don’t receive the injection may present with bruising, bleeding from the umbilical cord, or in severe cases, intracranial hemorrhage with seizures and poor feeding.
Signs of Deficiency
Vitamin K deficiency is uncommon in healthy adults who eat a varied diet, but when it occurs, the signs center on bleeding. Easy bruising is often the first clue. Small red or purple spots on the skin (petechiae), larger patches of discoloration, and prolonged bleeding from cuts, surgical sites, or blood draws are characteristic. In severe cases, internal bleeding can occur without any obvious trauma.
Deficiency shows up in blood tests as a prolonged prothrombin time, meaning the blood takes significantly longer than normal to form a clot. People on long-term antibiotics, those with liver disease, and individuals with conditions that impair fat absorption (such as celiac disease, Crohn’s disease, or cystic fibrosis) are most susceptible.
Vitamin K and Blood Thinners
If you take warfarin or a similar blood-thinning medication, vitamin K intake matters more than it does for most people. Warfarin works by blocking the recycling of vitamin K in your liver, which reduces the production of clotting factors. Eating more vitamin K-rich foods counteracts this effect, while eating less amplifies it.
The key is consistency rather than avoidance. You don’t need to stop eating green vegetables. You need to eat roughly the same amount of them from week to week so your medication dose stays properly calibrated. Sudden changes, like starting a new diet heavy in salads or cutting out vegetables entirely, can push your clotting levels out of the safe range. If you’re on warfarin, your care team monitors your clotting time regularly and adjusts your dose based in part on your dietary habits.
Vitamin K’s Role in the Brain
Vitamin K is also involved in the nervous system, though this function gets far less attention. It’s required for the production of sphingolipids, a class of fats found in high concentrations in brain cell membranes. These compounds aren’t just structural. They play active roles in cell signaling, helping brain cells communicate and respond to their environment.
Research exploring the link between vitamin K status and cognitive function is still limited, but early findings suggest that both dietary intake and medications that interfere with vitamin K (like warfarin) may influence brain health. This is an area where the science is young, but the biological rationale for vitamin K’s importance in the nervous system has been recognized for over 40 years.