The Benefits and Risks of Dietary Vitamin K

When you bite into a forkful of dark, leafy greens like kale or spinach, you ingest lots of vitamin K. That’s good, because K is crucial for your health, especially for blood clotting and bone strength.

But vitamin K is sometimes misunderstood. Because of its clotting properties, it can interfere with an anticoagulant (blood thinner) called warfarin (Coumadin). New blood thinners, such as rivaroxaban (Xarelto), dabigatran (Pradaxa), and apixaban (Eliquis), do not interact with vitamin K.

But, because of its effect on warfarin, some people try to avoid eating foods rich in vitamin K. In most cases, however, that’s unnecessary.

What Is Vitamin K?

Vitamin K is an umbrella term that refers to phylloquinone (K1), which comes from plants, and a series of menaquinones (K2), which are converted from K1 by bacteria in the gut. K2 is then stored in the liver and fatty tissue. Vitamin K2 helps build bones.

High vitamin K intake is associated with increased bone density in women and a reduced risk of hip fracture in men and women.

Vitamin K also assists with the production of four of the 13 proteins needed for clotting. Blood clotting is essential to prevent uncontrolled bleeding if you have a cut or need surgery. It also may help prevent hardening of the arteries, reduce the risk of developing coronary artery disease, and increase insulin sensitivity.

It’s Not Potassium

Sometimes vitamin K is confused with potassium because the letter K is the symbol for potassium on the periodic table of elements. But vitamin K and potassium are different. Potassium counters the effects of sodium and lowers blood pressure. It also is important for muscle contraction, balancing body fluids, and maintaining a steady heartbeat. Vitamin K is a fat-soluble vitamin.

Warfarin and Vitamin K

People take warfarin to help prevent dangerous clots from forming in the arteries and veins. Conditions that can lead to such clots include deep vein thrombosis (a clot in the deep veins in the leg) and atrial fibrillation (when the top chambers of the heart beat erratically, causing the blood to pool and possibly clot).

Warfarin decreases the production of vitamin K-dependent clotting factors, which makes it harder for your body to form clots. Consuming too much vitamin K may decrease warfarin’s effectiveness. Consuming too little can make it difficult for your blood to clot at all.

If you take warfarin, you can still eat foods that contain vitamin K. However, you must keep your K intake consistent and report it to your doctor. This, along with test results that measure how fast your blood clots, will help your doctor determine your warfarin dosage.

Vitamin K Intake

Adequate intake of vitamin K for people age 19 or older is 120 micrograms (mcg) for men and 90 mcg for women. That can be easily achieved through diet.

One of the most common dietary sources of vitamin K1 is leafy vegetables. You’ll find 530 mcg of vitamin K1 in a half cup of frozen, boiled collard greens; 426 mcg in a half cup of frozen, boiled turnip greens; 145 mcg in a cup of raw spinach; 113 mcg in a cup of raw kale; and 110 mcg in a cup of chopped, boiled broccoli.

Vegetable oils and some fruits also contain vitamin K1. There’s 10 mcg in a tablespoon of canola oil and 8 mcg in a tablespoon of olive oil. A half cup of fresh blueberries has 14 mcg, while a half cup of grapes has 11 mcg.

Some meat, dairy foods, and eggs contain modest amounts of vitamin K2 (vitamin K1 that’s been metabolized by animals). Three ounces of rotisserie chicken breast has 13 mcg, and 3 ounces of broiled ground beef has 6 mcg.

Eating a variety of fresh, healthy foods should be enough to help you consume enough vitamin K each day. If you have concerns about whether you are taking in too much or too little vitamin K, talk to your doctor or a registered dietitian nutritionist.

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