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Harvard Commentaries
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Reviewed by the Faculty of Harvard Medical School

Food for Thought Food for Thought

Don't Overlook Vitamin K

January 16, 2013

By Linda Antinoro, R.D., L.D.N., J.D., C.D.E.
Brigham and Women's Hospital

Four Ways Vitamin K May Protect Health

Unless you are taking the blood thinner warfarin (Coumadin), you probably have not paid too much attention to your intake of vitamin K. That's because this vitamin is best known for helping your blood clot so you stop bleeding. New research is finding other ways this vitamin may protect your health.

    1. Brittle bones – Vitamin K helps the body make a protein called osteocalcin, which can improve bone mass and reduce fractures. In one study, as blood levels of vitamin K increased so did levels of osteocalcin. Other research has found that individuals getting 250 micrograms of vitamin K a day had fewer hip fractures than those who got only one-fifth of that amount (50 micrograms).
    2. Diabetes – Osteocalcin may also have a role in regulating insulin activity. (Insulin is needed to help move sugar out of the bloodstream and into the body's cells.) This vitamin-K dependant protein may signal fat cells to release a hormone associated with increasing insulin sensitivity. Research so far has only included animal studies. Research on humans is needed to confirm this connection.
    3. Arthritis – A limited intake of vitamin K can damage cartilage and trigger osteoarthritis. (Cartilage helps to support and cushion your joints.) In one study involving over 600 men and women, those with higher blood levels of vitamin K were less likely to suffer from osteoarthritis of the knee. Vitamin K’s ability to protect against inflammation is another reason it may ease arthritis.
    4. Wrinkles – Some data suggest that this vitamin may protect the elasticity of the skin. This has the potential to prevent skin aging.

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Are Vitamin K Requirements Adequate?

The recommended intake for vitamin K is 90 micrograms a day for women and 120 micrograms for men. These guidelines are based on average amounts people eat. They don't reflect what the body actually needs and may not be enough, given the research on bone mass mentioned above.

Another reason that we probably need more vitamin K than recommended is that we no longer think that the body makes much vitamin K from natural bacteria in our guts. And even though vitamin K is a fat-soluble vitamin, which means that your body can store extra amounts, it's only for a short period of time. In fact, some experts argue that we should think of vitamin K as a water-soluble vitamin. This suggests that a regular intake of vitamin K would be needed to help prevent deficiencies.

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Sources of Vitamin K

Vitamin K is found in a variety of foods. But the amounts tend to be small. The exception is dark, leafy green vegetables — the real vitamin K stars.


Amount (micrograms)

Kale, 1/2 cup, cooked
Collards, 1/2 cup, cooked
Spinach, 1/2 cup, cooked
Swiss chard, 1/2 cup, cooked
Spinach, 1 cup, raw
Broccoli, ½ cup, cooked
Lettuce, 1 cup, raw
Parsley, 1 Tbsp., raw
Kiwi, 1 medium, raw
Blueberries, 1 cup, raw
Soybean oil, 1 Tbsp.

Source: USDA Nutrient Database, release 20. 2007

Vitamin K is also found in multivitamin or bone supplements. Some multivitamin supplements have zero but other popular brands have 20 to 30 micrograms per pill. Read vitamin labels closely. The amounts can vary within the same brand. For example, One A Day® WeightSmart has 80 micrograms of vitamin K per pill, while other multivitamins have considerably less. It's better to get your vitamin K by eating more leafy greens. Avoid taking a separate vitamin K supplement.

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The Coumadin Connection

Since vitamin K helps blood to clot, people who take the blood-thinning drug Coumadin need to be careful. Research has shown that as few as 25 micrograms of vitamin K a day are enough to interfere with Coumadin’s action.

So how can people who take Coumadin reap the benefits of vitamin K? The key is to be consistent with the amount of vitamin K you normally consume. That way, your doctor can usually modify your dose of Coumadin accordingly. It is always best though to let your doctor know if you plan to increase your intake of vitamin K.

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The Bottom Line

Most of us can afford to consume more vitamin K. The best way to make sure you're getting an adequate and consistent amount, is to include dark leafy greens in your diet regularly and take a daily multivitamin that contains vitamin K.

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Linda Antinoro, R.D., L.D.N., J.D., C.D.E., is a senior nutritionist at Brigham and Women’s Hospital. She is also a certified diabetes educator. Ms. Antinoro counsels patients at the Nutrition Consultation Service.

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