Vitamin K is well known for its role in blood clotting, but supplements containing vitamin K are growing in popularity for other uses. Some people, especially older women, are taking vitamin K as a supplement to improve bone strength and reduce fractures. While there is evidence to support this use, it’s important to help patients pick the right supplement to ensure they are taking the right form of vitamin K.
Vitamin K1 (phylloquinone) and vitamin K2 (menaquinones) are available in North America as supplements. Vitamin K1 is the most common form of vitamin K found in the diet, so the recommended daily intake (RDI) for vitamin K (120 mcg daily for men and 90 mcg daily for women) is based mainly on vitamin K1. Most people following a Western diet can easily achieve the RDI for vitamin K by eating leafy greens like kale and spinach. A cup of raw spinach contains 145 mcg of vitamin K, which is more than 180% the daily value.
Vitamin K2 intake, on the other hand, is generally lower in the Western diet. For instance, a 3-ounce serving of chicken breast or ground beef contains vitamin K2, but only in amounts ranging from 6-13 mcg. Recently, it has been suggested that K1 and K2 be split into separate RDIs to ensure adequate intake of both. This might be important because vitamin K2 is distributed throughout the body, whereas vitamin K1 is absorbed by the liver. And vitamin K2 appears to be most promising for some of the more popular uses, including bone health.
The vast majority of supplements on the market contain vitamin K1, so patients wanting to take vitamin K2 should check product labels closely and avoid products that contain an unspecified type of vitamin K. Right now, it’s not clear which dose of vitamin K2 is the most effective for bone health, but most studies have used the MK-4 form of vitamin K2 at 45 mg daily. Some product labels might list this form of vitamin K2 as “menatetrenone” rather than MK-4. Also, make sure your patients understand that vitamin K supplements won’t provide the necessary dose for treating coagulation disorders. So, if they’re taking prescription vitamin K, Mephyton, they can’t replace it with a supplement. For more details about vitamin K for bone health, check out our recently updated monograph.
Reviewed February 2023
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