While calcium and vitamin D often are thought of as the most important nutrients for bone health, vitamin K also plays a role in preventing bone loss — but to what extent?
Studies suggest getting enough of this vitamin through food sources is likely needed to prevent excess bone loss and help prevent osteoporosis, but vitamin K continues to gain traction in the supplements aisle. Certainly a player in the bone health puzzle, it deserves to be considered, though research is conflicting about the importance of its role.
In food, vitamin K is synthesized by plants and found mainly in green leafy vegetables — such as spinach, Swiss chard, parsley, kale and broccoli — and some fruits — such as kiwi, avocado and grapes — in the form of vitamin K1 or phytonadione. Vitamin K1 also is available in supplement or "free form" in the United States, and research shows it is better absorbed than vitamin K1 from food sources. This vitamin is also one of the few that is produced in the lower gut of humans by bacteria in the colon in the form of vitamin K2 or menaquinone. Vitamin K2 is also found in some animal products, such as dairy and fermented foods.
Vitamin K intake requirements (percent daily value for adults is 80 micrograms μg) are easily met from food. For example, one cup of raw spinach or kale provides nearly twice the DV for adults. While dietary sources offer abundant vitamin K, some research studies suggest the supplemental "free form" is more readily absorbed by the body because it is bound to compounds within plants. Some terms used on labels include K1 as phylloquinone or phytonadione and K2 as MK-4 or MK-7.
According to the Natural Medicines Database, there is "insufficient reliable evidence to rate" vitamin K's effectiveness in the treatment of osteoporosis. There is, however, some promising research about this vitamin and bone health because it supports one of the main proteins that strengthens bones. A collection of 19 studies that included more than 6,000 participants found significant improvement in bone mineral density of the spine with the addition of vitamin K2 supplementation. Some studies suggest a higher dietary vitamin K intake is connected with a lower risk of bone fracture and higher bone mineral density, while others do not show a benefit.
Drug-nutrient interactions of moderate and major effects are important to discuss with patients interested in vitamin K supplementation. These include anti-diabetes drugs, blood thinning medications, the weight-loss drug Orlistat, bile acid medications, certain antibiotics and several vitamins. A tolerable upper limit for K1 and K2 has not been established due to lack of known toxicity, though risk is reportedly low and adverse effects are rare.
Unless patients have demonstrated vitamin K deficiency, there is little current evidence that additional vitamin K supplementation is helpful for the prevention of osteoporosis and fractures. Even in studies that show vitamin K benefits bone health, it's important to note that other bone-building compounds, such as vitamin D and calcium, are present. Consumers who choose to supplement should discuss it with their physician, use caution and look for independent quality assessment seals and third-party verification for added safety, as dietary supplements do not require FDA approval.