Taking extra vitamin K stops excessive blood loss in people with a deficiency in the nutrient because of illness or medications such as antibiotics. The vitamin also promotes strong bones. But it's easy to take too much vitamin K. A 1/2 cup of kale, for example. provides six days' worth of vitamin K. For some people, excessive vitamin K may be dangerous.
Foods and Supplements
If you need to limit your vitamin K intake, doctors suggest that you eat foods ranked high for the nutrient no more than once a day, and foods ranked moderately high no more than three times a day. Foods high in vitamin K include fresh parsley, kale, collards, spinach, mustard greens, turnip greens and Swiss chard. Dietary sources considered moderately high include dark salad greens, broccoli and Brussels sprouts. If you take a vitamin supplement, discuss with your doctor whether using one that contains vitamin K is appropriate for your health needs.
Overall Risk
According to the Linus Pauling Institute, it's important to know which form of vitamin K you are taking. The supplement comes in three forms, known as K-1, K-2 and K-3. For healthy adults, no known health problems exist for high amounts of K-1 and K-2. But K-3, also known as menadione, may cause liver damage, anemia or jaundice when taken in high amounts, especially by injection. K-3 is no longer given in high doses as a treatment for vitamin K deficiency because of this potential toxicity. K-3 is sometimes included in multivitamins, however, so use caution when taking vitamin supplements.
Drug Interaction
If you take blood thinners, how much vitamin K you consume each day is important, according to the Drug-Nutrient Interaction Task Force of the National Institutes of Health. Vitamin K prevents excessive blood loss in most people. But for people at risk of life-threatening blood clots, high doses of vitamin K can be problematic. The vitamin interferes with the effectiveness of blood thinners like warfarin, also known as Coumadin. If your blood-thinning medications don't work properly, you may be at increased risk for heart disease and stroke.
General Recommendations
For the general population, the recommended daily amount of vitamin K is 75 mcg for teens, 90 mcg for women, and 120 mcg for men. For optimal bone health, especially in older adults, your doctor may suggest that you take as much as 250 mcg a day, which you can attain through 1 cup of leafy greens and a multivitamin containing vitamin K. If you are taking blood thinners, talk to your doctor about whether you need to consume less vitamin K. In some cases, your doctor may wish you to have a higher amount of vitamin K, but to balance your warfarin prescription accordingly. It's important to take the prescribed amounts of both the vitamin and the medication consistently, notes the Drug-Nutrient Interaction Task Force.
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