Vitamin K is needed to make proteins that help you clot. It also helps regulate bone growth and density.
Because vitamin K is used to make clotting proteins, a deficiency of vitamin k leads to clotting problems, specifically excessive bleeding or taking a long time to stop bleeding. There is also some evidence that low vitamin K intakes leads to osteoporosis, but we’re not sure if that’s because of the vitamin K specifically or just the result of an overall poor diet.
Your body does not store very much vitamin K and a lot of what you take in is excreted, either through urine or feces. There’s no known toxicity with either of the types of vitamin k found in food, K1 or K2. So there’s no concern about getting too much vitamin k.
The current recommendation from the National Institute of Health is for adult men to get 120 micrograms a day and adult women to get 90 micrograms a day. But a study in 2000 found that ~250mcg/day of vitamin K helped prevent hip fractures and 120 mcg/day did not. Since there’s evidence that 250mcg/day is healthier than 120mcg/day, and since there’s no evidence that too much vitamin k is harmful, I recommend everyone get at least 250mcg/day of vitamin K. This is easily obtained with just a small amount of dark, leafy greens or supplements.
Dark, leafy greens! Since vitamin K is used for photosynthesis, green vegetable leaves have the highest concentrations. Some oils have a small amount of vitamin k, and most other foods don’t have any appreciable amount. The bacteria in your large intestine make Vitamin K, but we’re not sure if they make enough to meet your needs.
Vitamin K can be supplemented in either K1 or K2 forms, which are equally effective. However, the form K3 (menadione) should not be used because it can be toxic. It is a man-made form of vitamin K that does not occur in nature. It has been banned in the US because it can be toxic.
There is no toxicity from Vitamin K. It does not cause clotting disorders. However, if you already have a clotting disorder, you might be on medicines that inhibit Vitamin K, and in this case excessive vitamin K might overwhelm your medicine. Your doctor will recognize if you’re on a medicine that might interact with Vitamin K. Otherwise, there are no dangers from Vitamin K.