You Almost Certainly Are Not Low on Vitamin K

Start with the sentence the aisle never prints. The NIH Office of Dietary Supplements states that vitamin K deficiency is very rare, and that most people in the United States get enough from the food they eat. Bacteria in your colon make some of it too.

There are real exceptions, and they are specific. Newborns who do not get the vitamin K shot at birth. People with cystic fibrosis, celiac disease, ulcerative colitis or short bowel syndrome, because those conditions cut how much they absorb. And people who have had weight loss surgery. If none of those is you, a shortage is not your problem.

Vitamin K is not one thing. K1 is phylloquinone, and it is the form in green leafy vegetables and vegetable oils. K2 is menaquinone, and it turns up in meat, cheese, eggs and soybeans. On a supplement label the same three names keep appearing: phylloquinone or phytonadione for K1, then MK-4 and MK-7 for K2.

So what does the extra form buy you? Here the honest answer is a shrug. ODS reports that people who eat more vitamin K rich foods tend to have stronger bones, that some supplement trials found better bone strength and fewer fractures, and that other trials did not. Its own line is that more research is needed. The heart question is at an even earlier stage.

It helps to remember what the bottle itself is. The NIH guidance on dietary supplements notes that the FDA does not check whether a supplement works before it goes on sale, and that a third party seal from a group like NSF or USP confirms what is inside the capsule without promising the capsule helps.

This is the part to read twice. Vitamin K can have a serious interaction with the blood thinner warfarin. ODS is precise about the rule: if you take warfarin, keep the amount of vitamin K you get from food and supplements about the same every day. A sudden change can cause dangerous bleeding if you take in less, or blood clots if you take in more.

Note what that rule is not. It is not "avoid greens." It is "do not swing." Other drugs pull in the same direction quietly: antibiotics taken for more than a few weeks, bile acid binders used for cholesterol, and the weight loss drug orlistat all reduce how much vitamin K you end up with. NCCIH's Know the Science module exists for exactly this kind of overlap.