Vitamin K

What Is Vitamin K?

Vitamin K takes its letter from Koagulation, the German spelling of coagulation, the scientific word for clotting. Its job is to switch on proteins. An enzyme uses vitamin K to attach an extra acid group to certain proteins, and only then can they bind calcium and do their work.

Four of the clotting factors made in the liver, prothrombin among them, need that step, which is why a shortage shows as easy bruising and bleeding. Bone uses the same step to activate osteocalcin, a protein that binds calcium into bone, and blood vessels use it for matrix Gla protein, which may help keep calcium out of artery walls.

Vitamin K1 and K2: Where Each Comes From

Vitamin K1 (phylloquinone) is made by plants as part of photosynthesis, so the greener the leaf, the more it has. It is most of the vitamin K in a Western diet. Vitamin K2 is a family of menaquinones, named MK-4 to MK-13 by the length of their side chain. Bacteria make most of them, so K2 turns up in fermented foods, cheese, egg yolk and meat. MK-4 is the exception: animal and human tissues make it from K1.

Natto, Japanese fermented soybeans, is the standout: about 850 mcg of K2 in 3 oz, according to the NIH. Bacteria in the colon also make K2, but how much of it the body uses is not known.

In leaves, K1 is held tightly inside the chloroplasts, so the body absorbs less of it than from oils or supplements. Vitamin K is fat-soluble: greens eaten with an oil dressing or alongside a meal with fat give up more of it.

Vitamin K in a Day of Food With One Salad

One ordinary day built only from foods in the Caltide food library, in usual portions, with a side salad and broccoli but no spinach or kale. No supplements.

Vitamin K and Warfarin

Warfarin thins the blood by blocking the recycling of vitamin K. Its dose is set around what a person usually eats, so the risk comes from change, not from vitamin K itself. People on warfarin are advised to keep their intake of greens and any supplement containing vitamin K about the same from week to week, and to tell their doctor before changing it.

The sample day above gives 177 mcg. Swapping its lettuce for a cup of raw spinach adds about 110 mcg more, the kind of jump that can change how warfarin works.

Who Is Most Likely to Be Low in Vitamin K

Deficiency is rare in healthy adults. Newborns are the main exception: little vitamin K crosses the placenta and breast milk has little, so a single vitamin K shot at birth is standard care. In adults, low levels come with fat malabsorption, such as celiac disease, cystic fibrosis or after bariatric surgery, and sometimes with long courses of antibiotics, which can reduce the gut bacteria that make K2.