Vitamin K

What it does

Vitamin K is best known for its essential role in blood clotting — the "K" comes from the German Koagulation. It is required to activate several of the clotting factors the body uses to stop bleeding after an injury. Without it, these factors cannot do their job and even minor injuries can bleed excessively, which is why newborn babies, who have low stores, are routinely offered vitamin K to prevent rare but serious bleeding.

Vitamin K also has an important role in bone health. It activates proteins such as osteocalcin that help bind calcium into the bone matrix, contributing to bone strength. Related proteins in blood vessel walls use vitamin K to help keep calcium where it belongs (in bone) rather than being deposited in arteries, linking the vitamin to cardiovascular as well as skeletal health.

It comes in two main forms: vitamin K1 from green leafy vegetables such as kale, spinach and broccoli, and vitamin K2 from fermented foods and some animal products, with a further contribution from bacteria in the gut. Because vitamin K directly affects clotting, people taking warfarin — a blood thinner that works by blocking vitamin K — are advised to keep their intake steady rather than swinging between very high and very low amounts.