Vitamin K is a crucial nutrient that's often overlooked. It plays a central role in helping your blood clot, and it contributes to the maintenance of normal bones – yet it rarely gets the attention of the better-known vitamins. Here's how vitamin K works, where to find it, and when a supplement might help.
What does vitamin K do?
Vitamin K has two well-established, authorised roles. The first is in blood clotting: vitamin K contributes to normal blood clotting, the process that allows your blood to thicken and form a clot so that a wound can close and heal. It's such a defining function that the "K" even comes from Koagulation, the German spelling of coagulation.
Its second key role is in your skeleton: vitamin K contributes to the maintenance of normal bones. This is also why you'll often see it paired with vitamin D and calcium in bone-support supplements, since those nutrients work together in this area.
What are the best sources of vitamin K?
We get vitamin K mainly from the food we eat, and some of it also comes from the good bacteria that live in the gut. It exists in two main forms: vitamin K1, found primarily in dark leafy green vegetables, and vitamin K2, which comes from animal products and fermented foods.
The richest sources of K1 are leafy greens like kale, spinach, broccoli and Brussels sprouts – just half a cup (59g) of cooked kale provides well over your daily requirement. Vitamin K2 is found in lean meats, certain cheeses, and fermented foods like natto (fermented soybeans).
Here's a genuinely lovely detail: plants depend on vitamin K1 for photosynthesis – so when you eat kale, spinach or broccoli, you're essentially consuming the very nutrient plants use to turn sunlight into energy. The NHS recommends approximately 1µg of vitamin K per kilogram of body weight a day – so someone weighing 75kg would need around 75µg daily.
Should I take a vitamin K supplement?
Deficiency is rare, so if you eat a varied, healthy diet with plenty of leafy greens, you're likely getting all the vitamin K you need. That said, some groups may find a supplement helpful – for example, if you have a condition that affects how well you absorb nutrients, or your diet is very limited. In those cases, it's worth speaking with a healthcare professional about whether a supplement is right for you.
There's one especially important point with vitamin K. If you take blood-thinning (anticoagulant) medication such as warfarin, you should always speak to your GP or pharmacist before taking a vitamin K supplement. Because vitamin K is directly involved in blood clotting, it can affect how these medicines work – so this isn't one to start on your own if you're on those medications. It's genuinely important, rather than a routine disclaimer.
Vitamin K is often combined with other nutrients like vitamin D and calcium to support bones. Our vitamin K supplements come in various forms, including capsules and oral sprays, and are suitable for vegetarians and vegans, so you can choose what works best for you.
Explore further
This piece from our Discover blog picks up where this article leaves off:
- Vitamin D3 vs K2: which supplement do you need? – how vitamin K2 and vitamin D work together, and how to choose.
Explore our range of vitamin K supplements to support normal blood clotting and the maintenance of normal bones.