Vitamin K is a fat-soluble vitamin your body needs for normal blood clotting and for building healthy bones. Most healthy adults get enough from food, but people on blood thinners such as warfarin need to understand it well.
This guide explains how much vitamin K you need, which foods are rich in it, what deficiency looks like, and which medicines interact with it.
What does vitamin K do
Vitamin K helps the liver make proteins that allow blood to clot. Without enough of it, even a small cut can bleed for longer than normal. It also helps proteins in bone bind calcium, so it supports bone health.
- K1 (phylloquinone): found mainly in green leafy vegetables and vegetable oils.
- K2 (menaquinones, such as MK-4 and MK-7): found in some animal foods and fermented foods, and also made by gut bacteria.
Daily vitamin K requirement
The US National Institutes of Health lists the following recommended amounts (mcg per day):
- Children 1 to 3 years: 30 mcg
- Children 4 to 8 years: 55 mcg
- Children 9 to 13 years: 60 mcg
- Teens 14 to 18 years: 75 mcg
- Adult men: 120 mcg
- Adult women: 90 mcg
- Pregnant or breastfeeding women: 90 mcg (75 mcg for teens)
No upper limit has been set for vitamin K because it has not been shown to cause harm at normal intakes.
Best food sources of vitamin K
Green leafy vegetables are the richest source.
- Spinach, kale, broccoli and lettuce
- Vegetable oils such as soybean oil
- Blueberries and figs
- Meat, cheese and eggs
- Soybeans
Vitamin K deficiency: signs and who is at risk
Deficiency is very rare in healthy adults. When it happens, it causes easy bruising and bleeding, and possibly weaker bones over time.
People at higher risk include:
- Newborns who do not receive the vitamin K shot at birth
- People with cystic fibrosis, celiac disease, ulcerative colitis or short bowel syndrome
- People who have had weight-loss (bariatric) surgery
If you notice unusual bleeding or bruising, see a doctor rather than self-treating with supplements.
Medicine interactions: warfarin and others
Vitamin K and medicines can affect each other. Talk to your doctor before changing your diet or taking supplements if you use any of these:
- Warfarin: it works by blocking vitamin K, so keep your vitamin K intake consistent from week to week instead of avoiding green vegetables.
- Long-term antibiotics: can lower vitamin K made by gut bacteria.
- Bile acid sequestrants (cholestyramine, colestipol) and orlistat: can reduce absorption.
Frequently Asked Questions
How much vitamin K do adults need per day?
The NIH lists 120 mcg per day for adult men and 90 mcg per day for adult women.
Which foods are highest in vitamin K?
Green leafy vegetables such as spinach, kale, broccoli and lettuce are the richest sources, along with vegetable oils.
Can I eat green vegetables while on warfarin?
Yes, but keep the amount fairly consistent every week. Sudden big changes can alter how well warfarin works, so follow your doctor’s advice.
Is vitamin K toxic in high doses?
No upper limit has been set and vitamin K has not been shown to cause harm, but supplements should still be taken only with medical advice, especially with blood thinners.
Why do newborns get a vitamin K shot?
Newborns have low vitamin K stores, so the shot helps prevent serious bleeding in the first weeks of life.
Related reading: Calcium daily needs and supplement safety | Vitamin D daily needs and deficiency | Magnesium daily needs and supplement safety
Sources
Medical disclaimer: this article is for general information and does not replace advice from a qualified doctor. Do not start, stop or change any supplement or medicine without medical advice.