Vitamin K1 is a fat-soluble vitamin found in some leafy greens. It’s primarily responsible for helping your blood clot.

Further down the alphabet, after vitamins A through E, lies one more critical nutrient: vitamin K. This micronutrient helps your blood clot and may have other benefits, such as promoting bone health and preventing the hardening of arteries.

You’ve probably heard of vitamin K, but you may not realize that it comes in two forms: vitamin K1 and vitamin K2.

Vitamin K1 is found in certain leafy green vegetables, whereas vitamin K2 is found in animal foods. Though they perform many of the same functions in the body, they also have subtle differences.

Here’s what to know about vitamin K1.

Learn more about vitamin K.

Though vitamin K1 isn’t required to be listed on nutrition facts labels, it’s still an important nutrient for human health. Its main role is helping blood clot properly. In fact, the K in its name comes from the German “Koagulation.”

In everyday circumstances, typical clotting allows you to develop scabs that protect you from bleeding excessively. This is also the case in more serious wounds. Without clotting, bleeding could become life threatening.

Limited research has examined vitamin K1’s effects on other areas of wellness, including bone health. Though some studies have found a correlation between high vitamin K intake and low bone mineral density, others indicate that elevated intake could be protective.

In some research, consuming more vitamin K has led to higher bone mineral density and a lower risk of hip fracture.

Reduced risk of heart disease is another possible benefit of vitamin K1. This nutrient is involved in the production of a group of proteins that prevent arterial hardening. So, some studies have examined whether vitamin K1 supplements could help prevent heart disease.

More research is needed to draw firm conclusions about the connection between vitamin K and cardiovascular risk.

The only way to know if you’re getting enough vitamin K1 is to have a blood test. However, most people can consume enough through a varied diet.

According to the National Institutes of Health, adequate vitamin K intake is as follows:

  • Children birth to 6 months old: 2 micrograms (mcg)
  • Children 7–12 months old: 2.5 mcg
  • Children 1–3 years old: 30 mcg
  • Children 4–8 years old: 55 mcg
  • Children 9–13 years old: 60 mcg
  • Children 14–18 years old: 75 mcg
  • Adult males 19 and over: 120 mcg
  • Adult females 19 and over, including those pregnant and nursing: 90 mcg

Vitamin K1 deficiency is more common in children (especially newborns) than adults. This is because the placenta does not transfer vitamin K very well, and breastmilk may also be low in the vitamin. Still, anyone can have too little.

Symptoms of vitamin K deficiency include bleeding excessively, bruising easily, heavy periods, and/or blood in the stool.

It’s extremely rare to get too much vitamin K1. For this reason, the Dietary Reference Intakes (DRIs) set no tolerable upper limit for consumption.

If you eat lots of foods high in vitamin K1, such as leafy greens, you’re more likely to experience gastrointestinal upset from excessive fiber than any side effects from too much vitamin K1.

For people who take certain medications, though, such as blood thinners, too much vitamin K1 may be problematic. Be sure to discuss your vitamin K1 intake with your doctor or dietitian if you take a blood thinner.

When it comes to vitamin K1, think: greens, greens, and more greens. Leafy greens are the top sources of this nutrient. Cooked greens tend to contain more than fresh (but this isn’t always the case). Because vitamin K1 is fat-soluble, including a source of fat, such as salad dressing, boosts absorption.

Some of the foods with the highest levels of vitamin K1 include:

  • cooked kale
  • cooked turnip greens
  • cooked collard greens
  • cooked spinach
  • raw dandelion greens
  • raw Swiss chard
  • raw arugula
  • dried herbs, such as basil, thyme, oregano, and marjoram

Blood thinners like warfarin (brand name Coumadin) typically come with a warning not to take vitamin K1 (and even to avoid foods high in vitamin K) during use. Besides these drugs, antacids, some antibiotics, and certain drugs for cancer, seizures, and high cholesterol may be affected by vitamin K1.

For certain groups of people, extra vitamin K1 can be harmful. This is especially true for people on blood thinners since vitamin K boosts blood clotting.

People on dialysis for kidney disease, people with liver or gallbladder disease, and those with intestinal problems should also discuss vitamin K1 usage with their doctor.

You may also be advised to avoid vitamin K1 supplements during pregnancy since they may cause jaundice in infants.

What is vitamin K1 used for?

Vitamin K1 is primarily used to enhance blood clotting and prevent excessive bleeding. However, some people may be instructed by their doctor to take it as part of a heart health regimen.

Do vitamins K1 and K2 do the same thing?

Though vitamins K1 and K2 come from different food sources, they serve many of the same functions in the body. That said, K2 may be better at reducing the risk of heart disease than K1. More research is needed to be certain.

What food is highest in vitamin K1?

Dark leafy greens like kale are some of the highest in vitamin K1. For example, 1 cup of cooked kale contains 1,062 micrograms, over 10 times the daily recommended dose.

Who should not take vitamin K1?

If you take certain medications, such as blood thinners (including warfarin, sometimes called Coumadin), you should not take vitamin K1. People with rare metabolic conditions that affect blood clotting, such as glucose-6-phosphate dehydrogenase deficiency, should also steer clear of vitamin K.

Most people can consume enough vitamin K1 from a healthy, varied diet. Focus on including plenty of dark green leafy vegetables — and if you have concerns that your levels might be too low, talk with your doctor.