Vitamin K is well-known for its role in blood clotting.
But you may not know that its name actually refers to a group of several vitamins that provide health benefits far beyond helping your blood clot.
This article will review the differences between the two main forms of vitamin K found in the human diet: vitamin K1 and vitamin K2.
You’ll also learn which foods are good sources of these vitamins and the health benefits you can expect from eating them.
What Is Vitamin K?
Vitamin K is a group of fat-soluble vitamins that share similar chemical structures.
Vitamin K was accidentally discovered in the 1920s and 1930s after restricted diets in animals led to excessive bleeding ().
Although there are several different types of vitamin K, the two most often found in the human diet are vitamin K1 and vitamin K2.
Vitamin K1, also called phylloquinone, is mostly found in plant foods like leafy green vegetables. It makes up about 75–90% of all vitamin K consumed by humans ().
Vitamin K2 is found in fermented foods and animal products, and is also produced by gut bacteria. It has several subtypes called menaquinones (MKs) that are named by the length of their side chain. They range from MK-4 to MK-13.
Summary: Vitamin K refers to a group of vitamins that share a similar chemical structure. The two main forms found in the human diet are K1 and K2.
Food Sources of Vitamin K1
Vitamin K1 is produced by plants. It is the predominant form of vitamin K found in the human diet.
The following list includes several foods that are high in vitamin K1. Each value represents the amount of vitamin K1 in 1 cup of the cooked vegetable ().
- Kale: 1,062 mcg
- Collard greens: 1,059 mcg
- Spinach: 889 mcg
- Turnip greens: 529 mcg
- Broccoli: 220 mcg
- Brussels sprouts: 218 mcg
Summary: Vitamin K1 is the main type of vitamin K in the human diet. It is most commonly found in leafy green vegetables.
Food Sources of Vitamin K2
Food sources of vitamin K2 vary by subtype.
One subtype, MK-4, is found in some animal products and is the only form not produced by bacteria. Chicken, egg yolks and butter are good sources of MK-4.
MK-5 through MK-15 are forms of vitamin K2 with longer side chains. They are produced by bacteria and often found in fermented foods.
Natto, a popular Japanese dish made from fermented soybeans, is particularly high in MK-7.
Certain hard and soft cheeses are also good sources of vitamin K2, in the form of MK-8 and MK-9. Additionally, a recent study discovered several pork products contain vitamin K2 as MK-10 and MK-11 ().
The vitamin K2 content for 3.5 ounces (100 grams) of several foods is listed below (, , ).
- Natto: 1,062 mcg
- Pork sausage: 383 mcg
- Hard cheeses: 76 mcg
- Pork chop (with bone): 75 mcg
- Chicken (leg/thigh): 60 mcg
- Soft cheeses: 57 mcg
- Egg yolk: 32 mcg
Summary: Vitamin K2 food sources vary by subtype, though they include fermented foods and certain animal products.
Differences Between K1 and K2 in the Body
The main function of all types of vitamin K is to activate proteins that serve important roles in blood clotting, heart health and bone health.
However, because of differences in absorption and transport to tissues throughout the body, vitamin K1 and K2 could have profoundly different effects on your health.
In general, the vitamin K1 found in plants is poorly absorbed by the body. One study estimated that less than 10% of the K1 found in plants is actually absorbed ().
Less is known about the absorption of vitamin K2. Yet experts believe that because K2 is often found in foods that contain fat, it may be better absorbed than K1 ().
This is because vitamin K is a fat-soluble vitamin. Fat-soluble vitamins are much better absorbed when eaten with dietary fat.
Additionally, vitamin K2’s long side chain allows it to circulate in the blood longer than K1. Where vitamin K1 may stay in the blood for several hours, some forms of K2 can remain in the blood for days ().
Some researchers believe that the longer circulation time of vitamin K2 allows it to be better used in tissues located throughout the body. Vitamin K1 is primarily transported to and used by the liver ().
These differences are crucial to identifying the different roles vitamin K1 and K2 play in the body. The next sections investigate this topic further.
Summary: Differences in absorption and transportation of vitamin K1 and K2 in the body may lead to differences in their effects on your health.
Health Benefits of Vitamin K1 and K2
Studies investigating the health benefits of vitamin K have suggested that it may benefit blood clotting, bone health and heart health.
Vitamin K and Blood Clotting
Several proteins involved in blood clotting depend on vitamin K to get their job done. Blood clotting may sound like a bad thing, and sometimes it is. Yet without it, you could bleed excessively and end up dying from even a minor injury.
Some people have blood clotting disorders and take a medication called warfarin to prevent the blood from clotting too easily. If you take this medication, you should keep your vitamin K intake consistent due to its powerful effects on blood clotting.
Although most of the attention in this area focuses on food sources of vitamin K1, it may also be important to monitor vitamin K2 intake.
One study showed that a single serving of natto rich in vitamin K2 altered measures of blood clotting for up to four days. This was a much larger effect than foods high in vitamin K1 ().
Therefore, it is probably a good idea to monitor foods high in vitamin K1 as well as vitamin K2 if you are on the blood-thinning medication warfarin.
Vitamin K and Bone Health
Many experts believe vitamin K activates proteins required for bone growth and development ().
Several observational studies have correlated low levels of vitamin K1 and K2 with a higher risk of bone fractures, though these studies are not as good at proving cause and effect as controlled studies ().
Most controlled studies examining the effects of vitamin K1 supplements on bone loss have been inconclusive and shown little benefit ().
However, one review of controlled studies concluded that vitamin K2 supplementation as MK-4 significantly reduced the risk of bone fractures. Nevertheless, since this review, several large controlled studies have shown no effect (, ).
Overall, the available studies have been somewhat inconsistent, but the current evidence was convincing enough for the European Food Safety Authority to conclude that vitamin K is directly involved in the maintenance of normal bone health ().
More high-quality, controlled studies are needed to further investigate the effects of both vitamin K1 and K2 on bone health and determine whether there are any real differences between the two.
Vitamin K and Heart Health
In addition to blood clotting and bone health, vitamin K also seems to play an important role in preventing heart disease.
Vitamin K activates a protein that helps prevent calcium from depositing in your arteries. These calcium deposits contribute to the development of plaque, so it’s not surprising that they are a strong predictor of heart disease (, ).
Several observational studies have suggested that vitamin K2 is better than K1 at reducing these calcium deposits and lowering your risk of heart disease (, , ).
However, higher quality controlled studies have shown that both vitamin K1 and vitamin K2 (specifically MK-7) supplements improve various measures of heart health (, ).
Nevertheless, further studies are needed to prove that supplementing with vitamin K actually causes these improvements in heart health. Additionally, more research is needed to determine if K2 is truly better for heart health than K1.
Summary: Vitamin K1 and K2 are important for blood clotting, bone health and possibly heart health. Further research is needed to clarify if K2 is better than K1 at performing any of these functions.
Vitamin K Deficiency
True vitamin K deficiency is rare in healthy adults. It typically only occurs in people with severe malnutrition or malabsorption, and sometimes in people taking the medication warfarin.
Symptoms of deficiency include excessive bleeding that won’t stop easily, though this could also be caused by other things and should be evaluated by a physician.
Although you might not be deficient in vitamin K, it is possible that you aren’t getting enough vitamin K to help prevent heart disease and bone disorders like osteoporosis.
For this reason, it is important you get the appropriate amount of vitamin K your body needs.
Summary: True vitamin K deficiency is characterized by excessive bleeding and is rare in adults. However, just because you don’t have a deficiency doesn’t mean that you are getting enough vitamin K for optimal health.
How to Get Enough Vitamin K
The recommended adequate intake for vitamin K is based only on vitamin K1 and is set at 90 mcg/day for adult women and 120 mcg/day for adult men ().
This can easily be achieved by adding a cup of spinach to an omelet or salad, or by adding a 1/2 cup of broccoli or Brussels sprouts as a side for dinner.
Furthermore, consuming these with a source of fat like egg yolks or olive oil will help your body absorb the vitamin K better.
There is currently no recommendation on how much vitamin K2 you should be eating. It is best to try to incorporate a variety of vitamin K2-rich foods into your diet.
Below are some tips on how to do this.
- Try natto: Natto is a fermented food that is extremely high in vitamin K2. Some people don’t like the taste, but if you can stomach it, your K2 intake will skyrocket.
- Eat more eggs: Eggs are fairly good sources of vitamin K2 that can easily be added to your daily breakfast.
- Eat certain cheeses: Fermented cheeses, such as Jarlsberg, Edam, Gouda, cheddar and blue cheese, contain vitamin K2 formed by the bacteria used during their production.
- Consume dark meat chicken: The dark meat of chicken, such as leg and thigh meat, contains moderate amounts of vitamin K2 and may be better absorbed than the K2 found in chicken breasts.
Both vitamin K1 and vitamin K2 are also available in supplement form and often consumed in large doses. Although there are no known toxicities, further research is needed before specific recommendations for supplements can be given.
Summary: It is best to incorporate a variety of food sources of both vitamin K1 and K2 in your diet to obtain the health benefits that these vitamins offer.
The Bottom Line
Vitamin K1 is primarily found in leafy green vegetables, while K2 is most abundant in fermented foods and some animal products.
Vitamin K2 may be absorbed better by the body and some forms may stay in the blood longer than vitamin K1. These two things may cause K1 and K2 to have different effects on your health.
Vitamin K likely plays an important role in blood clotting and promoting good heart and bone health. Some research suggests that K2 may be superior to K1 in some of these functions, but further research is needed to confirm this.
For optimal health, focus on increasing food sources of both vitamin K1 and K2. Try to include one green vegetable daily and incorporate fermented foods and K2-rich animal products into your diet.