What to know about vitamin K-2
Last reviewed Mon 29 April 2019
By Jamie Eske
Reviewed by Katherine Marengo LDN, RD
- What is it? Functions K-1 vs. K-2 Dietary sources Health benefits Daily intakeDeficiency symptoms Supplements Summary
Vitamin K is an essential vitamin that supports blood clotting and healthy bones. It occurs in two forms, K-1 and K-2.
Vitamin K-1 is the primary form, and it mainly comes from leafy green vegetables. Vitamin K-2 occurs in animal proteins and fermented foods. The bacteria in the human gut also produce small quantities of K-2.
In this article, we discuss vitamin K-2, its functions, and how it differs from K-1. We also describe dietary sources, health benefits, the recommended daily intake, deficiency symptoms, and supplements.
What is it?
Sauerkraut is a good dietary source of vitamin K-2.
Vitamin K refers to a family of fat-soluble vitamins that the body needs to produce a protein called prothrombin, which promotes blood clotting and regulates bone metabolism.
The vitamin comes in two main forms:
- Vitamin K-1, or phylloquinone, occurs naturally in dark leafy green vegetables and is the main dietary source of vitamin K.
- Vitamin K-2, or menaquinone, is present in small quantities in organ meats and fermented foods. Gut bacteria also produce vitamin K-2.
The body needs both types of vitamin K to produce prothrombin, a protein that plays crucial roles in blood clotting, bone metabolism, and heart health. Vitamin K also helps facilitate energy production in the mitochondria of cells.
Vitamin K-1 is primarily involved in blood coagulation. K-2 may have a more diverse range of functions in the body.
In a long-term study involving 36,629 participants, researchers observed an association
between high intakes of vitamin K-2 and a reduced risk of developing peripheral arterial disease(PAD), particularly in people with high blood pressure. However, the authors concluded that K-1 had no effect on PAD risk.
Vitamin K has antioxidant properties. It protects cellular membranes from damage due to excess free radicals, in a process known as peroxidation. Blood thinning medication, such as warfarin, can lower the antioxidative potential of vitamin K.
K-1 vs. K-2
Vitamins K-1 and K-2 have different chemical structures. Both types have a phytyl side chain, but K-2 also has isoprenoid side chains.
K-2 has several subtypes, called menaquinones (MKs), which scientists have numbered MK-4 through MK-13, based on the length of their side chains.
K-1 is the primary form of the vitamin, and it is mainly present in leafy green vegetables. However, the body has difficulty absorbing vitamin K-1 from plants.
According to a 2019 review, research suggests that the body absorbs 10 times more vitamin K-2, in the form of MK-7, than vitamin K-1.
Vitamin K is fat-soluble, so eating dietary fats, such as butter or plant oils, may enhance the body's absorption of vitamin K-1 from plants.
Bacteria in the gut can synthesize vitamin K-1 into vitamin K-2. Also, fermented foods, meat, and dairy products contain modest amounts of vitamin K-2.
The body stores vitamins K-1 and K-2 differently. K-1 accumulates in the liver, heart, and pancreas. K-2 occurs in high concentrations in the brain and kidneys.
Best dietary sources
Leafy green vegetables contain vitamin K-1.
Several foods are rich in vitamin K-1, and vitamin K-2 is much less common. Bacteria in the gut can convert some K-1 into K-2.
Fermented foods are a good source of vitamin K-2.
Also, because it is fat-soluble, organ meats and high-fat dairy products contain fairly substantial quantities of vitamin K-2. Conversely, lean meats, such as poultry, are not good sources of K-2.
Dietary sources of vitamin K-1 include:
- dark leafy green vegetables, such as spinach, kale, and collards
- vegetable oils
Dietary sources of vitamin K-2 include:
- natto, a traditional Japanese dish of fermented soybeans
- dairy products, especially hard cheeses
- liver and other organ meats
- egg yolks
- fatty fish, such as salmon
Health benefits of vitamin K-2
In addition to its crucial role in blood clotting and wound healing, vitamin K-2 has a number of other health benefits. We discuss some of these below.
Vitamin K-2 may lower the risk of cardiovascular damage and improve overall heart health.
According to a 2015 review article, K-2 activates a protein that prevents calcium deposits from forming in the walls of blood vessels. The author cited findings suggesting that a diet high in natural vitamin K2 may decrease the risk of coronary heart disease.
Vitamin K-2 promotes healthy bone mineral density by carboxylating osteocalcin, a protein that binds calcium to bones.
A 2019 study investigated the effects of taking MK-4 supplements in 29 postmenopausal females who had experienced hip or vertebral compression fractures.
The researchers concluded that taking 5 milligrams of an MK-4 supplement daily reduced the levels of undercarboxylated osteocalcin to that "typical of healthy, premenopausal women.
A 2017 study from Japan examined whether vitamin K-2 enhances the effects of standard medication for osteoporosis in adult females aged 65 or older. According to the results, vitamin K-2 did not appear to enhance the effects of the osteoporosis medication.
Anxiety and depression
High blood glucose levels may increase a person's risk of developing depression, anxiety, and cognitive impairment.
A 2016 study investigated the effects of vitamin K-2 in rats with metabolic syndrome, high blood glucose levels and symptoms of anxiety, depression, and memory deficit.
After 10 weeks, treatment with vitamin K had normalized blood glucose and reduced symptoms of anxiety and depression. However, it did not improve memory deficit in the rats.
Vitamin K-2 has antioxidant properties that may help protect against cancer. In addition, findings suggest that K-2 may suppress genetic processes that lead to tumor growth.
According to a 2018 study, vitamin K-2 that scientists had modified with a sialic acid-cholesterol conjugate significantly suppressed tumor growth in mouse cells.
A 2019 study suggests that K-2 significantly reduces the activity of hypoxia-inducible factor 1-alpha (HIF-1A) in hepatocellular carcinoma cells. HIF-1A is an important target for cancer drug therapy.
Recommend daily intake
The Office of Dietary Supplements (ODS) recommend a daily intake of 120 micrograms (mcg) of vitamin K for adult males and 90 mcg for adult females. There is no specific recommendation for vitamin K-2.
According to the ODS, vitamin K deficiency affects very few adults in the United States. Newborns and people with certain gastrointestinal disorders,