Vitamin K: Benefits, Food Sources and Intake

Readers who understand the difference between vitamin K1 and K2, where each form comes from in food, and what the evidence actually says about bone and heart outcomes can apply that knowledge to a balanced diet. Vitamin K is one of the fat-soluble nutrients that has accumulated a confusing mix of strong claims and weak evidence over decades, so separating the two is the starting point for using it well.

Vitamin K comes in two main dietary forms. Vitamin K1 (phylloquinone) is found mostly in leafy green vegetables. Vitamin K2 (menaquinone) is a family of related forms, including MK-4 and MK-7, that appears in fermented foods and some animal products. The compound was discovered in 1929 by a Danish researcher working with chickens fed a fat-free diet, who found that the birds developed severe hemorrhages and slow-clotting blood. He named it the “koagulation” vitamin, German spelling, shortened to vitamin K.

What vitamin K does in the body

The nutrient is involved in blood clotting, bone metabolism and protein building, and it works in the liver and in other tissues including the brain, pancreas, heart and skeletal system. Unlike the other fat-soluble vitamins, only very small amounts of vitamin K circulate in the blood at any given time, and the body recycles and reuses some of it multiple times.

A 2010 study identified the human enzyme UBIAD1, which converts K1 from food into MK-4, a form of K2, directly in human tissues. The conversion does not require gut bacteria, according to studies in germ-free animals.

Vitamin K1 and K2 in everyday foods

Vitamin K1 is primarily found in leafy greens such as spinach, kale and cabbage. Some research suggests people absorb only a fraction of the vitamin K1 they get from leafy greens compared with a supplement. A small 1999 trial found that absorption from raw spinach, lettuce and broccoli was roughly one-sixth that of an equivalent dose in tablet form.

Vitamin K2 covers a family of related forms, including MK-4 and MK-7. MK-7 is found in fermented soy foods such as natto, a traditional Japanese dish made from fermented soybeans. K2 also appears in animal products like butter, beef liver, aged cheese and egg yolks. K2 is typically produced in the large intestine by bacteria, and unless someone has intestinal damage that limits absorption, dietary deficiency is rare. Long-term use of broad-spectrum antibiotics may reduce the vitamin-producing bacteria population.

Absorption of vitamin K requires dietary fat and bile. Conditions affecting the gut, liver or gallbladder can limit how much the body takes in, and eating some fat alongside leafy greens supports absorption.

What the evidence shows for bones and heart

Both vitamin K1 and K2 are involved in blood coagulation, and inadequate intake is linked to excessive bleeding from wounds. Inadequate intake has also been linked to an increased risk of osteoporosis and bone fractures.

A 2006 meta-analysis, mostly of trials of older Japanese adults, reported that menaquinone-4 reduced hip fractures by 77%, spinal fractures by 60% and non-spinal fractures by 81% in Japanese trials. Thirteen years later, researchers reran the numbers and found a roughly 28% reduction in clinical fracture risk. When the analysis was restricted to the best-conducted trials, the effect was no longer statistically significant, and vertebral fractures showed no benefit at all.

A 2009 observational study of more than 16,000 women found that those who consumed more vitamin K2 had a lower risk of coronary heart disease. Every 10-microgram-per-day increase in K2 intake was associated with a 9% lower risk, though the finding sat at the edge of statistical significance.

A 2008 study published in the American Journal of Clinical Nutrition found that men with a high intake of vitamin K2 had a 63% lower risk for advanced prostate cancer. A 2002 review in the Journal of Cellular Physiology stated that K vitamins have been shown to inhibit cell growth, and other studies suggest K2 may support brain health and reduce liver cancer recurrence.

How much vitamin K is recommended

The National Academy of Medicine sets an adequate intake for vitamin K1 from foods at 90 micrograms per day for women and 120 micrograms per day for men. The World Health Organization recommends an average of 55 micrograms per day for women and 65 micrograms per day for men. Survey data collected in 2011-2012 found that only 43% of men and 63% of women reached the adequate intake level, with older men reporting the lowest intakes.

There are no official recommendations for K2 specifically. K2 stays in circulation longer than K1 and reaches bone and blood vessels more readily, which is why it is the form studied for those tissues. The closest guidance comes from cohort studies linking K2 intake to cardiovascular health. Participants in those studies took in an average of about 30 micrograms of K2 per day, which describes what people ate rather than a recommendation. A single teaspoon of natto provides 50 micrograms of vitamin K2.

Where to find vitamin K in food

Vitamin K1 sources include leafy greens, cruciferous vegetables, asparagus, prunes, peas, parsley and scallions. A medium cucumber with peel provides over 41% of the daily value for vitamin K.

Plant-based K2 sources include natto, tempeh, sauerkraut, kimchi and fermented pickles. Animal products such as butter, egg yolks, organ meats and high-fat dairy also contain K2, more so if they are grass-fed or pasture-raised.

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FAQ

What is the difference between vitamin K1 and K2?

Vitamin K1 (phylloquinone) is found mostly in leafy green vegetables and supports blood clotting. Vitamin K2 (menaquinone) is a family of related forms found in fermented foods such as natto and in some animal products, and it reaches bone and blood vessels more readily than K1.

How much vitamin K should I get each day?

The National Academy of Medicine sets an adequate intake for K1 at 90 micrograms per day for women and 120 micrograms per day for men. The World Health Organization recommends 55 micrograms per day for women and 65 micrograms per day for men. There is no official recommendation for K2.

What foods are the richest sources of vitamin K?

Leafy greens such as spinach, kale and cabbage are the richest sources of K1. Natto, tempeh, sauerkraut, kimchi and fermented pickles are the main plant-based K2 sources, while butter, egg yolks, organ meats and high-fat dairy provide K2 from animal products, especially when grass-fed.

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