Different types of leafy greens that are a good source of vitamin K.

When you think about vitamins that are important for health, there’s one that might not come to mind but should. “Older adults—particularly older men—are the age group that consume the least amount of vitamin K,” says Sarah Booth, Ph.D., director of the Vitamin K Laboratory at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University in Boston.

Some new research is showing how the nutrient can affect health in significant ways. An analysis of data combined from three studies, recently published in The American Journal of Clinical Nutrition, found that older adults with low blood levels of vitamin K (less than 0.5 nanomoles per liter) had a 19 percent higher risk of dying early from any cause.

The analysis, the largest of its kind so far, included almost 4,000 people ages 54 to 76 who were followed for 13 years. Thirty-five percent of the group was racially and ethnically diverse, mostly African American, Hispanic, or Asian.

“We know low vitamin K is linked to arterial calcification,” or calcium buildup in the arteries, says Kyla Shea, Ph.D., lead study author and a scientist on the vitamin K team at the Tufts. “When that happens in the coronary arteries, it’s called atherosclerosis [stiffening of the arteries], which is a manifestation of cardiovascular disease. It can also happen in arteries throughout the body, and arterial stiffening is linked to mortality.”

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In a previous study, published in The Journal of Nutrition, Shea and her colleagues found that Chinese Americans had the highest blood levels of vitamin K, Hispanics had the lowest, and non-Hispanic Blacks and whites had similar levels. “In this current study, we weren’t able to look at the relationship between vitamin K levels and cardiovascular disease events or mortality by race,” Shea says. “The numbers weren’t large enough to draw conclusions about the associations within specific race and ethnic groups. However, since the analysis included a racially and ethnically diverse group, the results seem to be generalizable to a wider population.”

Other research has shown that too little vitamin K is associated with various age-related concerns, such as cognitive function and mobility.

For example, in one 2019 study by Booth and her colleagues, older people with low blood levels of the vitamin were more likely to have difficulty being physically active than those with higher levels.

And a 2015 study of people 65 and older, published in the journal Nutrients, found that those who consumed more vita­min K (207 micrograms or more per day from foods) performed better on cognition tests than their peers who ate less. 

What Vitamin K Does

“Vitamin K is important because it’s involved in blood clotting,” says Geoffrey Barnes, M.D., a cardiologist at the University of Michigan Samuel and Jean Frankel Cardiovascular Center in Ann Arbor.

It’s also required to make proteins that affect bone, cartilage, and blood vessel health. “It is emerging that vitamin K has a critical role in keeping tissues you don’t want to calcify from calcifying,” Booth says. As Shea said, that can be a problem in arteries throughout the body, and calcification can also have other effects. For example, calcified cartilage in the knees is linked to osteoarthritis.

There are two main types of vitamin K: K1 is found mostly in dark green vegetables. K2 (which comprises several forms) is found in animal products, such as egg yolks, beef, and fermented foods, says Julie Stefanski, R.D.N., a spokesperson for the Academy of Nutrition and Dietetics who has educated healthcare providers on vitamin K. K2 is also produced by the healthy bacteria in our guts.

But it’s more important to focus on getting enough of the vitamin than on the types. “There is a misconception that vitamin K2 is the superior form,” Booth says. “But the scientific evidence shows that vitamin K1 is safe, readily available [in foods], and is converted to the right forms of vitamin K2 where needed.” 

How to Get Enough

The recommended intake of vitamin K is 120 micrograms for men and 90 mcg for women. A cup a day of many dark green veggies will give you that or more.

Consuming veggies with some fat will enhance absorption of the vitamin, Stefanski says, so consider sautéing your vegetables with olive oil or another healthy fat, or drizzling some on before serving.

Some good sources of vitamin K include:

Collard greens (773 mcg per cup), kale (494 mcg per cup), broccoli (254 mcg per medium stalk), Brussels sprouts (219 mcg per cup), spinach (145 mcg per cup, raw), pesto sauce (118 mcg per 1⁄4 cup), asparagus (91 mcg, per cup), kiwifruit (73 mcg per cup), green beans (60 mcg per cup), bok choy (58 mcg per cup), avocado (46 mcg per cup), lettuce (46 mcg per cup), blueberries (29 mcg per cup), and soybean oil (25 mcg per tablespoon). 

Blood Thinners and Vitamin K

The anticoagulant warfarin (Coumadin) prevents harmful blood clots by blocking the action of vitamin K, so people taking it need to regulate their intake of the vitamin.

But that doesn’t mean you should avoid foods rich in vitamin K—they supply other important nutrients, such as antioxidants, fiber, folate, and potassium.

“If it’s green, it probably has vitamin K in it, and you should be eating these foods, but it has to be a consistent amount every day,” Barnes says.

If you decide to add more vitamin K-containing foods to your diet, be sure to let your doctor know so that he can check your blood and alter your warfarin dose if necessary.

Newer anti­coagulants, such as apixaban (Eliquis) and rivaroxaban (Xarelto), aren’t affected by vitamin K intake, Barnes says.

Editor’s Note: This article, which originally appeared in the December 2019 issue of Consumer Reports On Health, has been updated to include the findings of a new study on vitamin K and mortality.