A patient on the medication Coumadin should monitor their intake of which vitamin

If you are a heart patient who is taking blood thinners, such as warfarin (Coumadin®), you need to be careful not to overdo vitamin K.

Advertising Policy

Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy

Blood thinners are often prescribed for people at risk for developing harmful blood clots.

If you suddenly increase your intake of vitamin K intake in your diet, it can have an unintended consequence. It can actually decrease the effect of warfarin, says cardiologist Leslie Cho, MD. 

“This is because vitamin K is an essential part of the chemical process for forming blood clots in your body,” she says.

Don’t cut vitamin K out completely

You don’t want to cut out vitamin K completely, as it is present in a variety of healthy, nutrient-rich foods. These include leafy greens and many vegetables. Instead, be smart about how much vitamin K you consume, be consistent, and work with your doctor to find just the right balance.

For example, if you eat a diet rich in vitamin K, you may need to check your blood a little more frequently or take more warfarin. If you change your diet and eat fewer foods containing vitamin K, you may need to take less warfarin.

Work with your doctor to find the right dose for you.

Advertising Policy

Here are three tips to help you safely manage your vitamin K intake:

1. Pay attention to food labels to keep your vitamin K intake consistent

“Vitamin K foods can be included in your diet on a regular basis as long as you are mindful of the portion and keep the overall intake of vitamin K-rich foods consistent, says Julia Zumpano, RD, LD.

For instance, you can choose to have a vitamin K rich food every day, every week, or three times in a week — as long as you keep this portion and frequency consistent, she says.

“If necessary, you can discuss including regular sources of vitamin K in your diet with your doctor in case your warfarin dosage would need to be adjusted,” she adds. You will want to tell your physician how often you eat foods high in vitamin K and how much of those foods you eat. Being knowledgeable about vitamin K is a key to managing it in your diet.

There are a variety of vegetables that contain lower amounts of vitamin K. These include:

• Tomatoes.
• Peppers.
• Carrots.
• Cauliflower.
• Cucumbers.
• Potatoes.
• Sweet potatoes.
• Squash (both summer and winter).

Iceberg lettuce is low and romaine is also fairly low, so most people can eat either if them daily. In addition, be sure to read labels on multivitamins as they have varying amounts of vitamin K. Talk to your doctor about what vitamins you should take.

Advertising Policy

2. Beware of herbal supplements and omega-3 supplements (EPA/DHA)

You may need to avoid certain supplements and vitamins to keep your blood values stable. Talk to your doctor about any and all supplements you take to be sure they are not interfering with your blood thinners.

3. Take blood thinners in consistent way

Another way to manage how well your blood thinners work is to take your dose of warfarin at the same time each day, and from day to day, make sure your vitamin K intake is consistent, Dr. Cho says.

To be sure you’re on track, have your blood values checked regularly (usually once per month with your physician; this may be more often during dose adjusting).

Vitamin K in popular foods

Below, find more details on the amount of vitamin K present in different foods, including leafy greens, vegetables and other foods as provided by the U.S. Department of Agriculture.

When it comes to blood thinners, the more you know the better you can manage your diet. Be sure to talk to your doctor if you have any concerns.

People taking blood-thinning medications have been told in the past to limit their vitamin K intake. Some researchers now say that may not be the best advice.

Share on PinterestSome experts say people taking warfarin should be told to increase their intake of vitamin K, not decrease it. Getty Images

Patients taking blood-thinning drugs such as warfarin are told by doctors to reduce their intake of vitamin K because it’s believed too much of this vitamin can decrease the drug’s effectiveness.

This is due to the belief that the vitamin interacts with the body’s clotting process and can interfere with the drug’s blood-thinning properties.

But what if this advice is wrong?

According to a new clinical trial, people taking these drugs should actually be told to increase the amount of vitamin K they consume.

This clinical trial is the first randomized controlled trial testing how people taking warfarin responded to dietary changes aimed at increasing vitamin K intake.

The study included nearly 50 patients with a history of anticoagulation instability, which is an inability to maintain healthy levels of blood clotting.

Half of the participants were provided dietary counseling sessions and cooking lessons that offered general nutritional advice.

The rest attended counseling sessions and received cooking lessons that focused specifically on increasing consumption of vitamin K–rich vegetables, oils, and herbs.

“Green and leafy vegetables such as spinach, broccoli and lettuce are rich in vitamin K. Also, foods such as kiwi, asparagus and soybeans are good sources of vitamin K,” Dr. Brandie Williams, FACC, a cardiologist at Texas Health Stephenville, told Healthline.

Six months after the study began, 50 percent of the participants who were taught to increase their vitamin K intake were all able to maintain stable anticoagulation levels.

Only 20 percent of those receiving general nutritional counseling achieved a similar improvement.

Guylaine Ferland, lead study author and professor of nutrition at Université de Montréal and scientist at the Montreal Heart Institute Research Centre, said the findings suggest patients on warfarin would significantly benefit from consuming at least 90 micrograms of vitamin K per day for women and 120 micrograms per day for men.

Warfarin is used to prevent blood clots from forming and is often used for medical conditions such as an irregular heartbeat, clots in the veins of the body called deep-vein thrombosis or pulmonary embolism, and after a heart valve replacement.

Williams explained the reasoning behind the current recommendations.

“Vitamin K is part of the complex process needed for the body to make clots, and warfarin blocks this process,” she said. “So eating too many foods rich in vitamin K is believed to cause warfarin to become less effective and cause more clotting in the body.”

Consistent vitamin K intake is key

“I think all warfarin-treated patients would benefit from increasing their daily vitamin K intake,” Ferland said in a statement.

She added that “given the direct interaction between dietary vitamin K and the action of the drug, it is important that [higher] daily vitamin K intakes be as consistent as possible.”

“Our hope is that healthcare professionals will stop advising warfarin-treated patients to avoid green vegetables,” she said.

Williams agreed.

“Clinically, this may prevent patients on warfarin from having too many highs and lows on their INRs, the International Normalized Ratio blood test used to monitor how thick or thin the blood is,” she said. “This could give more consistency to the patient’s blood clotting ability.”

But she cautioned that “while the information in this trial is thought-provoking for physicians, larger studies will need to be conducted before significant changes can be made in patient care.”

Precautions with warfarin

Apart from the findings of this clinical trial, there are other medications and vitamins that can affect how warfarin works. These include:

• prescription medications, such as the common antibiotics azithromycin andciprofloxacin
• nonprescription medications such as naproxen, aspirin, and ibuprofen, which are common nonsteroidal anti-inflammatory drugs (NSAIDs)
• vitamin preparations containing large amounts ofvitamin Eor vitamin C

Alcohol consumption may also influence the way your body metabolizes warfarin.

TheAmerican Heart Association (AHA)recommends that men shouldn’t have more than one or two drinks per day and women should limit themselves to about one drink per day when taking warfarin.

Warfarin side effects

Like almost any prescription drug, warfarin users may experience side effects.

Symptoms that indicate you should call your doctor include:

• bleeding and major hemorrhage
• bruisingeasily
• “purple toe syndrome,” a type of skin destruction (necrosis)

For patients who are told they need to take a blood thinner and are concerned about these issues, there are newermedicationsavailable called direct-acting oral anticoagulants (DOACs).

This class of drugs includesapixaban,rivaroxaban, and others.

DOACs are shorter acting than warfarin, don’t require blood test monitoring for bleeding and clotting risk, and have fewer drug and food interactions than warfarin.

The bottom line

Warfarin is a drug prescribed to patients at risk of dangerous blood clots.

It can slow the body’s production of clotting factors, which are produced using vitamin K.

New research finds levels of vitamin K in a person’s diet can improve, rather than impede the effects of warfarin.

However, there are other vitamins and medications that will affect people taking warfarin, and care must be taken when using them.