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.

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Some 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.”

“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.”

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

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

The American 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.

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

Symptoms that indicate you should call your doctor include:

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

This class of drugs includes apixaban, 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.

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.