When prescribed the anticoagulant drug warfarin, many patients are told to limit foods rich in vitamin K, such as green vegetables. The results of a new clinical trial call that advice into question and suggest patients on warfarin actually benefit from increasing their vitamin K intake—as long as they keep their intake levels consistent.

Warfarin is widely used to prevent the dangerous blood clots that cause heart attacks and strokes. The drug’s dosage must be carefully calibrated to balance the risk of clots against the risk of uncontrolled bleeding. Because warfarin counteracts the activity of vitamin K in the blood, large swings in vitamin K intake can disrupt this balance.

The current recommendation to keep daily vitamin K intakes consistent often translates into patients limiting vitamin K intake. According to the new trial, patients would be better advised to increase the amount of vitamin K in their diet.

“I think all warfarin-treated patients would benefit from increasing their daily vitamin K intake, said lead study author Guylaine Ferland, professor of nutrition at Université de Montréal and scientist at the Montreal Heart Institute Research Centre. “That said, 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.”

Ferland will present the research at Nutrition 2019, the American Society for Nutrition annual meeting, held June 8-11, 2019 in Baltimore.

“Our hope is that healthcare professionals will stop advising warfarin-treated patients to avoid green vegetables,” said Ferland. She explained that eating plenty of green vegetables and other nutritious vitamin-K rich foods can help stabilize anticoagulation therapy and offers many other health benefits.

The study is the first randomized controlled trial to test how patients on warfarin respond to a dietary intervention aimed at systematically increasing vitamin K intake. Researchers enrolled 46 patients with a history of anticoagulation instability. Half attended dietary counseling sessions and cooking lessons that provided general nutrition information, while half attended counseling sessions and cooking lessons focused on increasing intake of green vegetables and vitamin-K rich oils and herbs.

After six months, 50% of those counseled to increase their vitamin K intake were maintaining stable anticoagulation levels, compared with just 20% of those who received the general nutritional counseling, a significant improvement. The results suggest patients taking warfarin would benefit from consuming foods that provide a minimum of 90 micrograms of vitamin K per day for women and 120 micrograms per day for men, Ferland said.