Dark leafy greens such as kale, spinach, and fresh green herbs as well as dark green vegetables such as broccoli are often touted as vitamin powerhouses and superfoods that should be a regular contributor to everyone’s daily intake of vegetables. And generally speaking, this advice is truthful. Dark green vegetables and herbs are full of healing and health-promoting antioxidant nutrients. But among this array of nutrients stands a unique vitamin that is often targeted by a subset of the population, causing them to avoid these nutritious foods. The vitamin is K. The subset of people are those on anticoagulation therapy such as warfarin (Coumadin).

Vitamin K is richly sourced in the aforementioned foods as well as some lighter green vegetables such as cabbage. Its unique feature (among vitamins) is its essential role in the synthesis of proteins that function in the coagulation and anticoagulation systems. Specifically, vitamin K is the required coenzyme for gamma-glutamyl carboxylase which activates coagulation factors VII, IX, X, prothrombin and proteins S and C. Vitamin K deficiencies are associated with hemorrhages in deep soft tissues. Dietary insufficiency may contribute to inadequate levels of this vitamin, but since the bacteria of a healthy microbiome will also manufacture vitamin K, a true deficiency more likely arises from external factors such as drug therapy, cystic fibrosis,  hepatobiliary diseases such as primary biliary cholangitis, primary sclerosing cholangitis, biliary atresia, liver failure, and metabolic disorders. Warfarin (Coumadin) and Cephalosporin antibiotics are the most common pharmaceutical culprits leading to a vitamin K deficiency.

As an anticoagulant pharmaceutical, warfarin (Coumadin) interacts with vitamin K such that patients taking Coumadin are educated to limit foods high in vitamin K. However, eliminating such a large variety of vegetables can be a challenge. Also, some dark green leafy vegetables such as kale have very high amounts of vitamin – a whooping 531 μg in a

cooked, half-cup serving! Since warfarin has such a narrow therapeutic index, it can be difficult for patients to remain in that range without closely monitoring their dietary intake. As a result, many patients simply avoid vitamin K foods altogether, but according to recent studies, this traditional practice may actually be contributing to the problem of remaining in a safe “therapeutic” drug range.

According to an article published in JAMA in September 2019, encouraging patients on warfarin (Coumadin) to increase their dietary intake of vitamin K foods may help to stabilize their anticoagulation therapy, keeping it in the desired “therapeutic” range.

The article was based on a randomized controlled trial in which 46 patients treated with warfarin for more than 6 months and unable to maintain a “therapeutic” range less than half the time were divided into two groups. One group received general dietary information and the other received dietary counseling on foods that would increase their vitamin K consumption by 150 μg daily as a means of achieving more stability with their anticoagulation therapy. At the end of 20 weeks, 50 percent of the group receiving counseling were able to reach their dietary goal and maintain their “therapeutic” range more than 70 percent of the time, which was the targeted end goal. At this percentage, the risk of stroke and death is significantly lower. Of the group that received general dietary information, only 20 percent were able to meet the end goal.

The proposed mechanism by which increased dietary vitamin K may help stabilize anticoagulant therapy is that by raising levels of dietary vitamin K, the liver stores greater quantities of the vitamin, which can then be tapped into on days when dietary intake is low, allowing the body’s anticoagulation pathways to remain steady despite dietary intake.

Although the JAMA study is small and still hasn’t reached full publication status, other studies support the validity of its findings. In 2016, a systematic review of the interaction between dietary vitamin K and the stability of anticoagulation therapy was conducted and it was found that maintaining a steady dietary intake led to far greater stability in the anticoagulation therapy compared to individuals following the traditional advice to avoid vitamin-K foods.

Anticoagulation therapy is among the most common long-term pharmaceuticals, meaning a large portion of the population has been educated to avoid superfoods such as kale, broccoli, spinach, cabbage, and fresh herbs. As studies show that this advice may no longer be relevant (and actually counterproductive), let’s begin spreading the word that vitamin K is no longer just for kale lovers, but for those taking Coumadin, as well.