People taking the blood thinning medication warfarin may need to moderate vitamin K levels in their diets. Vitamin K may interfere with the effectiveness of warfarin.

A doctor may prescribe warfarin to someone who has had a blood clot in the past, as they are at a higher risk of blood clots in the future. Other factors that increase the chances of a blood clot include:

• obesity
• pregnancy
• long periods of inactivity
• smoking
• irregular heartbeat
• trauma
• older age
• chronic inflammatory diseases
• diabetes
• high blood pressure
• high cholesterol

Warfarin works by slowing the production of clotting factors, which the body makes by using vitamin K from food. Levels of vitamin K in a person’s diet could influence the effects of warfarin.

It is possible that vitamin-K rich diets can reduce the effectiveness of warfarin.

The warfarin diet

Vitamin K, which is in some foods, has an important role in blood clotting, and how warfarin works.

The liver uses vitamin K to produce clotting factors, which are cells that help to control bleeding and enable blood clots to form.

Warfarin disrupts this clotting process by inhibiting an enzyme in the liver that uses vitamin K to form clotting factors.

Warfarin can reduce the chances of a dangerous blood clot forming by increasing the time it takes for the liver to produce clotting factors.

It is possible that eating a diet rich in vitamin K could reduce the effect of warfarin on clotting factors.

The American Heart Association (AHA) suggest that eating vitamin K-rich foods may counteract the effects of warfarin, and lower the prothrombin time. This is the time it takes for a blood clot to form.

The AHA’a list of 19 foods high in vitamin K includes:

• amaranth leaves
• asparagus
• broccoli
• Brussels sprouts
• coleslaw
• collard greens
• canned beef stroganoff soup
• endive
• garden cress
• kale
• kiwifruit
• lettuce
• mustard greens
• soybeans
• spinach
• Swiss chard
• tuna fish in oil
• turnips
• vegetable drinks

It is not necessary to cut out foods that contain vitamin K entirely. The foods that contain vitamin K also have other nutritional properties that contribute to a healthful diet.

It is important to note that the guidance varies on how much vitamin K people on warfarin can consume.

For example, a recent systematic review suggests a diet that restricts vitamin K intake is unlikely to improve the efficacy of warfarin. The authors suggest that keeping vitamin K levels consistent may be more beneficial.

The average person only needs a small amount of vitamin K, around 60 to 80 micrograms (mcg)per day. As this amount is so small, it can be easy for vitamin K levels to fluctuate across different days, creating a problem for people on warfarin.

Keeping vitamin K levels stable, and within a normal range, may reduce its effect on the actions of warfarin. Keeping a food diary and being aware of foods that are high in vitamin K can help a person keep track.

Alcohol and warfarin

Alcohol can also affect the action of warfarin and, therefore, the risk of developing blood clots.

High levels of alcohol consumption can alter the way the body metabolizes warfarin.

The AHA suggest that, on average, men should drink no more than one or two drinks per day, and women should drink no more than one drink per day.

Examples of one drink are a 12-oz beer, a 4-oz glass of wine, 1/5 oz of 80-proof spirits, or 1 oz of 100-proof spirits.

Drinking too much alcohol can be particularly harmful for people taking warfarin. A study of 570 people in 2015 found that alcohol misuse has links to a higher risk of major bleeding in people taking warfarin.

Summary

Warfarin can help to prevent dangerous blood clots. It works by slowing the production of clotting factors, which the body makes by using vitamin K from food. Levels of vitamin K in a person’s diet could influence the effects of warfarin.

People taking warfarin must avoid eating too many foods that are high in vitamin K, but it is not necessary to avoid these foods entirely. A stable diet, containing around 60 to 80 mcg of vitamin K is desirable.

People on warfarin must also ensure that they only consume alcohol in moderation. High levels of alcohol may affect the metabolism of warfarin and increase the risk of major bleeding.