Dark green leafy vegetables -- a rich source of vitamins, phytonutrients, and fiber -- are a vital part of a healthy diet. They are particularly rich in vitamin K, which is necessary for proper blood clotting. Due to their high vitamin K content, green leafy vegetables can cause blood thinning medications to be less effective. If you take blood thinners, your doctor may ask that you restrict your intake of dark green leafy vegetables. These foods themselves, however, play no role in increasing risk of bleeding.
Blood Thinning Medications
Blood thinning medications -- also called anticoagulants -- inhibit clotting factors in the blood, and are prescribed to lower the risk of blood clots, heart attack and strokes. Warfarin -- sold under the brand name Coumadin -- is a common anticoagulant. Warfarin can cause you to bleed more easily; patients with bleeding disorders or upcoming surgeries should not take it. Certain medications, herbal supplements and foods can interfere with warfarin and other blood thinners; consult your doctor before taking any new medications, herbal remedies or supplements, and before making changes in your diet. Drugs.com notes that you should avoid eating large quantities of dark green leafy vegetables -- as well as excessive amounts of liver and vegetable oils -- while taking warfarin.
Vitamin K -- a fat-soluble vitamin -- derives its name from the German word "koagulation." In addition to facilitating blood clotting, Vitamin K plays a role in bone formation; there is evidence that vitamin K may help protect against osteoporosis and hip fractures in elderly people. Vitamin K appears in dark green leafy vegetables and other plant foods as phylloquinone; a secondary form of vitamin K is synthesized by animals and humans from phylloquinone in the body. Oral anticoagulants inhibit blood clotting by working against vitamin K. Although ingesting large amounts of dietary vitamin K can overcome warfarin's anticoagulant effect, Linus Pauling Institute at Oregon State University advises that individuals taking warfarin still meet dietary recommendations of 90 to 120 mcg of vitamin K per day.
Dark green leafy vegetables generally contain extremely high levels of vitamin K. A cup of shredded romaine lettuce provides 48.2 mcg; the same amount of chopped broccoli raab yields 89.6 micrograms. A cup of raw spinach contains 144.9 mcg, while a cup of chopped kale yields a whopping 547.4 micrograms. Other foods yield much more modest amounts. A slice of rye bread contains a negligible .4 mcg of vitamin K, while a large poached egg provides a scant .1 microgram.
Leafy Vegetable Benefits
Dark green leafy vegetables are reliably rich not only in vitamin K, but in vitamin A and beta-carotenes, which are converted into vitamin A in the body. A cup of raw spinach contains 2,813 IU of vitamin A and 1,688 mcg of beta carotene. Vitamin A, an antioxidant, is essential for proper vision and supports a healthy immune system. Other vitamins provided in healthy amounts by dark leafy greens are vitamin E -- which boosts the immune system -- and folate, a B vitamin that helps to build red blood cells by synthesizing nucleic acid. In addition, green leafy vegetables are low in calories and fat and high in beneficial dietary fiber.