Vitamin K is a fat-soluble vitamin – a substance essential for the human body. The K comes from the German word for coagulation – or blood clotting. Plants synthesize vitamin K1 (phylooquinone); when we eat green vegetables or certain vegetable oils, bacteria in our guts transform the K1 from food to K2, the main active form in our bodies. Vitamin K is essential for blood clotting and bone strength.
Blood Clotting: As soon as a blood vessel is damaged, the body begins to prevent serious bleeding by many mechanisms. Blood platelets form a plug at the site of injury, and proteins in the blood - called clotting factors – are activated to form fibrin strands which strengthen the platelet plug. Vitamin K is essential for the formation of many of the clotting factors in this process. Since these clotting factors are made in the liver, severe liver disease may result in lower blood levels and a risk of uncontrolled bleeding. Alternatively, some people are at risk of forming clots which could block the flow of blood in arteries to the heart, brain or lungs, resulting in heart attack, stroke or pulmonary embolism. If they take Coumadin as an anticoagulant, the drug works by antagonizing the action of vitamin K. Doctors advise patients on Coumadin to have a reasonably constant amount of green vegetables containing vitamin K in their diets, but not to avoid them..
Bone mineralization: Osteoblasts are bone-forming cells that make proteins essential for bone mineralization. Vitamin K plays a role in the synthesis of these proteins (their activity is also regulated by vitamin D). Population studies have shown a relationship between foods containing vitamin K and hip fracture – the Nurses’ Health Study followed 72,000 women for 10 years and found that those with the highest intake of vitamin K had a significantly lower risk of hip fracture than those with lesser intake. Lettuce was the food contributing the most to vitamin K intake. A similar finding was seen in the Framingham Heart Study of men and women – a 65% lower risk of hip fracture in those in the highest quartile of vitamin K intake from food, compared to those in the lowest, despite no association with bone mineral density. Researchers in Japan used very high doses of supplemental vitamin K in patients on hemodialysis and in osteoporotic women and reported significant reductions in bone loss and fracture risk. However, using supplemental K at doses attainable in the diet has shown mixed results with respect to bone density. There are inconsistencies here that need further research.
Foods containing Vitamin K: A good diet that includes a daily intake of green leafy vegetables is endorsed by all vitamin K researchers. Kale, chard, spinach and broccoli are high in K, and lettuce is moderately high. . Levels of K are not reduced by many cooking methods, however it is unwise to cook most vegetables in abundant water if you discard the cooking water. Canola, soy and olive oils all contain vitamin K. To eat the amount of foods high in K associated with a decreased risk of hip fracture in the Framingham Heart Study, an individual would need to eat at least a cup of cooked dark green vegetables a day, or a large salad of mixed greens. Many multivitamins now include small amounts of Vitamin K1; while this may be helpful, it should not substitute for leafy greens in the diet. Using olive, canola or soy oils instead of butter will increase K intake and provide less saturated fat. Prolonged use of broad spectrum antibiotics may decrease the synthesis of K by intestinal bacteria, so extra care will be needed to get adequate levels from food and multivitamins. Here in California we have access to leafy greens throughout the year, often locally grown. A really easy green vegetable to grow yourself is the fava bean – and you don’t have to wait for the beans to form. Their green leaves are delicious and are becoming a culinary trend in salad or cooked.
Sadja Greenwood, MD, MPH. Past issues on this blog. Leave me a comment if you wish.