Читать книгу The Vitamin Cure - Monte Lai - Страница 33

13

VITAMIN K (PHYLLOQUINONE)

Vitamin K is the anticoagulation vitamin. In 1929, Henrik Dam used oil to extract and remove all lipid-soluble substances from animal feed so that it contained only water-soluble substances and then fed that to chickens. He found that the chickens suffered muscle bleeding and slow blood coagulation. Apparently, some molecule in the lipid-soluble substances was important for blood clotting. In 1935, Dam isolated a lipid-soluble substance from the animal feed and called it vitamin K. K came from the first letter of the German word Koagulation, meaning “blood clotting.” In subsequent years, scientists confirmed that the chemical structure of vitamin K was phylloquinone.

Lipid-soluble vitamin K is an essential nutrient for blood coagulation in the body. Vitamin K consists of a group of structurally related naphthoquinones. Among them, vitamins K1 and K2 are most important to human health. Vitamin K1, also known as “phylloquinone,” comes from plant-based foods, particularly dark-green leafy vegetables. Vitamin K2, also known as “menadione,” is produced mainly by bacteria living in the human colon.

What Are the Major Functions of Vitamin K?

 • Blood coagulation. Blood coagulation is required to stop bleeding after injury. Vitamin K is an essential cofactor for enzymes participating in the blood coagulation process. Vitamin K deficiency can adversely affect blood coagulation and bring about uncontrollable bleeding.

 • Bone density. Vitamin K helps maintain bone health. Supplementation with vitamin K augments bone density and curtails the risk of bone fractures in patients with osteoporosis.

 • Calcification of blood vessels. Vitamin K enhances the activity of matrix Gla proteins, which inhibit calcification of blood vessels and prevent the formation of atherosclerosis.

What Are the Symptoms of Vitamin K Deficiency?

 • Symptoms of vitamin K deficiency include nosebleeds, bleeding gums, and/or blood in the urine and stool. Fortunately, deficiency is rare because vitamin K is widely distributed in plant-based foods, and the body is able to recycle oxidized vitamin K through a regenerative oxidation-reduction mechanism. In addition, bacteria living in the colon can produce vitamin K2, at least part of which is available for human use.

What Are the Risk Factors for Vitamin K Deficiency?

 • Anticoagulants. Oral anticoagulant drugs are the major cause of vitamin K deficiency. Warfarin, a commonly prescribed anticoagulant drug, is an inhibitor of vitamin K epoxide reductase, an enzyme that allows the body to recycle and recover vitamin K. Long-term warfarin users are at a high risk of vitamin K deficiency.

 • Vitamin K–deficient bleeding in newborns. Bleeding caused by vitamin K deficiency in new babies is a classic example of vitamin K deficiency–related disease in humans. A number of factors—including low vitamin K in the placenta, defective coagulation factors and germ-free digestive tracts in infants, and low vitamin K in breast milk—can cause vitamin K–deficient bleeding in newborns. Vitamin K–deficient bleeding often occurs during the first week of birth. The American Pediatrics Association recommends that newborns be given an injection of 0.5–1 mg of vitamin K to avoid vitamin K–deficient bleeding.

 • Diseases. A number of conditions—including liver disease and inflammatory bowel diseases (such as Crohn’s disease and ulcerative colitis), as well as the long-term use of antibiotics—can trigger vitamin K deficiency. All these conditions could kill bacteria living in the colon.

Prevention and Treatment of Diseases

 • Prevention. Vitamin K helps prevent osteoporosis (94), cardiovascular disease (48), and calcification of blood vessels.

 • Treatment. Vitamin K may treat osteoporosis.

Which Food Items Are Vitamin K Rich?

Dark-green leafy vegetables are excellent sources of vitamin K1. Spinach, broccoli, kale, green beans, oats, and wheat contain vitamin K. Animal-based foods—such as meat, milk, and eggs—also contain vitamin K2. The Japanese food natto is rich in vitamin K2 as well.

This list of vitamin K–rich food items is adapted from information provided by the USDA.

Daily reference value of vitamin K is 80 mcg according to the 2013 FDA food-labeling guidelines.

What Are the Recommended Dietary Allowances for Vitamin K?

There is currently no upper intake limit for vitamin K.

Vitamin K Supplements

 • Dosage and types. The three major types of vitamin K supplements are vitamin K, vitamin K2, and vitamin K7. Unless stated otherwise, vitamin K refers to vitamin K1 (phylloquinone). The most common dosages of vitamin K supplements are 30–1,000 mcg. Vitamin K2 (menaquinone) supplements are more expensive compared to vitamin K1 supplements. Vitamin K7 comes from Japanese natto, a food made from fermented soybeans, and supplements of it are the most expensive. The advantage of vitamin K7 is that its half-life in the body is seven to eight times longer than that of vitamin K1. MK-7 (menaquinone-7) is a form of vitamin K2. The common dosage for MK-7 is in the range of 30–200 mcg.

 • Osteoporosis. Vitamin K may mitigate age-related bone loss. Studies from the Netherlands have shown that taking 180 mcg of MK-7 daily reduced the risk of bone loss in postmenopausal women. Vitamin K1 at a daily dose of 500 mcg curtailed the risk of osteoporosis.

 • Other diseases. Sufficient dietary intake of vitamin K may further reduce the risk of coronary artery disease, prostate cancer, and lung cancer.

Safety Issues

 • Side effects. There is no known side effect associated with vitamin K intake from foods or supplements.

 • Vitamin A and vitamin E. Taking high doses of vitamin A together with high-dose vitamin E reduces the blood level of vitamin K. High-dose vitamin A decreases the absorption of vitamin K by the intestines, while high-dose vitamin E interferes with the coagulation activity of vitamin K in the bloodstream.

What Types of Drugs May Interact with Vitamin K?

 • Anticoagulants. Vitamin K may decrease the efficacy of anticoagulant drugs. Patients who take such drugs should not use vitamin K supplements.

 • Other drugs. Antibiotics and anticonvulsant drugs may reduce blood levels of vitamin K.