Vitamin K, a fat-soluble vitamin, is needed for the proper clotting of blood (it plays a major role in the carboxylation of clotting proteins II, VI, IX, and X and proteins C and S). It may also help prevent osteoporosis, as it is needed for the synthesis of the bone protein (osteocalcin) involved in calcium crystallization (via the incorporation of calcium phosphates in growing bone). Vitamin K exists as vitamin K1 (phylloquinone) and K2 (menaquinone), the natural forms found in food, and as vitamin K3, the synthetic form called menadione. Both vitamins K1 and K2 are converted to dihydrovitamin K upon digestion.
Intestinal (colonic) bacteria manufacture a large amount of the vitamin K (K2) present in tissues throughout the body; therefore, supplementation with dietary vitamin K is usually not necessary in people and pets.
Deficiency of vitamin K causes excess internal or external bleeding due to a failure of the body to properly clot blood. In dogs and cats, this most commonly occurs as a result of rodent poisons containing warfarin or warfarin-type chemicals. Diseases causing maldigestion and malabsorption as well as destruction of bacteria in the colon by antibiotic therapy can also cause vitamin K deficiency. Supplementation with probiotic bacteria can help restore vitamin K production.
Severe liver deficiency may decrease the activation of vitamin K in the liver, resulting in defective carboxylation of vitamin- K dependent coagulation cofactors, resulting in bleeding disorders due to faulty blood clotting.
Vitamin K also aids in converting glucose into glycogen for energy storage in the liver. Healthy liver function is also promoted by vitamin K.
Therapeutic Uses For Vitamin K
Protection against cancers that involve the inner lining of body organs is attributed to vitamin K; vitamin K may also promote longevity.
While vitamin K has been recommended for use in people with osteoporosis, so far the evidence that it actually works is somewhat slim. There are no well-established therapeutic uses of vitamin K, other than its conventional use as an antidote for blood-thinning medications.
A study of people suggests that an intake of vitamin K higher than the RDA, in the range of 110 mcg daily, might be helpful for preventing osteoporosis. Research has found that people with osteoporosis have much lower blood levels of vitamin K than other people. For example, in a study of 71 postmenopausal women, participants with reduced bone mineral density showed lower serum vitamin K1 levels than those with normal bone density. Similar results have been seen in other studies. A recent report from 12,700 participants in the Nurses’ Health Study found higher dietary intake of vitamin K is associated with a significantly reduced risk of hip fracture. Interestingly, the most common source of vitamin K used by individuals in the study was iceberg lettuce, followed by broccoli, spinach, romaine lettuce, Brussels sprouts, and dark greens. Women who ate lettuce each day had only 55 percent the risk of hip fracture of those who ate it only weekly. However, among women taking estrogen, no benefit was seen, probably because estrogen is so much more powerful. Research also suggests supplemental vitamin K can reduce the amount of calcium lost in the urine. This is indirect evidence of a beneficial effect on bone. Taken together, these findings suggest vitamin K supplements might help prevent osteoporosis.
Vitamin K Sources
Vitamin K (in the form of K1) is found in dark green leafy vegetables. Kale, green tea, and turnip greens are the best food sources, providing about ten times the daily human adult requirement in a single serving. Spinach, broccoli, lettuce, and cabbage are very rich sources as well. Vitamin K is also found in such common foods as oats, green peas, whole wheat, green beans, Brussels sprouts, egg yolks, liver, oatmeal, safflower oil, soybeans, wheat, watercress, and asparagus. Green foods, alfalfa, and the herbs green tea, nettle and shepherd’s purse are also sources.
Vitamin K Dosages
There is no AAFCO recommended level of vitamin K in dog foods. However, there have been reports of vitamin K deficiency in cats fed several commercial foods containing high levels of salmon or tuna. The AAFCO recommends supplementation for any cat eating a diet containing greater than 25 percent fish (0.1 ppm vitamin K recommended and is usually included in the processed diets).
Vitamin K Safety Issues
Vitamin K is probably quite safe at the recommended therapeutic dosages, since those quantities are easily obtained from food.
Certain drugs can interfere with the action or absorption of vitamin K, including the antituberculosis drug isoniazid (INH), phenytoin (for seizures), cholestyramine (for high cholesterol), and even high doses of vitamin E. Additional vitamin K may be needed in these situations.
The blood-thinning drugs Coumadin (warfarin) and dicumarol work by antagonizing the effects of vitamin K. Conversely, vitamin K supplements, or intake of foods containing high levels of vitamin K, blocks the action of these medications, and is used as an antidote.
Excess vitamin K (vitamin K toxicity) is unlikely to be a problem in pets. However, menadione (synthetic vitamin K3) toxicity can occur and cause fatal anemia and jaundice.
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