What is vitamin K?
Vitamin K is a general name for a few related compounds that possess vitamin K activity. Phylloquinone is the form of vitamin K found naturally in plants; menaquinones are the form of vitamin K derived from bacteria; and menadione, which is not natural, is the synthetic (laboratory derived) form of vitamin K.
What foods provide vitamin K?
Humans receive vitamin K not only from various foods but also from bacteria in the colon. Good sources of vitamin K include broccoli, spinach, cabbage, Brussels sprouts, turnip greens, cauliflower, beef liver, and asparagus. Foods lower in vitamin K such as cheeses, eggs, corn oil, sunflower oil, and butter also make a respectable contribution to our vitamin K intake because of their frequency of consumption.
How much Vitamin K do we need?
The RDA (AI) for men and women is 120 and 90 micrograms of vitamin K daily. The RDA for pregnant and lactating women over the age of 18 is the same as non-pregnant adult women, however for pregnant females 18 and younger the recommendation is only 75 micrograms daily. Like other vitamins the RDA is the minimum recommendation of intake.
It has been estimated that as much as one-half of the vitamin K absorbed from the digestive tract was originally made by intestinal bacteria. Being a fat-soluble substance, vitamin K relies somewhat upon the activities of normal lipid digestion for optimal absorption. Vitamin K must also be transported from the intestines by way of chylomicrons, which ultimately reach the liver. Once in the liver, vitamin K can be packaged into VLDL and carried throughout the body.
What does vitamin K do in the body?
For years the only recognized activity of vitamin K was its involvement in proper normal blood clotting. In fact, rumor has it that vitamin K was so named by Danish researchers with respect to blood coagulation, a word spelled with a “K” in Danish. The liver is responsible for making the proteins, or clotting factors, that circulate in the blood. These proteins are activated when there is a hemorrhage and allow blood to clot at that site.
When clotting factors are initially made by liver cells, but before they are released into circulation, several of these proteins are modified by vitamin K. The modification occurs only in few amino acids; however, it changes the design and function of the proteins significantly. With this slightly modified design, these and other clotting factors are released into circulation. Once in circulation, these proteins await the signal to initiate clot formation. The signal is a tear in a blood vessel wall producing a hemorrhage. In light of vitamin K’s involvement with blood clotting, the vitamin K status of a patient is typically determined prior to any surgical procedure.
Vitamin K also seems to be active in other tissue besides the liver. In bone, muscles, and kidneys, vitamin K appears to be necessary for activities similar to those in the liver. At least two proteins in bone and one in the kidneys have been identified as needing modification by vitamin K to function properly.
Can too little or too much vitamin K be consumed?
Unlike other fat-soluble vitamins, vitamin K is not stored very well in the body and appreciable amounts are lost in urine and feces every day. This certainly presents the opportunity for a more rapid onset to deficiency. However, since vitamin K is abundant in the human diet and vitamin K is produced by bacteria in the digestive tract, vitamin K deficiency is uncommon in adults. The typical American adult may eat five to six times the RDA daily.
Opportunities for vitamin K deficiency do arise during infancy. There does not seem to be an appreciable transfer of vitamin K from the mother to the infant prior to birth. Thus newborns enter the world with very limited stores of vitamin K. Also, a newborn’s digestive tract is sterile and will not develop a mature bacterial population for a couple months. Further, maternal breast milk is not a good source of vitamin K. All of these factors place infants at greater risk for developing vitamin K deficiency, which can lead to poor blood clotting and hemorrhage, among other considerations. With these concerns in mind, newborns are commonly provided with vitamin K shortly after birth.
One other situation may raise concern regarding the development of a vitamin K deficiency. People using antibiotics for long periods of time are at a greater risk for vitamin K deficiency. Certain antibiotics can reduce the number of vitamin K-producing bacteria from our colon which puts us at a greater risk of deficiency, especially if a person eats a low vitamin K diet and/or is experiencing problems with lipid digestion. But the combination of these factors is indeed rare.
Vitamin K is relatively nontoxic in natural forms; however, there have been situations of toxicity from chronic use of excessive vitamin K in the synthetic menadione form.