Activity

Phylloquinone is present primarily in green leafy vegetables and is the main dietary form of vitamin K [3]. Menaquinones, which are predominantly of bacterial origin, are present in modest amounts in various animal-based and fermented foods [1,4]. Almost all menaquinones, in particular the long-chain menaquinones, are also produced by bacteria in the human gut [5,6]. MK-4 is unique in that it is produced by the body from phylloquinone via a conversion process that does not involve bacterial action [7].

Vitamin K functions as a coenzyme for vitamin K-dependent carboxylase, an enzyme required for the synthesis of proteins involved in hemostasis (blood clotting) and bone metabolism, and other diverse physiological functions [3,5]. Prothrombin (clotting factor II) is a vitamin K-dependent protein in plasma that is directly involved in blood clotting. Warfarin (Coumadin�) and some anticoagulants used primarily in Europe antagonize the activity of vitamin K and, in turn, prothrombin [8]. For this reason, individuals who are taking these anticoagulants need to maintain consistent vitamin K intakes.

Matrix Gla-protein, a vitamin K-dependent protein present in vascular smooth muscle, bone, and cartilage, is the focus of considerable scientific research because it might help reduce abnormal calcification [9]. Osteocalcin is another vitamin K-dependent protein that is present in bone and may be involved in bone mineralization or turnover [5].

Like dietary lipids and other fat-soluble vitamins, ingested vitamin K is incorporated into mixed micelles via the action of bile and pancreatic enzymes, and it is absorbed by enterocytes of the small intestine [10]. From there, vitamin K is incorporated into chylomicrons, secreted into the lymphatic capillaries, transported to the liver, and repackaged into very low-density lipoproteins [2,10]. Vitamin K is present in the liver and other body tissues, including the brain, heart, pancreas, and bone [2,3,11].

In the circulation, vitamin K is carried mainly in lipoproteins [2]. Compared to the other fat-soluble vitamins, very small amounts of vitamin K circulate in the blood. Vitamin K is rapidly metabolized and excreted. Based on phylloquinone measurements, the body retains only about 30% to 40% of an oral physiological dose, while about 20% is excreted in the urine and 40% to 50% in the feces via bile [2,11]. This rapid metabolism accounts for vitamin K�s relatively low blood levels and tissue stores compared to those of the other fat-soluble vitamins [11].

Little is known about the absorption and transport of vitamin K produced by gut bacteria, but research indicates that substantial quantities of long-chain menaquinones are present in the large bowel [7]. Although the amount of vitamin K that the body obtains in this manner is unclear, experts believe that these menaquinones satisfy at least some of the body�s requirement for vitamin K [6,7].

In most cases, vitamin K status is not routinely assessed, except in individuals who take anticoagulants or have bleeding disorders. The only clinically significant indicator of vitamin K status is prothrombin time (the time it takes for blood to clot), and ordinary changes in vitamin K intakes have rarely been shown to alter prothrombin time [5]. In
There are two main kinds of vitamin K. Vitamin K1 (phylloquinone) comes from plants, especially leafy green vegetables like spinach and kale. Vitamin K2 (menaquinone) is naturally created in the intestinal tract and works similarly to K1. , fasting concentrations of phylloquinone in plasma have been reported to range from 0.29 to 2.64 nmol/L [12]. However, it is not clear whether this measure can be used to quantitatively assess vitamin K status. People with plasma phylloquinone concentrations slightly below the normal range have no clinical indications of vitamin K deficiency, possibly because plasma phylloquinone concentrations do not measure the contribution of menaquinones from the diet and the large bowel [12]. No data on normal ranges of menaquinones are available [2].

Recommended Intakes

Intake recommendations for vitamin K and other nutrients are provided in the Dietary Reference Intakes (DRIs) developed by the Food and Nutrition Board (FNB) at the Institute of Medicine of the National Academies [3]. DRI is the general term for a set of reference values used for planning and assessing nutrient intakes of healthy people. These values, which vary by age and gender, include:

Recommended Dietary Allowance (RDA): Average daily level of intake sufficient to meet the nutrient requirements of nearly all (97%�98%) healthy individuals; often used to plan nutritionally adequate diets for individuals.

Adequate Intake (AI): Intake at this level is assumed to ensure nutritional adequacy; established when evidence is insufficient to develop an RDA.

Estimated Average Requirement (EAR): Average daily level of intake estimated to meet the requirements of 50% of healthy individuals; usually used to assess the nutrient intakes of groups of people and to plan nutritionally adequate diets for them; can also be used to assess the nutrient intakes of individuals.

Tolerable Upper Intake Level (UL): Maximum daily intake unlikely to cause adverse health effects.

Insufficient data were available to establish an EAR for vitamin K, so the FNB established AIs for all ages that are based on vitamin K intakes in healthy population groups [3]. Table 1 lists the current AIs for vitamin K in micrograms (mcg). The AIs for infants are based on the calculated mean vitamin K intake of healthy breastfed infants and the assumption that infants receive prophylactic vitamin K at birth as recommended by American and Canadian pediatric societies