vitamin K

What is vitamin K?

Vitamin K is not a single chemical substance but rather a family of chemically related substances that go by the general name of ???vitamin K.??? Over the past 20 years, no vitamin family has undergone a greater change in terms of our scientific understanding of its chemistry and function. In the past, members of the vitamin K family have traditionally been referred to as vitamin K1, vitamin K2, and vitamin K3. This terminology is largely being replaced by a different set of terms to describe what has now been determined to be a more complicated set of vitamin K compounds.

All types of vitamin K fall into a large chemical category of substances called naphthoquinones. Within this naphthoquinone category, there are two basic types of vitamin K. The first type, called phylloquinones, is made by plants. The second basic type, called menaquinones, is made by bacteria. (The only exception to this rule involves a special group of bacteria, called cyanobacteria, which make phylloquinones instead of menaquinones.) Contrary to some previous scientific assumptions, we get most of our dietary vitamin K in the form of phylloquinones from plant foods. In fact, up to 90% of our dietary vitamin K comes in this form, and within that 90%, over half comes from vegetables???especially green leafy vegetables. Many different types of bacteria in our intestines can make vitamin K in the form of menaquinones. While this synthesis of vitamin K in our digestive tract can contribute to our vitamin K requirements, this contribution is less than previously thought.

What are the functions of vitamin K?

In terms of health research, vitamin K is best known for its role in healthy blood clotting. In fact, use of the letter ???K??? in the very name of this vitamin originally came from the German word koagulation .

Although blood clotting may not sound like a body process that is critical for our everyday health, it is, in fact, essential. At one end of the spectrum, whenever we get a skin wound (even a simple cut) we need sufficient blood clotting ability to close the wound and prevent excessive bleeding. At the other end of the spectrum, we do not want too much blood clotting ability because when we are not wounded, we do not want our cardiovascular system to ???throw a clot??? and mistakenly block an otherwise functioning blood vessel. Vitamin K is one of the key nutrients for keeping our blood clotting ability at the exact right level.

We owe much of our understanding about vitamin K and clotting to early experiments with the prescription drug warfarin. Also known under the brand name Coumadin, warfarin is a widely used anticoagulant drug that works by inhibiting the body???s synthesis of clotting factors (including clotting factors II, VII, IX, and X).

Vitamin K sits right at the center of this clotting process. If clotting factors are to successfully close a wound, they need some way to stick onto the nearby tissue surfaces. What provides them with this ???stickiness??? is a chemical event called carboxylation. One of the amino acids in the clotting factors, called glumatic acid, is the component of the clotting factors that gets carboxylated. Two enzymes are needed to keep this process running smoothly. Warfarin works as an anticoagulant and interrupts this process by blocking one of those enzymes ( vitamin K epoxide reductase ). When this enzyme is blocked, vitamin K can no longer be recycled and ???recharged??? to help the clotting factors achieve their proper stickiness. For individuals with an excessive tendency to form blood clots, anticoagulant drugs like warfarin can be life saving. These warfarin-related discoveries have led to our current understanding of vitamin K as a key nutrient for healthy blood clotting.

The relationship of vitamin K to bone health has been fairly well researched, and in the big picture, vitamin K has emerged as a critical nutrient for bone health. Most convincing is research showing protection from bone fractures that occurs when vitamin K is consumed in adequate amounts. Individuals who are vitamin K deficient have been clearly shown to have a greater risk of fracture. In addition, for women who have passed through menopause and have started to experience unwanted bone loss, vitamin K has been clearly shown to help prevent future fractures. These bone-related benefits of vitamin K appear to depend on at least two basic mechanisms.

The first of these mechanisms involves a type of bone cells called osteoclasts. Osteoclasts are bone cells in charge of bone demineralization???they help take minerals out of the bone and make them available for other body functions. While the activity of these cells is important for proper health, we do not want too many osteoclasts (or too much activity by osteoclasts) since those imbalances would mean too much bone demineralization. Vitamin K makes it possible for our body to keep this process in check. One of the menaquinone forms of vitamin K (MK-4, also called menatetrenone) has repeatedly been show to block formation of too many osteoclasts and perhaps also to initiate their programmed cell death (a process called apoptosis).

A second mechanism involves the role of vitamin K in a process called carboxylation. (This process is the same one discussed earlier in relationship to the stickiness of clotting factors required for proper blood clotting.) For our bones to be optimally healthy, one of the proteins found in bone???a protein called osteocalcin???needs to be chemically altered through the process of carboxylation. (Osteocalcin is not just any typical bone protein. It is a protein especially linked to our bone mineral density???BMD???and for this reason, it is often measured in our blood when doctors are seeking to determine the health of our bone.) When too few of the osteocalcin proteins in our bone are carboxylated, our bones have increased risk for fracture. This unwanted risk appears to be particularly important with respect to hip fracture. Scientists refer to this bone problem as one involving ???undercarboxylated osteocalcin,??? and they have determined that vitamin K can greatly improve the situation. Since vitamin K is required for proper activity of the carboxylase enzyme that allows carboxylation of the osteocalcin proteins in our bone, vitamin K can restore these bone proteins to their proper place in our bone structure and strengthen the composition of the bone. It is the MK-4 menaquinone form of vitamin K that has been best researched in this regard.

Prevents calcification of blood vessels or heart valves

One common problem in many forms of cardiovascular disease is unwanted calcification, the build-up of calcium inside a tissue that is normally soft. This build-up of calcium causes the tissue to harden and stop functioning properly. When calcium builds up inside the arteries, it is typically referred to as hardening of the arteries. One direct way to inhibit the build-up of calcium along the arteries is to maintain ample supplies of a special protein called MGP in the body. MGP, or matrix Gla protein, directly blocks the formation of calcium crystals inside the blood vessels. For MGP to function in this way, it must first be present in its carboxylated form; vitamin K is required for this carboxylation process. In other words, the heart-protective benefits of MGP in prevention of calcification depend upon vitamin K. In animal studies, both basic forms of vitamin K???i.e., phylloquinones and menaquinones???have been found to provide excellent calcification-preventing benefits. Researchers have determined that individuals with vitamin K deficiency are at greater risk for hardening of the arteries than individuals with healthy vitamin K intake.

Researchers continue to explore a wide range of health-supportive roles for vitamin K. At the forefront of this research are roles in three basic areas: (1) protection against oxidative damage; (2) proper regulation of inflammatory response; and (3) support of brain and nervous system structure. With respect to protection against oxidative damage, vitamin K does not appear to function directly as an antioxidant in the same manner that other antioxidant vitamins (like vitamin E and vitamin C) do. Yet, both phylloquinone and menaquinone forms of vitamin K appear helpful in protecting cells???particularly nerve cells???from oxidative damage. In terms of inflammatory response, several markers of pro-inflammatory activity???including, for example, release of interleukin-6 (IL-6)???are significantly lowered by healthy vitamin K levels. Finally, with regard to brain and nervous system structure, vitamin K is known to be required for synthesis of a very important family of brain and nervous system fats called sphingolipids. These fats are critical in the formation of the myelin sheath that forms an outer wrapping around the nerves, and both phylloquinone and menaquinone forms of vitamin K have been found effective in supporting synthesis of these key nervous system components. All of the above roles for vitamin K have been investigated primarily in laboratory studies on animals or in laboratory studies on human cell samples.

What are deficiency symptoms for vitamin K?

Persons deficient in vitamin K are first and foremost likely to have symptoms related to problematic blood clotting or bleeding. These symptoms can include heavy menstrual bleeding, gum bleeding, bleeding within the digestive tract, nose bleeding, easy bruising, blood in the urine, prolonged clotting times, hemorrhaging, and anemia. A second set of vitamin K deficiency-related symptoms involves bone problems. These symptoms can include loss of bone (osteopenia), decrease in bone mineral density (osteoporosis), and fractures???including common age-related fractures like that of the hips. Yet another set of vitamin K deficiency-related symptoms involves excess deposition of calcium in soft tissues. These calcification-based problems include hardening of the arteries or calcium-related problems with heart valve function.

What are toxicity symptoms for vitamin K?

Since no adverse effects have been reported for higher levels of vitamin K intake from food and/or supplements, there are no documented toxicity symptoms for vitamin K. Levels as high as 340 micrograms per day have been reported in U.S. diets, and if dietary supplements are included, daily intake levels as high as 367 micrograms have been reported. In animal studies, vitamin K has been provided in amounts as high as 25 micrograms per kilogram of body weight (or for an adult human weighing 154 lbs, the equivalent of 1,750 micrograms of vitamin K) without noticeable toxicity. For these reasons, the Institute of Medicine at the National Academy of Sciences chose not to set a Tolerable Upper Limit (UL) for vitamin K when it revised its public health recommendations for this nutrient in 2000.

One important exception to these toxicity results involves a synthetic form of vitamin K called menadione. While this form of vitamin K can sometimes be converted by the body into non-toxic forms, research studies have shown unwanted risk stemming from intake of menadione. This risk involves excessive oxidative stress and resultant damage to a variety of cell types, including kidney and liver cells. Based on these findings, the U.S. Food and Drug Administration (FDA) does not allow vitamin K to be sold as a dietary supplement in its menadione form. (Menadione is also commonly referred to as Vitamin K3.)

How do cooking, сторидж, or processing affect vitamin K?

As a general rule, vitamin K is a resilient nutrient and is fairly well retained in most cooked or stored foods. We realize that some websites caution heavily against the freezing of some vegetables due to potential loss of vitamin K, but we have not seen research that documents this risk. In fact, the vast majority of research studies show a range of vitamin K values for raw/fresh, frozen, and cooked foods that varies by about 20-30% for any particular food. It is difficult to draw any hard and fast conclusions from this range of values because there can be at least a 20-30% variation in vitamin K between different varieties of the same food, especially when grown under different circumstances (for example, in different countries).

With respect to cooking, studies at the Nutrient Data Laboratory (part of the Agricultural Research Service at the U.S. Department of Agriculture???s facility in Beltsville, MD) have shown heating to cause no major loss of vitamin K in vegetables. In some cases, cooking actually appears to increase the measurable amount of vitamin K. Researchers have speculated that this increase in vitamin K following heating may be due to the location of the vitamin K in the vegetables. Because the phylloquinone forms of vitamin K are located in the chloroplast components of the plant cells, cooking might be able to disrupt the plant cell walls and release some of the vitamin K, which then would get measured in the laboratory where it would otherwise go undetected. Whether this release of vitamin K from the chloroplasts improves the availability of vitamin K in our body has not been determined. But in any event, the cooking of vegetables does not appear to affect their vitamin K content in a negative way.

Commercial processing, however, is another matter. Particularly with respect to fruits and their commercial processing into fruit juice, we???ve seen evidence of dramatic vitamin K loss. While we have not seen evidence about the juicing of fresh fruits at home, we suspect that home juicing would have far less impact on the vitamin K found in fruits (or vegetables, if fresh vegetables were being juiced).

In summary, research shows that the freezing and storing of vegetables and fruits and the heating of these foods are practices that do not cause excessive loss of vitamin K. Therefore, excellent vitamin K nourishment does not depend on consumption of raw/fresh foods (even though raw/fresh foods may be outstanding components of a diet for many other reasons).

What factors might contribute to a deficiency of vitamin K?

Any health problems that compromise digestion and/or absorption of nutrients can contribute to deficiency of vitamin K. These problems include health conditions like inflammatory bowel disease, ulcerative colitis, celiac disease, short bowel syndrome, and digestive tract surgeries (like intestinal resection). Problems with pancreatic function, liver function, or gallbladder function can also increase our risk of vitamin K deficiency.

Because our intestinal bacteria help supply us with vitamin K, any drugs that alter our normal intestinal bacteria can compromise our vitamin K status. At the top of this drug list would be antibiotics b...