Vitamin K:
The Overlooked Bone Builder and Heart Protector
Susan E. Brown, Ph.D., CCN
Director the Osteoporosis Education Project
© 2004

GLA (y-carboxyglutamic acid) is one of the many important amino acids (proteins) produced in the body. The only known role of vitamin K within the body is a cofactor for the formation of glutamate residues in certain precursors of this amino acid (GLA). Specifically, it is a cofactor for the vitamin K dependent y-carboxylation reaction.

Vitamin K is well known for its action facilitating blood coagulation. This action is due to vitamin K's role in GLA protein production. Since its discovery in the mid-1970s vitamin K was identified exclusively for its role in blood coagulation. Recently, however, vitamin K dependent GLA formation has also been identified as critical to strong bones and healthy hearts. Specifically, the vitamin K dependent y-carboxylation process is important for the formation and proper functioning of a bone protein known as "osteocalcin." If osteocalcin is properly carboxylated more mineral compounds are bound into bone. So in simple terms adequate vitamin K helps calcium and phosphorus bind onto the bone protein matrix. Several epidemiological studies show the link between low vitamin K intakes and higher rates of hip fracture. While we recommend leafy green vegetables like kale, collards, beet and mustard greens, and spinach for their high vitamin K content, even just regular consumption of common lettuce helps strengthen bones and reduce fracture risk.

In heart arterial health the same GLA protein is important, but in a different way. Did you every wonder how it is the body knows to deposit calcium in the bones, but not in other soft tissue, like the arteries? The vitamin K dependent protein GLA is part of the answer. When GLA is again properly carboxylated (by adequate supply of vitamin K) there is a strong inhibition of calcium deposition in the walls of blood vessels. When GLA is undercarboxylated (due to low vitamin K supply) calcium is allowed to drift from bone into arteries and other soft tissue. These calcium deposits in the artery increase the risk of heart attack. Recent studies in both animals and humans suggest that higher doses of vitamin K may help prevent cardiovascular diseases.

Vitamin K: The Best Food Sources

Vitamin K is not a single nutrient, but the name given to a group of vitamins of similar composition. The two main groups of vitamin K that occur naturally are vitamin K1 (phylloquinone) and K2 (menaquinone). K1 is found in many vegetables and K2 is produced by bacteria.

Dark green leafy vegetables, and fermented foods are high in Vitamin K. While you probably have enough vitamin K to provide for normal blood clotting, you most likely do not consume enough vitamin K to allow for optimum bone health. For example, studies document that for optimization of the bone protein osteocalcin adults would do well with a daily K1 intake of 1000 mcg. Currently the average K1 intake in this country is only 75 to 125 mcg.

The table below lists the foods highest in vitamin K1. It is worth noting that the absorption of vitamin K from vegetables is likely enhanced by the presence of dietary fat in the same meal, just as occurs with two other fat soluble vitamins, vitamins D and E. Unless you are a "greens" lover you might find it difficult to consume 1000 mcg of vitamin K in foods. Take heart and do the best you can. The prestigious Framingham Heart Study found that those with the highest vitamin K1 intake (250 mcg/day in this study compared to the recommended intake of about 75 mcg/day) had a threefold reduction in hip fracture risk. Even if your vitamin K1 intake is lower than 250 mcg a day you can also note that the 72,327 person Nurses Health Study found that those consuming a mere 109 mcg vitamin K1 a day had a lower risk of hip fracture over a ten year period than those consuming less vitamin K1.

Vitamin K2 is the most biologically active form of vitamin K. It is also the most beneficial for bone health maximization. Vitamin K2, however, is produced by bacteria and is mainly found in fermented foods such as ripe cheese, yogurt and a fermented soy food known as natto. Having a very strong odor, natto is often called the "Limburger Cheese" of Japan. Natto is by far the best food source for Vitamin K2. I suspect that one day soon you will see encapsulated natto K2 supplements on the market in this country.

Vitamin K3 (menadione) is a synthetic form that is manmade and not recommended for use as a nutritional supplement.

Foods containing vitamin K are nutritious and increasing their intake can provide many health benefits. If you suspect your vitamin K intake is low, you can also use vitamin supplements. Studies, in fact, suggest that the absorption of K1 from a tablet is considerably higher than the absorption from vegetables. You might use the following table to determine if your intake of vitamin K1 reaches the optimum level of 1000 mcg a day. If you fall short, consider increasing consumption of foods high in vitamin K, or discuss vitamin K supplementation with your health professional.

A note of caution:
Those taking medications to thin the blood, such as Coumadin, should avoid use of supplemental vitamin K as it can reverse the effects of this medication. Also those on this medication should consult with their physician before increasing consumption of foods high in vitamin K.

Foods Highest in Vitamin K1 (phylloquinone)

RDA 70-140	Mcg Vit. K1 per 1/2 cup
Kale, cooked	630
Collard greens, cooked	520
Spinach, cooked	510
Turnip greens, cooked	425
Beet greens, cooked	350
Mustard greens, cooked	270
Brussels sprouts, cooked	210
Broccoli, cooked	110
Dandelion greens, cooked	102
Parsley, 10 sprigs	90
Lettuce, boston and bibb, raw	90
Asparagus, cooked	75
Sauerkraut	70
Lettuce, green leaf, raw	50
Lettuce, romaine	25

Reference: USDA National Nutrient Database for Standard Reference, Release 16