28 March 2017Vitamin K Update

The term "vitamin K" refers to several different molecules that all have a basic quinone structure at one end. They vary in the chemical structures at the other end, which determine their biochemical function. Vitamin K molecules play essential roles in numerous biochemical processes, including blood clotting and activation of bone building proteins. In fact, "K" comes from the German word 'koagulation'.

Blood clotting is probably vitamin K's best-known function. Clotting is a complex multi-step process, and vitamin K plays a critical part, thanks to its ability to bind calcium and activate clotting factors. People who are vitamin K deficient can have impaired blood clotting, leading to easy bruising, nosebleeds and bleeding gums. People prone to blood clots and stroke are typically given warfarin, a drug that interferes with vitamin K in the clotting process.

The latest buzz about vitamin K is about bone health. The discovery of vitamin K-dependent proteins in bone led researchers to investigate a possible role for this vitamin in bone mineralization. But supplementation studies using vitamin K1 - phylloquinone - for 1-3 years showed no benefit for bone health. Japanese cuisine offered another possible clue: natto, a fermented soy food and an especially good source of the menaquinone form of vitamin K, MK-7 (another form, MK-4, is found in some other foods). The theory is that the menaquinone form of vitamin K is the most effective form for activating bone-building proteins

Natto is popular in certain regions of Japan. Research shows that elderly women in those regions who eat natto have higher bone density than non-natto-eaters. In another study, older men who consume the highest amounts of natto had higher bone density than men who consumed little. Is it the MK-7? Studies using MK-4 or MK-7 supplements have shown mixed results. Some studies showed improved bone density; some showed no difference when compared to placebo. Despite these results, researchers do agree on one thing: more and better studies are needed to clarify the potential vitamin K benefit.

Finally, the vitamin K-calcium connection may be linked to another serious condition: vascular disease. As atherosclerotic plaque builds up inside blood vessels, the plaque is calcified, which increases risk for rupture and heart attack. Some research suggests that vitamin-K dependent proteins may prevent this calcification. Vitamin K supplementation studies of older subjects showed that arterial calcification was significantly slowed in supplemented groups. In some cases, the people with the most improvement after 3 years of supplementation had started with the most arterial stiffness.

By now you might be thinking it would be a good idea to get enough vitamin K in your diet. How do you do that? The daily Adequate Intake recommendations are 120 micrograms for males and 90 mcg for females. Keep in mind, at the moment those recommendations do not distinguish between K1 and other forms of vitamin K, even though the different forms may have different functions in the body.

Food Sources

Vitamin K1, phylloquinone, is concentrated in plant foods, notably leafy greens. Some of the best sources include:

  • Kale
  • Collard greens
  • Spinach
  • Beet Greens
  • Turnip Greens
  • Chard
  • Brussels sprouts
  • Broccoli
  • Cabbage
  • Asparagus
  • Peas
  • Green beans

One cup of cooked kale leaves has over 1100 micrograms of vitamin K1, roughly 10X the daily requirement (NOTE: one cup of cooked kale is a LOT of kale). 1 cup of cooked spinach has 890 mcg, while one cup of raw spinach has about 150 mcg. So as you can see, it's relatively easy to get a day's recommended dose of K1 from food. There's one catch: vitamin K is a fat soluble vitamin, so it's better absorbed if eaten with a meal that contains fat. Perhaps that's a good reason to put oil-based salad dressing on tossed green salads.

People on the anticoagulant warfarin are typically told to limit vitamin K-containing foods and high dose supplements. Warfarin acts by interfering with vitamin K. If your typical intake of vitamin K from foods is relatively stable, your drug dose can be adjusted to account for that. Problems can occur if you suddenly start eating lots of vitamin K for some reason. Example: you develop a taste for fresh spinach salads in spring and you go from eating no spinach to 2 cups of raw spinach everyday. The physician who prescribes your medication will have more advice about moderating vitamin K intake. Some newer anticoagulant drugs are not vitamin K antagonists, so diet restrictions would be unnecessary.

When it comes to vitamin K2, or menaquinones, the food data is less complete, and the amounts in food are smaller. In addition to natto, K2 is found in fermented dairy foods and some meats. According to USDA data, 1 oz of cheddar cheese has only 3 micrograms of MK-4. 3 oz of cooked chicken thigh meat has 21 mcg. Despite the known high menaquinone content of natto, the USDA database has no information on that amount. According to a monograph from Natto Pharma, manufacturer of vitamin K2 supplements, 100 grams of natto has 1500 mcg of K2 (other sources give slightly lower values). I don't know what a typical serving of natto weighs; 100 grams sounds like a lot for a strongly fermented food. In some of the studies mentioned above, the supplement dose was 180 mcg/day of K2, just over 10 grams of natto.

Take Away Message

Vitamin K is not just for blood clotting. It has several other important metabolic roles, which are currently being investigated, including bone mineralization and vascular health. As with all things nutrition, vitamin K alone is not the one answer to strong bones or healthy blood vessels. And there is no evidence that more is better. Include vitamin-K rich greens in your regular diet. Hard cheeses and other fermented dairy foods will contribute some K2. If you're curious, perhaps try natto? I admit I have not done that as yet. For now, I'll stick with cheese.

Donna P Feldman MS RDN

is author of "Feed Your Vegetarian Teen", writes about food and nutrition at Radio Nutrition and is co-host of the Walk Talk Nutrition podcast series.

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Nutrients/Other Vitamins & Minerals Other Health Issues/Cardiovascular Disease

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