When you think about vitamins for bone health, calcium and vitamin D probably come to mind immediately. For heart health, you might consider omega-3s or CoQ10. But there's a critical vitamin that influences both of these vital systems, and chances are you've never given it a second thought. Vitamin K, often called the "forgotten vitamin," may be the missing piece in your health puzzle.
Despite being discovered nearly a century ago, vitamin K has remained in the shadows while its more famous counterparts grabbed headlines. That's beginning to change as research reveals the remarkable roles this vitamin plays in keeping calcium where it belongs - in your bones, not your arteries.
Understanding Vitamin K: K1 vs K2
Vitamin K isn't a single compound but a family of related molecules. The two most important forms for human health are vitamin K1 (phylloquinone) and vitamin K2 (menaquinone). While they share some functions, they have distinct roles and sources that are important to understand.
Vitamin K1 is found primarily in green leafy vegetables. It's the form most people get from their diet, and its primary function is supporting blood clotting. When you eat spinach, kale, or broccoli, you're getting vitamin K1. Your body can convert small amounts of K1 to K2, but this conversion is inefficient.
Vitamin K2 is where things get really interesting. This form is found in fermented foods and animal products, and it plays a crucial role in directing calcium to the right places in your body. K2 comes in several subtypes, with MK-4 and MK-7 being the most studied. MK-4 is found in animal products like egg yolks and chicken, while MK-7 comes from bacterial fermentation and is abundant in foods like natto (fermented soybeans).
The Calcium Paradox: Why K2 Matters
Here's a troubling paradox that has puzzled researchers: why do some people who take calcium supplements for bone health actually experience increased cardiovascular risk? The answer lies in understanding what happens to calcium in your body.
Calcium doesn't automatically go to your bones just because you swallow it. Without proper direction, calcium can deposit in your arteries, contributing to atherosclerosis and arterial stiffness. This is where vitamin K2 becomes essential. K2 activates two crucial proteins:
- Osteocalcin: This protein binds calcium and incorporates it into your bone matrix, strengthening bones and increasing bone mineral density.
- Matrix GLA Protein (MGP): This protein prevents calcium from depositing in soft tissues, including your arteries, kidneys, and cartilage.
Without adequate vitamin K2, these proteins remain inactive, and calcium can end up in all the wrong places. You might be building up arterial plaque while your bones remain fragile - the worst of both worlds.
Vitamin K and Bone Health: The Evidence
The connection between vitamin K and bone health is supported by substantial research. Multiple studies have shown that higher vitamin K intake is associated with greater bone mineral density and reduced fracture risk.
A landmark study in the American Journal of Clinical Nutrition followed over 72,000 women for 10 years. Those with the lowest vitamin K intake had a 30% higher risk of hip fracture compared to women with the highest intake. Other research has shown that vitamin K2 supplementation can:
- Slow bone loss in postmenopausal women
- Improve bone strength markers beyond just mineral density
- Work synergistically with vitamin D and calcium for optimal bone building
- Reduce fracture risk in osteoporotic patients
In Japan, where natto consumption is common, vitamin K2 (as MK-7) has been used therapeutically for osteoporosis treatment for decades. Regions of Japan with high natto consumption have notably lower rates of hip fractures compared to areas where natto is less popular.
Cardiovascular Protection: Keeping Arteries Clean
Perhaps even more exciting is the emerging research on vitamin K2 and cardiovascular health. The Rotterdam Study, which followed nearly 5,000 participants for over seven years, found that those with the highest vitamin K2 intake had a 57% lower risk of dying from heart disease compared to those with the lowest intake. Importantly, this benefit was specific to K2 - vitamin K1 showed no such association.
Arterial calcification is a significant predictor of cardiovascular events. Research has shown that vitamin K2 supplementation can:
- Slow the progression of existing arterial calcification
- Improve arterial flexibility and reduce stiffness
- Support healthy blood pressure
- Complement the benefits of statin medications
For people taking high-dose calcium or vitamin D supplements, adding vitamin K2 may be particularly important to ensure that extra calcium ends up strengthening bones rather than hardening arteries.
Signs You Might Be Deficient
Vitamin K deficiency severe enough to impair blood clotting is rare in healthy adults. However, subclinical deficiency - enough to affect bone and cardiovascular health without obvious symptoms - may be surprisingly common. Signs that you might not be getting enough vitamin K include:
- Easy bruising or excessive bleeding from minor cuts
- Heavy menstrual periods
- Bleeding gums despite good dental hygiene
- Osteoporosis or osteopenia, especially if not responding to calcium/vitamin D
- Arterial calcification visible on imaging
Certain groups are at higher risk for vitamin K deficiency:
- People with digestive disorders: Conditions like Crohn's disease, celiac disease, and ulcerative colitis can impair vitamin K absorption.
- Those on long-term antibiotics: Gut bacteria produce some vitamin K2, and antibiotics can disrupt this production.
- People who've had bariatric surgery: Fat malabsorption following surgery reduces absorption of fat-soluble vitamins.
- Individuals taking certain medications: Cholesterol-lowering drugs, some antibiotics, and high-dose vitamin A can interfere with vitamin K status.
- People avoiding animal products and fermented foods: K2 is primarily found in these foods.
Food Sources of Vitamin K
The best way to get vitamin K is through food. Here are the top sources for each form:
Vitamin K1 sources (aim for at least one serving daily):
- Kale (1 cup cooked): 1,062 mcg
- Spinach (1 cup cooked): 888 mcg
- Collard greens (1 cup cooked): 773 mcg
- Swiss chard (1 cup cooked): 573 mcg
- Broccoli (1 cup cooked): 220 mcg
- Brussels sprouts (1 cup cooked): 219 mcg
Vitamin K2 sources:
- Natto (3.5 oz): 1,000+ mcg (primarily MK-7)
- Goose liver (3.5 oz): 369 mcg
- Hard cheeses like Gouda (3.5 oz): 75 mcg
- Soft cheeses like Brie (3.5 oz): 57 mcg
- Egg yolks (1 large): 32 mcg
- Chicken (3.5 oz dark meat): 60 mcg
- Butter from grass-fed cows (1 tbsp): 15 mcg
Notice that natto is dramatically higher in K2 than any other food. If you can acquire a taste for this sticky, pungent Japanese dish, it's an unparalleled source. However, most Western diets provide very little K2, which is why supplementation may be worth considering.
Should You Supplement?
While eating plenty of leafy greens can provide adequate K1, getting sufficient K2 through diet alone is challenging unless you regularly eat natto or organ meats. Supplementation may be beneficial if you:
- Take calcium or vitamin D supplements
- Have osteoporosis or osteopenia
- Have cardiovascular disease or risk factors
- Don't regularly eat fermented foods or organ meats
- Have digestive issues affecting fat absorption
- Take medications that interfere with vitamin K
When choosing a supplement, MK-7 is generally preferred over MK-4 for several reasons: it has a longer half-life in the body (several days vs. several hours), it's more effective at lower doses, and it reaches more tissues including bones and arteries. A typical therapeutic dose is 100-200 mcg of MK-7 daily.
Important Considerations and Warnings
If you take blood-thinning medications like warfarin (Coumadin), vitamin K supplementation requires careful medical supervision. Warfarin works by blocking vitamin K's role in blood clotting, so taking vitamin K supplements can reduce the medication's effectiveness. This doesn't mean you can't have any vitamin K - but your intake needs to be consistent and monitored.
Newer blood thinners like apixaban (Eliquis) and rivaroxaban (Xarelto) work differently and don't interact with vitamin K. If you're on these medications, vitamin K supplementation is generally considered safe, but always confirm with your healthcare provider.
For everyone else, vitamin K has an excellent safety profile. There's no established upper limit for vitamin K intake because toxicity hasn't been observed even at high doses. However, as with any supplement, more isn't necessarily better - stick to recommended amounts unless directed otherwise by a healthcare professional.
Putting It All Together
Vitamin K may be the missing piece in many people's bone and heart health strategies. If you're taking calcium and vitamin D but not seeing the bone benefits you expected, or if you're concerned about cardiovascular calcification, adding vitamin K2 to your regimen could make a meaningful difference.
Start by increasing your intake of leafy greens for K1 and exploring fermented foods for K2. Consider a K2 supplement, particularly if you fit into any of the higher-risk categories. And most importantly, view vitamin K as part of a comprehensive approach to bone and cardiovascular health - one that includes adequate protein, weight-bearing exercise, and attention to other key nutrients like magnesium and vitamin D.
The "forgotten vitamin" is finally getting the attention it deserves. Don't let it remain forgotten in your health routine.