Key Takeaway
Vitamin K2 activates two proteins that decide where calcium goes: osteocalcin, which deposits it into bone, and matrix Gla protein, which keeps it out of your arteries. That makes K2 the natural partner to vitamin D3, which boosts calcium absorption but does not control its destination. My Nutricost capsules are MK-4, the form found in animal foods; the other common form, MK-7, lasts far longer in the blood per dose. Most Western diets are low in K2 because the richest source, natto, is rare here. For a 38-year-old taking 5,000 IU of D3 daily, adding K2 is a logical insurance policy for both my skeleton and my arteries. The evidence is strongest for bone and arterial calcium handling; just do not take it if you are on warfarin.
Most people lump all vitamin K together and assume it is the "blood clotting vitamin" they learned about once and never thought about again. That is half the story, and the half they are missing is the half that matters for anyone serious about long-term health: vitamin K2 is one of the most important and most overlooked nutrients for keeping your skeleton strong and your arteries clear.
I started taking K2 specifically because I take a high dose of vitamin D3, and once you understand how the two interact, taking D3 without K2 starts to feel like only doing half the job. Let me explain the logic, walk through the evidence, sort out the confusing MK-4 versus MK-7 question, and tell you where this fits for a lifter.
Vitamin K1 vs. K2: Two Different Jobs
"Vitamin K" is actually a family of related compounds, and the two that matter are K1 and K2.
Vitamin K1 (phylloquinone) is the form found in leafy green vegetables: spinach, kale, broccoli, collards. It is the form your body uses primarily for blood clotting, and it is the reason people on the blood thinner warfarin are told to keep their green vegetable intake consistent. Most people get adequate K1 from diet, and the liver preferentially uses it for clotting factors.
Vitamin K2 (menaquinone) is a different animal. It is found in fermented foods and animal products, and it is the form that goes to work in your bones and blood vessels rather than just your liver. K2 itself comes in several subtypes named by the length of their side chain: MK-4, MK-7, MK-9, and others. The two you will see on supplement labels are MK-4 and MK-7.
The critical insight, established by researchers like Cees Vermeer in the Netherlands, is that the body handles K1 and K2 quite differently. K1 stays mostly in the liver and serves clotting. K2 circulates longer and reaches the peripheral tissues, bone and arteries, where it activates the proteins that manage calcium. So even if your green-vegetable K1 intake is fine, you can still be functionally short on the K2 your bones and arteries depend on.
What K2 Actually Does: The Calcium Traffic Cop
Here is the mechanism that makes K2 click. Vitamin K is a required cofactor for an enzyme that performs "carboxylation" — a chemical modification that switches certain proteins from inactive to active. Two of these vitamin-K-dependent proteins are the whole story for our purposes:
- Osteocalcin. Produced by bone-building cells (osteoblasts), osteocalcin is the protein that binds calcium and incorporates it into the bone matrix. But osteocalcin only works when it has been carboxylated by vitamin K. Without enough K2, you produce undercarboxylated osteocalcin that cannot do its job, and the calcium does not get properly laid into bone.
- Matrix Gla protein (MGP). Found in the walls of blood vessels, MGP is the body's most powerful natural inhibitor of vascular calcification. Its job is to actively keep calcium from depositing in arterial walls. Like osteocalcin, MGP must be carboxylated by vitamin K to function. Undercarboxylated MGP cannot protect your arteries.
Put those two together and you get the metaphor that actually explains K2: it is a traffic cop for calcium. It directs calcium into bone (via osteocalcin) and away from arteries and soft tissue (via MGP). When K2 is sufficient, calcium goes where you want it. When K2 is low, you get the worst of both worlds — calcium leaching from bones while simultaneously depositing in blood vessels. That paradox, soft bones and hard arteries at the same time, is sometimes called the "calcium paradox," and adequate K2 is central to avoiding it.
Why K2 and Vitamin D3 Belong Together
This is the reason K2 ended up in my stack, and it is worth making explicit.
Vitamin D3's main job in calcium metabolism is to increase how much calcium you absorb from your gut. Take a healthy dose of D3 and you pull more calcium out of your food and into your bloodstream. That is good — but absorption is only step one. The calcium then has to go somewhere, and D3 does not control that destination. That is K2's department.
So the logical pairing is this: D3 increases the calcium supply, and K2 directs where it lands. Taking a substantial dose of D3 (I take 5,000 IU) without adequate K2 means you are increasing circulating calcium without guaranteeing it gets routed properly to bone instead of arteries. The theoretical concern, that high-dose D3 alone could increase arterial calcium deposition in someone deficient in K2, is exactly why the D3 + K2 combination has become so popular. Many supplements now sell them together for this reason.
The Practical Pairing
Both vitamins D3 and K2 are fat-soluble, so take them together with a meal that contains some fat (eggs, avocado, fish, oils). This improves the absorption of both at once. I take my D3, K2, and fish oil with the same fat-containing meal, and the fish oil conveniently provides the fat. See the vitamin D3 guide and fish oil guide for the partners.
K2 and Bone Health
The bone evidence for K2 is the most developed, much of it coming out of Japan, where MK-4 has actually been used as a prescription treatment for osteoporosis at high doses (45mg per day).
The mechanism is the osteocalcin story above: K2 carboxylates osteocalcin so it can incorporate calcium into bone. Studies measuring undercarboxylated osteocalcin (a marker of poor vitamin K status) have linked higher levels to lower bone density and higher fracture risk, particularly in older adults. Supplementing K2 reliably reduces undercarboxylated osteocalcin, confirming it is doing its biochemical job.
For clinical outcomes, the strongest data comes from MK-7 trials. A landmark three-year randomized controlled trial by Knapen and colleagues found that 180 micrograms of MK-7 daily significantly improved bone mineral density and reduced the age-related loss of bone strength in postmenopausal women. The Japanese MK-4 osteoporosis research used much higher doses (45mg) and also showed fracture-reduction benefits in that clinical population.
For a lifter, bone is not a side concern. Progressive overload loads the skeleton heavily, and strong bones are part of the infrastructure that lets you keep adding weight to the bar for decades. K2 is one of the nutrients that keeps the calcium you absorb actually building that infrastructure.
K2 and Arterial Calcification
This is the area generating the most excitement, and also the area where I want to be measured about what is proven versus promising.
The observational evidence is striking. The Rotterdam Study, a large prospective cohort, found that people with the highest dietary intake of K2 (menaquinone) had significantly lower rates of severe arterial calcification and lower cardiovascular mortality over roughly seven to ten years of follow-up. Notably, K1 intake showed no such association — it was specifically K2 that tracked with healthier arteries. A separate prospective study (Gast et al.) found higher K2 intake associated with reduced coronary heart disease risk in women.
The mechanism (K2 activating MGP to inhibit vascular calcification) makes these associations biologically credible rather than random. And some intervention trials, mostly using long-acting MK-7, have shown improvements in markers of arterial stiffness and inactive MGP.
Honest Limits of the Artery Data
Much of the strongest arterial evidence is observational, which shows association, not proof of cause. Intervention trials are fewer and mostly use MK-7, not MK-4. One trial using a very high dose of MK-4 (45mg/day) failed to reduce coronary artery calcium scores over a year. So while the mechanism and population data are genuinely compelling, K2 is not a proven drug for reversing arterial calcification. I take it as sound preventive logic, not as a guaranteed cardiovascular intervention.
MK-4 vs. MK-7: Which Form and Why
My Nutricost capsules are MK-4, so let me address the form question directly, because it is the single most confusing thing about K2.
Both MK-4 and MK-7 are vitamin K2 and both activate osteocalcin and MGP through the same mechanism. The difference is entirely in how they behave in the body, specifically their half-life:
| Feature | MK-4 | MK-7 |
|---|---|---|
| Source | Animal foods; synthesized in body from K1 | Fermented foods (natto); bacterial |
| Half-life | Short (~1-4 hours) | Long (~3 days) |
| Typical dose | Milligram-level (1.5mg or higher), or multiple times daily | Microgram-level (90-200mcg once daily) |
| Steady blood levels from one daily dose | Harder — clears quickly | Easier — stays elevated all day |
| Best evidence | High-dose Japanese bone/osteoporosis research | Nutritional-dose bone & arterial RCTs |
The practical implication: because MK-4 clears the blood within hours, a single small daily dose does not keep your levels elevated around the clock the way MK-7 does. This is why a lot of the modern nutritional research has shifted to MK-7, which keeps blood levels steady for days off a single microgram-level dose. If I were buying fresh today and wanted maximum convenience and steady coverage from one capsule, I would lean toward MK-7 at 100-200mcg.
That said, MK-4 is not a bad form — it is the form that occurs naturally in animal foods and the form your own body can make from K1. The 100mcg of MK-4 in my capsules is a modest nutritional dose. If I wanted to lean on MK-4 specifically, the more research-aligned approach would be a higher dose or splitting it across the day to account for the short half-life. For most people, the honest recommendation is: either form works, but MK-7 is the more convenient choice for steady, once-daily coverage.
What It Does For Me at 38
My Read, Personally
At 38, six feet, 172 pounds, I am not worried about osteoporosis tomorrow. I take K2 because I take 5,000 IU of vitamin D3 every day, and pairing the two is the logically complete version of supplementing either one. D3 raises my calcium absorption; K2 makes sure that calcium builds my bones and stays out of my arteries. I lift heavy, which means I want a skeleton that keeps adapting to load for decades, and I would like my arteries clear for just as long. K2 is preventive infrastructure for both, taken now while it is cheap and easy, not later when the damage is done.
The case for K2 in a healthy, training 38-year-old is preventive, not corrective. I do not feel K2 the way I feel caffeine, and I never will. Its entire value is in slow, long-horizon protection: bone density I want to still have at 70, and arteries I want to still be clear at 70. Those are not things you can perceive day to day, which makes K2 a discipline supplement — you take it on the strength of the mechanism and the population data, trusting that the quiet daily upkeep compounds over decades. Combined with the fact that it directly complements the D3 I already take, that is enough to keep it in the stack.
Food Sources of K2
Getting K2 from food is harder than getting K1, which is a big reason supplementation is popular. The single best source towers over everything else.
| Food | K2 Form | Approx. K2 per serving | Notes |
|---|---|---|---|
| Natto (fermented soybeans) | MK-7 | ~850-1000mcg per 100g | By far the richest source; an acquired taste |
| Goose liver / pate | MK-4 | ~360mcg per 100g | Organ meats are concentrated |
| Hard cheeses (Gouda, Edam) | MK-7/MK-8/MK-9 | ~75mcg per 100g | Fermentation produces menaquinones |
| Soft cheeses (Brie) | Mixed MK | ~55mcg per 100g | Another fermented source |
| Egg yolk | MK-4 | ~15-30mcg each | Pasture-raised yolks higher |
| Butter (grass-fed) | MK-4 | ~15mcg per 100g | Grass feeding raises content |
| Chicken (dark meat/skin) | MK-4 | ~10mcg per 100g | Practical everyday source |
The takeaway: unless you eat natto (almost no one outside Japan does), it is genuinely hard to get a robust K2 intake from a typical Western diet, especially if you avoid egg yolks, full-fat dairy, and animal fats in the name of "eating clean." Lifters who eat egg whites only and lean chicken breast are quietly cutting out their best everyday K2 sources. This is one of the clearer cases where a cheap supplement closes a real dietary gap.
Dosing and Safety
How Much
- MK-7: 90-200mcg once daily is the well-studied nutritional range; 100-180mcg is a sweet spot supported by RCTs.
- MK-4: nutritional doses around 100-1500mcg; the Japanese osteoporosis research used 45mg, which is a pharmaceutical dose, not what is in a general supplement.
- With D3: a common, sensible pairing is roughly 100mcg of K2 (MK-7) per 1,000-5,000 IU of D3.
Safety
Vitamin K2 is remarkably safe. There is no established toxicity at supplemental doses and no tolerable upper limit has been set, even at the 45mg doses used in osteoporosis research. It is fat-soluble but does not accumulate to harmful levels the way vitamins A or D can.
The One Hard Rule: Blood Thinners
If you take warfarin (Coumadin) or any other vitamin-K-antagonist anticoagulant, do not supplement vitamin K2 without explicit guidance from the doctor managing your medication. These drugs work by blocking vitamin K, and adding K2 can interfere with the careful dosing balance and your INR. This is the single most important safety consideration for vitamin K of any form. Newer anticoagulants (like apixaban or rivaroxaban) do not work through vitamin K, but you should still confirm with your prescriber.
The Bottom Line
Vitamin K2 is the calcium traffic cop. By activating osteocalcin and matrix Gla protein, it directs calcium into your bones and away from your arteries, which is precisely the job that vitamin D3 leaves unfinished. The bone evidence is strong, the arterial evidence is compelling but largely observational, and the safety profile is excellent for everyone except people on warfarin.
My capsules are MK-4, a perfectly legitimate nutritional form, though MK-7 is the more convenient choice for steady once-daily coverage thanks to its long half-life. Western diets are genuinely low in K2 unless you eat natto or plenty of egg yolks, full-fat dairy, and organ meats. For anyone taking meaningful doses of vitamin D3, adding K2 is the logical completion of the calcium story, and for a lifter who wants strong bones and clear arteries into old age, it is cheap, low-risk, long-game insurance. I take it for exactly that reason.
References
- Geleijnse, J.M., Vermeer, C., Grobbee, D.E., et al. (2004). Dietary intake of menaquinone is associated with a reduced risk of coronary heart disease: the Rotterdam Study. Journal of Nutrition, 134(11), 3100-3105.
- Knapen, M.H., Drummen, N.E., Smit, E., Vermeer, C., & Theuwissen, E. (2013). Three-year low-dose menaquinone-7 supplementation helps decrease bone loss in healthy postmenopausal women. Osteoporosis International, 24(9), 2499-2507.
- Schurgers, L.J., Teunissen, K.J., Hamulyak, K., Knapen, M.H., Vik, H., & Vermeer, C. (2007). Vitamin K-containing dietary supplements: comparison of synthetic vitamin K1 and natto-derived menaquinone-7. Blood, 109(8), 3279-3283.
- Gast, G.C., et al. (2009). A high menaquinone intake reduces the incidence of coronary heart disease. Nutrition, Metabolism and Cardiovascular Diseases, 19(7), 504-510.
- Inaba, N., Sato, T., & Yamashita, T. (2015). Low-dose daily intake of vitamin K2 (menaquinone-7) improves osteocalcin gamma-carboxylation. Journal of Nutritional Science and Vitaminology, 61(6), 471-480.
- Maresz, K. (2015). Proper calcium use: vitamin K2 as a promoter of bone and cardiovascular health. Integrative Medicine, 14(1), 34-39.