There’s a lot of chatter about pairing Vitamin D3 with Vitamin K – usually K2 – for “optimal” health, especially for bones and heart. I get it, the combo sounds like a dream team. But as a pharmacist, I’ve got some reservations. Here are 7 reasons I’d think twice before popping D3 with K together—at least without some serious consideration.
7 Reasons To Consider Just Taking D3
1. You Could End Up with Too Much Calcium in Your Blood
Vitamin D3 is a champ at pulling calcium from your gut into your bloodstream. That’s its job! But if you overdo it with high doses—like 10,000 IU daily—your blood calcium levels can climb too high, a dangerous condition called hypercalcemia. This might lead to kidney stones or even calcium deposits in your arteries.
Vitamin K2 is often touted as the fix, directing calcium to bones instead of soft tissues. And it’s true—K2 activates proteins like osteocalcin and MGP to help with that. But if your D3 dose is sky-high, even K2 might not keep up, leaving you at risk. I’d rather you stick to moderate D3 doses, and skip the combo guessing game. By “moderate” I am referring to somewhere between 2,000 and 5,000 IU daily (with routine blood work to determine your serum D3 levels).
2. It Might Not Protect Your Bones Like You Think
High doses of D3 alone can sometimes backfire on bones. Studies—like one in JAMA from 2019—showed that 10,000 IU daily didn’t strengthen bones in older adults and might even increase fracture risk by throwing off bone remodeling. Adding K2 could help in theory, since it supports calcium binding in bones, but the combo isn’t a magic bullet. If D3 is too high, the balance gets tricky. I’d rather see you get your D3 right first—say, 2,000 to 5,000 IU—than rely on K2 to patch it up.
3. Vitamin K Is Easy to Get from Food
You might not need a K supplement at all. Think of it! Vitamin K1 is in every leafy green out there—spinach, kale, you name it—and covers your clotting needs. K2, the one paired with D3, comes from fermented foods like natto or animal goodies like egg yolks, natto, butter/ghee, chicken, pork, cheese, and liver and maybe a few other things. Sure, K2 might be lower in Western diets, and some studies say 100-200 mcg daily helps bones and arteries. But if you eat a decent mix of foods, you’re probably getting enough K—why add a pill?
4. The Combo Could Make D3’s Effects Harder to Predict
D3 gets turned into 25(OH)D in your liver, then 1,25(OH)2D in your kidneys—the active form that cranks up calcium absorption. High D3 doses can push this system hard, raising calcium levels. K2 doesn’t directly boost that active form (despite some old theories), but it does handle the calcium D3 brings in. The wrinkle? If you’re piling on both, it’s tough to know how much calcium is floating around—or where it’s landing. I’d rather keep it simple with D3 alone and track how you feel. I’m not anti-K2 though, I don’t want you to think that, it’s totally fine. This is just my opinion and of the mindset that K2 is pretty easy to get into your diet.
5. Doctors Might Test the Wrong Vitamin D
Most docs check 25(OH)D to gauge your D status—it’s the stable “storage” form. But 1,25(OH)2D, the active one, matters too, especially if calcium’s running wild. High D3 doses can skew these levels, and while K2 won’t mess with the test directly, the combo complicates the picture—how much is too much? I’d test both forms if you’re experimenting with D3 and K together, but honestly, plain D3 is easier to monitor.
6. Your Body Already Balances Calcium Pretty Well
Your parathyroid glands, kidneys, and bones are a tight crew, keeping calcium in check. D3 nudges absorption up, and K2 can steer it to bones, sure. But if your diet’s solid and your D3 isn’t crazy-high, your body’s got this. Overloading with both might just throw a wrench in that natural rhythm—why risk it? If you want to read more about the dangers of too much calcium read my other blog, Calcium and Hypercalcemia Treatment Options.
7. More Isn’t Always Better
I see folks chasing megadoses—10,000 IU D3, 200 mcg K2—thinking more equals better. Nope. Excess D3 can push calcium too far, and while K2 might soften the blow, it’s not a cure-all. Moderation’s my mantra: 2,000-5,000 IU D3, food-sourced K, and you’re golden without the combo chaos.
Final Thoughts
I’m not saying Vitamin K2 is useless—it’s got its perks for bones and arteries. I like it, I definitely do.
As for D3? Essential, especially if you’re low. But pairing them isn’t always the golden ticket it’s cracked up to be by people who sell these types of supplements. High-dose D3 alone can stir up trouble—kidney stones, wonky bones—and K2 won’t fully fix that. Since I sell plain D3 (no K2 attached), I’d rather you nail your dose, eat some greens, and keep risks to a minimum. Talk to your doc, test your levels, and keep it simple. What do you think—am I being too cautious here?
Suzy Cohen, has been a licensed pharmacist for over 30 years and believes the best approach to chronic illness is a combination of natural medicine and conventional. She founded her own dietary supplement company specializing in custom-formulas, some of which have patents. With a special focus on functional medicine, thyroid health and drug nutrient depletion, Suzy is the author of several related books including Thyroid Healthy, Drug Muggers, Diabetes Without Drugs, and a nationally syndicated column.
