What Does The Data Say About Vitamin D3 And K2 For Bone Health?

Recently, a few of my Instagram posts about osteoporosis and supplemental calcium went viral. Thousands of you wanted to know more about these “bone” vitamins.

Before we dive in, let me reiterate: there has been extensive data out for well over 10 years that shows that supplemental calcium of any kind, when taken by the vast majority of post-menopausal women with osteoporosis:  

1. Does not lower the risk of fractures
2. Actually increases the risk of heart attacks and kidney stones  

That means most women should not take calcium in supplement form. They should get it from food. Now, with that out of the way, let’s talk about the vitamins everyone’s curious about: Vitamin D3 and K2.  

Vitamin D3: Helpful When It’s Correcting a Problem

First up: Vitamin D3. This vitamin helps your bones absorb calcium. When vitamin D is deficient, bones suffer. That part is not controversial. The question is whether everyone needs a supplement to prevent fractures.  

• The General View: The USPSTF (2024 draft) suggests that healthy older adults do not need routine Vitamin D supplements for fracture prevention, as the evidence shows no net benefit.  
• The Nuanced View: The Endocrine Society’s 2024 guideline suggests that certain groups (like adults over 75) may still benefit from supplementation, even if there’s uncertainty around optimal targets for broad “prevention” use.  

Translation: Vitamin D is most helpful when you’re deficient, have limited sun exposure, have malabsorption risk, osteoporosis, or other clear indications.  

Personally? I take D3 intermittently at a dose of 1000-2000 units a day. More is NOT better: 4,000 units a day is the maximum (or you can get 15 minutes of outdoor natural sunlight, but the tradeoff is sun/skin damage).  

Vitamin K2: The “Traffic Cop” Hypothesis

Think of Vitamin K2 as the “traffic cop” for your bones. It activates two key proteins that manage calcium in the body: osteocalcin, which helps bind calcium to bones and teeth, and matrix Gla protein (MGP), which prevents calcium from depositing in blood vessels.  

• Zoom In: K2 shuttles calcium into bones and teeth (where you want it) and keeps it out of arteries and soft tissues (where you don’t).  

Researchers have looked at K2 forms (like MK-4 or MK-7) to see if they can increase bone density. While most experts agree that Vitamin K plays a role in bone metabolism, the data on whether it actually prevents fractures is inconsistent. It’s not ready for primetime, as the saying goes.  

While we do have older trials in postmenopausal women with osteoporosis showing that combined D3 and K2 improved lumbar spine bone mineral density, we don’t have a slam-dunk, “everyone should take D3+K2” body of evidence.

Pros and Cons For Women Over 50

Potential Pros

• Better Absorption: Vitamin D3 may help your body better absorb calcium.  
• Bone Quality: Vitamin K2 may improve bone-related proteins and markers and support bone quality in some settings.  

Potential Cons & Cautions

• No “Magic Bullet”: For healthy adults without a deficiency, taking extra Vitamin D3 hasn't shown a clear fracture prevention benefit.  
• Medication Conflict: Vitamin K2 interacts with warfarin (and other vitamin K–antagonist anticoagulants). Do not start it without your clinician. This is a hard stop.  
• More is Not Better: High-dose supplementation can create imbalances and distract from what actually moves the needle: Protein, strength training, and fall prevention.

Practical Tips For Women Over 50

First, look at your own personal risk factors for low Vitamin D. That includes limited sun exposure, darker skin, a higher BMI, malabsorption, or osteoporosis. If these apply to you, supplementation may be more likely to help. The ideal blood level range hovers around 50.  

A few more tips:  

• Take Vitamin D3 with a meal that contains fat for better absorption (such as olive oil, salmon, avocado, and eggs).  
• Don’t let supplements replace the “big levers”: strength training, adequate protein, and calcium-rich foods like yogurt or kefir, sardines with bones, calcium-set tofu, and leafy greens.  
• If you’re considering K2, double-check your medications first. Always inform your doctor about the supplements you take.  

Bottom line: D3 makes sense when you need it/for many or most women. K2 might help some women (data does not clearly say who yet or how much), but it’s not a universal bone insurance policy, and at this time, we should get our K2 from whole food sources (Google your favorites) rather than supplements.

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