Vitamin D3 + K2

Vitamin D3 + K2 is a 7.5–8.0/10 fit for deficient adults, winter indoor workers, postmenopausal women, and people pairing vitamin D with calcium or bone goals, not a guaranteed benefit for already replete adults. The cleanest evidence anchors are Jolliffe 2021, which found acute respiratory infection risk reduction was strongest in deficient adults, and Knapen 2013, which used MK-7 180 mcg per day for bone outcomes. Diederichsen 2022 adds useful context: keeps cardiovascular calcification claims honest. The practical gap is the same one that shows up across the report: mechanism and early outcomes are more convincing than broad real-world certainty. In practice, Vitamin D3 + K2 belongs after the basics, works best when the target is specific, and deserves tracking around benefits, side effects, interactions, and cost before it becomes a standing protocol.

✅ Best for: Adults living above 35° latitude during winter months, indoor workers with minimal sun exposure, those with confirmed serum 25(OH)D below 30 ng/mL, postmenopausal women targeting bone protection, and adults with cardiovascular calcification risk who want both calcium-routing protection and arterial-stiffness benefits. The deficient majority sees clear, replicable benefit. People with darker skin, higher BMI, anticonvulsant or glucocorticoid users, and gastric bypass patients have higher requirements and benefit most.

❌ Avoid if: You are on warfarin, Coumadin, or other vitamin-K antagonists (K2 reverses anticoagulation; this is a hard contraindication unless your prescribing physician explicitly approves and monitors INR closely). You have sarcoidosis, granulomatous disease, primary hyperparathyroidism, Williams syndrome, or idiopathic infantile hypercalcemia (D3 absolutely contraindicated due to autonomous calcitriol production). You are sun-replete year-round with serum 25(OH)D consistently above 50 ng/mL. additional supplementation provides minimal benefit per VITAL primary outcomes. You take thiazide diuretics with lithium and high-dose D3 (additive hypercalcemia risk).