Why deficiency is nearly structural after 50
Vitamin D is unlike most nutrients. Your body can produce it — if you're exposed to sunlight. That mechanism degrades progressively after 50.
Skin synthesis of vitamin D drops 75% between age 20 and age 70. At 70, an hour of sun exposure produces four times less vitamin D than it did at 20, for the same exposure duration. Add reduced intestinal absorption with age, and a tendency to spend less time outdoors, and you have a structural deficiency risk.
A 2025 retrospective study of 320 adults over 50 — 138 with hip fractures, 182 community controls — measured significantly lower vitamin D levels in the fracture group, and identified that the deficit acts first by reducing muscle function, before it even reaches bone density. This is a key mechanism: vitamin D deficiency weakens muscles, which then can't protect bones from falls.
The updated 2026 guidelines: what the numbers actually mean
The Endocrine Society updated its clinical practice guidelines in 2026. Here's what matters:
Optimal range for musculoskeletal health: 30–50 ng/mL (75–125 nmol/L) of serum 25-hydroxyvitamin D. Below 20 ng/mL is insufficiency. Below 12 ng/mL is severe deficiency — with documented risks of fractures and significant muscle weakness.
If you've never had a blood panel that includes vitamin D, that's your starting point. Without knowing your level, any supplementation is flying blind. The test is 25-OH vitamin D (also called calcidiol). A standard blood panel can include it on physician request — or you can order it directly through several at-home testing services.
D3 or D2? And should you add K2?
Supplemental vitamin D comes in two forms: D2 (ergocalciferol) and D3 (cholecalciferol). Comparative studies show D3 is 87% more effective than D2 at raising serum 25-OH vitamin D levels. D3 is the form to take, consistently.
The K2 question is more nuanced. Vitamin K2 (particularly the MK-7 form) plays a role in calcium routing — it favors calcium deposition in bones rather than arteries. If you're taking high-dose vitamin D (above 4,000 IU per day), the D3+K2 combination is recommended by most bone health experts.
For most adults over 50 looking to maintain muscle and bone capital, 2,000–4,000 IU of D3 daily is a reasonable starting point — adjusted after a blood panel. Sun exposure (20–30 minutes midday on arms and legs) remains useful but insufficient on its own past 50.
What this changes if you're training
Vitamin D isn't just about bones. It's directly involved in muscle contraction, muscle protein synthesis, and intracellular calcium regulation — a central mechanism in force production.
People who are vitamin D deficient show lower grip strength, slower walking speed, and significantly reduced chair-stand performance compared to those with sufficient levels. In studies on adults over 60, correcting a vitamin D deficiency produces measurable improvements in muscle performance within 12 weeks.
The action plan is simple: get your level tested, correct it if you're below 30 ng/mL, and retest in 3 months. This isn't an abstract health expense — it's a prerequisite for getting full value from your training.