Vitamin D Isn’t Just About Bones, Here’s What the Evidence Really Shows
Vitamin D is one of those nutrients that people often take for granted, and yet in clinic, it rarely presents as a simple story. Fatigue that doesn’t fully lift, recurrent infections, low mood, metabolic strain, or bone density concerns plus, vitamin D often sits within a layered picture rather than being the sole cause of symptoms.
Most current vitamin D guidelines were developed with musculoskeletal health in mind, particularly the prevention of rickets and osteomalacia. From a public health perspective, the focus has been on defining a minimum serum level of vitamin D to prevent deficiency, and then estimating the intake required to reach that level under conditions of little or no sun exposure.
How we measure vitamin D
Vitamin D status is assessed using serum 25-hydroxyvitamin D (25[OH]D). This is the circulating storage form and the best indicator of how much vitamin D is available to the body.
Below 25–30 nmol/L (10–12 ng/mL) is widely considered deficient and should be corrected.
Levels between 50 and 75 nmol/L (20–30 ng/mL) are considered sufficient for bone health, though there is debate about whether higher targets are better for broader health outcomes.
What the evidence shows beyond bone health
While guidelines have traditionally focused on preventing bone disease, the extraskeletal roles of vitamin D are increasingly recognised. Observational studies, cohort analyses, and mechanistic research suggest that serum 25(OH)D above 75 nmol/L (30 ng/mL) is associated with the lowest risk of chronic disease and mortality.
For some conditions, even higher levels may be beneficial:
Vitamin D rich foods include eggs, meat and margarine BUT, they are not a easily converted as the synthesis that occurs when the sun interacts with the cholesterol on your skin and converts to vitamin D.
Type 2 diabetes in people with prediabetes: RCT data suggest ≥100 nmol/L (40 ng/mL) may be optimal.
Cancer and immune function: Some evidence points to benefits at similar or slightly higher serum levels.
It’s important to note that we cannot definitively claim causality. Vitamin D is rarely the only factor at play & it sits within a broader pattern of metabolic, inflammatory, and immune health.
Why recommended intakes vary
Vitamin D intake recommendations are usually calculated under assumptions of minimal sun exposure and average absorption.
400–800 IU/day is often enough to meet skeletal targets for most White adults.
However, many adults, particularly those with darker skin, higher body weight, digestive issues, or inflammatory conditions, may require higher doses to reach the same serum levels.
For example:
Obese individuals have a blunted response to supplementation. In the VITAL trial, 2000 IU/day raised 25(OH)D to 44, 41, 39, and 37 ng/mL in BMI categories <25, 25–29.9, 30–34.9, and ≥35 kg/m² respectively.
People with inflammatory bowel disease may require doses above 3500 IU/day to achieve similar blood levels.
Vitamin D form matters
Evidence supports vitamin D3 (cholecalciferol) over D2 (ergocalciferol) as it’s more effective and efficient at raising serum 25(OH)D.
Is higher supplementation safe?
A common question is whether targeting ≥75 nmol/L (30 ng/mL), which may require 2000 IU/day is safe. Large trials indicate that these doses are generally safe for adults. Individual monitoring remains important because vitamin D interacts with calcium, magnesium, and renal function, and responses vary widely between people. My preferred vitamin D supplement (if indicated) is here.
What this means in practice
Vitamin D deficiency rarely exists in isolation. It is part of a layered health pattern: immune activation, inflammation, metabolic strain, or chronic disease often sit alongside low vitamin D. Correcting levels can support energy, resilience, and recovery - but it rarely works as a standalone intervention.
Key points I discuss with clients:
25(OH)D is the most meaningful marker of vitamin D status.
Deficiency (<25–30 nmol/L) should always be addressed.
Observational evidence suggests ≥75 nmol/L (30 ng/mL) aligns with the lowest risk of chronic disease and mortality.
Certain outcomes (like prediabetes or some immune outcomes) may benefit from ≥100 nmol/L (40 ng/mL).
Many adults require closer to 2000 IU/day to reach 75 nmol/L, especially with higher BMI, darker skin, malabsorption, or inflammatory conditions.
Vitamin D3 is preferred.
Testing, context, and follow-up are essential — supplementation is not a “set and forget” solution.
Vitamin D is rarely about chasing a number. It’s about supporting the body’s ability to adapt and recover, gradually and sustainably, within the broader context of lifestyle, nutrition, and metabolic health. That’s the work I focus on in clinic -slow, steady, and personalised.
References:
Grant, W. B., Wimalawansa, S. J., Pludowski, P., & Cheng, R. Z. (2025). Vitamin D: Evidence-Based Health Benefits and Recommendations for Population Guidelines. Nutrients, 17(2), 277. https://doi.org/10.3390/nu17020277
Pludowski, P., Grant, W. B., Karras, S. N., Zittermann, A., & Pilz, S. (2024). Vitamin D Supplementation: A Review of the Evidence Arguing for a Daily Dose of 2000 International Units (50 µg) of Vitamin D for Adults in the General Population. Nutrients, 16(3), 391. https://doi.org/10.3390/nu16030391
