This is part 2 of my Vitamin D controversy series, if you haven’t read part 1, check it out here.
Between 2011 and 2024, the IOM–Endocrine Society controversy around Vitamin D thresholds continued to generate discussion in the scientific community. In fact, two chapters in a 2018 textbook were dedicated to the debate:
- Vieth & Holick (2018) — in support of the Endocrine Society
- Bouillon & Rosen (2018) — in support of the IOM
Not surprisingly, authors involved in the 2011 Endocrine Society guidelines strongly supported their original position. At the same time, other Institute of Medicine (the IOM) experts argued in favour of the IOM approach.
Where there is agreement
Despite the disagreement, both sides align on a few key points:
- Vitamin D is essential for preventing rickets and osteomalacia
- Individuals with darker skin pigmentation—especially in northern latitudes—may be at higher risk of deficiency
The core controversy remained:
👉 What is the optimal target level for vitamin D when testing is done?
What causes weaker bones in older adults?
While vitamin D is important for bone health, it is only one piece of a much more complex puzzle.
Earlier theories suggested a relatively simple pathway:
- Low vitamin D → increased PTH → increased bone turnover → fractures
However, newer evidence shows that bone health is influenced by multiple factors, including:
- Sex hormone deficiencies
- Osteoblast dysfunction
- Oxidative stress
- Decreasing muscle mass
- Reduced mobility
- Mechanical bone strain
What defines “adequate” vitamin D levels?
For vitamin D levels to be considered adequate, they should support:
- Vitamin D metabolite production
- Appropriate PTH levels
- Intestinal calcium absorption
- Bone mass maintenance
Once these needs are met, additional increases in vitamin D are unlikely to provide further benefit due to physiological feedback regulation.
What levels achieve these goals?
Vitamin D metabolites
The strongest evidence comes from studies in individuals with deficiency:
- Older adults: ~15 ng/mL (37.5nmol/L) is sufficient to normalize 1,25(OH)₂D
- CKD stages 3–5: Levels closer to 30 ng/mL (75nmol/L) may be required
Parathyroid hormone (PTH)
- Vitamin D deficiency increases PTH
- Supplementation studies suggest:
- ~20 ng/mL (50nmol/L) is sufficient to reduce PTH
- Above this level, PTH is no longer strongly vitamin D–dependent
Intestinal calcium absorption
- Impaired in vitamin D deficiency
- Evidence suggests optimal absorption occurs around:
- 15–20 ng/mL (37.5-50nmol/L) in healthy adults
Bone mineral density (BMD)
- Limited high-quality evidence
- Available data suggest:
- ~12–24 ng/mL (30-60nmol/L) is sufficient
Fracture risk
For patients with recent fractures:
- Vitamin D supplementation alone has inconsistent benefits for reducing fracture risk
- However when Vitamin D supplements are given WITH calcium:
- ~15% reduction in total fractures
- ~30% reduction in hip fractures
There is no strong high-quality evidence defining an optimal 25(OH)D level for fracture prevention, though some analyses suggest ~24 ng/mL (60nmol/L) may be sufficient.
What about non-skeletal effects?
Vitamin D has been linked to many non-skeletal outcomes—but the evidence is far less clear.
Cancer
- Plausible biological link
- No RCTs demonstrating a causal relationship
- ❗ No defined protective threshold
Immune function
- Mechanistic plausibility exists
- No consistent RCT evidence
- ❗ No defined optimal level
Diabetes and metabolic health
- No consistent effect on:
- Body weight
- Glycemic control
- Metabolic syndrome
There is slightly stronger evidence for:
- Slowing progression from prediabetes to type 2 diabetes
Some data suggest:
- <20 ng/mL (<50nmol/L) → higher risk
- ≥20 ng/mL (>50nmol/L)→ lower risk
Muscle health and mortality
- Some studies show a U-shaped or J-shaped relationship
- Increased risk observed at levels >50 ng/mL (125nmol/L)
- This supports 50 ng/mL (125nmol/L) as a potential upper limit
Pregnancy
- Limited evidence
- It appears reasonable that levels should exceed ~20 ng/mL (50nmol/L)
Key takeaways
- The disagreement is not about whether vitamin D matters—it clearly does
- The controversy is about how much is enough
- Across multiple outcomes, evidence consistently suggests:
- ~20 ng/mL (50nmol/L) meets most physiological needs for bone health
- Evidence for higher targets—especially for non-skeletal outcomes—is inconsistent or lacking
Up next
Part 3: The Vitamin D Controversy—What Changed in 2024?
