Low vitamin D rarely came with the biochemical pattern of osteomalacia

Design

Cross-sectional analysis with nested randomised placebo-controlled trial

Population

423 Norwegian adults with low 25-hydroxyvitamin D (mean 34 nmol/L); 382 in the trial

Primary outcome

Prevalence of biochemical patterns compatible with osteomalacia; change with vitamin D

Effect

Full triad 0%; most permissive pattern 8.3%; vitamin D lowered PTH and alkaline phosphatase vs placebo

Vitamin D deficiency thresholds rest largely on preventing osteomalacia, which strictly needs a bone biopsy to diagnose. This Norwegian study took 423 adults from the population-based Tromsø cohort with low vitamin D (mean 25-hydroxyvitamin D 34 nmol/L) and looked for biochemical patterns compatible with osteomalacia: raised PTH, raised alkaline phosphatase and low calcium. A nested randomised trial then gave 382 of them vitamin D (100 000 IU loading, then 20 000 IU weekly for four months) or placebo.

No participant had the full triad. Using the most permissive combination, 8.3% had a compatible pattern, and the prevalence depended heavily on the thresholds chosen. Vitamin D lowered PTH and alkaline phosphatase and raised calcium compared with placebo.

For laboratories, the message is that a low 25-hydroxyvitamin D is not a diagnosis of bone disease, and that raised alkaline phosphatase with low calcium in a patient with low vitamin D deserves attention because it is uncommon. The mean level here is not far from what is seen in many Indian populations, though dietary calcium intake differs.

• Do not interpret a low 25-hydroxyvitamin D alone as osteomalacia.
• Flag the combination of low calcium, raised alkaline phosphatase and raised PTH with low vitamin D for clinical review.
• Suggest bone-specific alkaline phosphatase when total alkaline phosphatase is raised and the source is unclear.
• Note that calcium intake matters; the Norwegian population's intake is higher than in many Indian diets.

Why it matters

It questions how often low vitamin D alone means clinically important bone disease.

Don't overread it

Biochemical patterns were not validated against bone histomorphometry, so the true prevalence of osteomalacia is unknown.

The statistics, in plain English

Prevalence swung from 0% to 8.3% depending on which cut-offs were used for 'high' and 'low'. That sensitivity to thresholds is itself the finding: biochemistry alone is a blunt tool for this diagnosis.

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