Parathyroid hormone (PTH) is only one measurable index of skeletal health, and we reasoned that a histomorphometric analysis of iliac crest biopsies would be another and even more direct approach to assess bone health and address the required minimum 25-Hydroxyvitamin D [25(OH)D] level.

A cohort from the northern European population with its known high prevalence of vitamin D deficiency therefore would be ideal to answer the latter question. We examined 675 iliac crest biopsies from male and female individuals, excluding all patients who showed any signs of secondary bone diseases at autopsy.

Structural histomorphometric parameters, including osteoid indices, were quantified using the Osteomeasure System according to ASBMR standards, and serum 25(OH)D levels were measured for all patients. Statistical analysis was performed by Student's t test. The histologic results demonstrate an unexpected high prevalence of mineralization defects, that is, a pathologic increase in osteoid. Indeed, 36.15% of the analyzed patients presented with an osteoid surface per bone surface (OS/BS) of more than 20%.

Based on the most conservative threshold that defines osteomalacia at the histomorphometric level with a pathologic increase in osteoid volume per bone volume (OV/BV) greater than 2% manifest mineralization defects were present in 25.63% of the patients. The latter were found independent of bone volume per trabecular volume (BV/TV) throughout all ages and affected both sexes equally. While we could not establish a minimum 25(OH)D level that was inevitably associated with mineralization defects, we did not find pathologic accumulation of osteoid in any patient with circulating 25(OH)D above 75 nmol/L.

Our data demonstrate that pathologic mineralization defects of bone occur in patients with a serum 25(OH)D below 75 nmol/L and strongly argue that in conjunction with a sufficient calcium intake, the dose of vitamin D supplementation should ensure that circulating levels of 25(OH)D reach this minimum threshold (75 nmol/L or 30 ng/mL) to maintain skeletal health.