An inverse relationship between serum vitamin D levels and serum parathyroid hormone (PTH) is well established, with higher PTH levels recognized by many scientists as an indicator of poor bone health. A cross-sectional study on Icelandic adults aged 30-85 was conducted to determine the relative importance of serum vitamin D on PTH levels, and therefore bone health, over a wide range of calcium intakes.
Using data collected from 944 healthy participants' food frequency questionnaires, Icelandic researchers discovered that whenever serum vitamin D levels were maintained above 18 ng/mL, PTH levels did not increase until calcium intakes dropped well below 80% of the daily value. However, comparatively minor reductions in serum vitamin D levels resulted in rather significant PTH increases. This was true even in the presence of excess calcium.
The researchers conclude that vitamin D may be more important than calcium intake in the preservation of bone health. Furthermore, because vitamin D deficiencies are common during the winter in northern climates, vitamin D supplements may be necessary to avoid vitamin D deficiency, increased PTH levels, poor bone health and an increased risk of bone fractures.
Steingrimsdottir, L et al. "Relationship Between Serum Parathyroid Hormone Levels, Vitamin D Sufficiency and Calcium Intake", JAMA, Nov. 9, 2005; Vol 294, No. 18, 2336-2341