Vitamin D increases circulating IGF1 in adults: potential implication for the treatment of GH deficiency

Objectives: Previous studies suggested that vitamin D modulates circulating IGF1. We investigated this effect in adults and its clinical relevance in the management of GH deficiency (GHD). Design and methods: IGF1 levels were prospectively measured before and after 12 weeks of treatment with oral vitamin D-3 (5000 or 7000 IU/week) vs no intervention in 39 subjects 61.9 +/- 7.9 years old. The frequency of IGF1 values >= 50th age-and sex-specific percentile in relation to vitamin D status, as determined by the concentration of 25-hydroxyvitamin D (25(OH)D), was retrospectively assessed in 69 GHD patients (57.4 +/- 16.6 years) on stable hormone replacement and with 25(OH)D and IGF1 concurrently measured. Results: Treatment with 5000 and 7000 IU vitamin D-3/week significantly raised 25(OH)D by 12.7 +/- 8.4 and 13.1 +/- 6.5 ng/ml respectively (both P<0.001 vs baseline). In the 7000 IU group, IGF1 levels also significantly increased by 31.3 +/- 36.7 ng/ml (P=0.01). Neither 25(OH)D nor IGF1 significantly varied in controls. IGF1 was >= 50th percentile more frequently in GHD patients with 25(OH) D levels >= 15 than <15 ng/ml (65.9 vs 40.0%, P<0.05). Logistic regression with adjustment for recombinant human GH (rhGH) dose, vitamin D supplements, gender, use of thyroid hormones, corticosteroids or estrogen/ testosterone, and season revealed a significant positive association between >= 15 ng/ml 25(OH)D and IGF1 >= 50th percentile (OR 4.4, 95% CI 1.0-18.8, P<0.05). A significant negative correlation between 25(OH)D concentrations and rhGH dose was found after correcting for age and IGF1 (beta -0.042, P<0.01), but not after further adjusting for sex, thyroid, adrenal or gonadal replacement, and season (beta -0.037, P=0.06). Conclusions: Vitamin D increases circulating IGF1 in adults. As a result, a better vitamin D status may ease the achievement of normal IGF1 values in GHD.