Myotonic dystrophy type 1 (DM1) is a multisystemic
disorder affecting, among others, the endocrine
system, with derangement of steroid hormones functions.
Vitamin D is a steroid recognized for its role in calcium
homeostasis. In addition, vitamin D influences muscle
metabolism by genomic and non-genomic actions,
including stimulation of the insulin-like-growth-factor 1
(IGF1), a major regulator of muscle trophism. To verify
the presence of vitamin D deficit in DM1 and its possible
consequences, serum 25-hydroxyvitamin D (25(OH)D),
calcium, parathormone (PTH), and IGF1 levels were
measured in 32 DM1 patients and in 32 age-matched
controls. Bone mineral density (BMD) and proximal
muscle strength were also measured by DXA and a
handheld dynamometer, respectively. In DM1 patients,
25(OH)D levels were reduced compared to controls, and a
significant decrease of IGF1 was also found. 25(OH)D
levels inversely correlated with CTG expansion size,
while IGF1 levels and muscle strength directly correlated
with levels of 25(OH)D lower than 20 and 10 ng/ml,
respectively. A significantly higher percentage of DM1
patients presented hyperparathyroidism as compared to
controls. Calcium levels and BMD were comparable
between the two groups. Oral administration of cholecalciferol
in 11 DM1 patients with severe vitamin D deficiency
induced a normal increase of circulating 25(OH)D,
ruling out defects in intestinal absorption or hepatic
hydroxylation. DM1 patients show a reduction of circulating
25(OH)D, which correlates with genotype and may
influence IGF1 levels and proximal muscle strength. Oral
supplementation with vitamin D should be considered in
DM1 and might mitigate muscle weakness