Conclusions
This article has discussed vitamin D's effects in skeletal muscle, commencing with a striking case report describing severe yet reversible muscle weakness in a veiled female with severe vitamin D deficiency, and including an assessment of clinical trials and biological mechanisms on vitamin D and muscle. As reflected in our case report, marked deficiency of vitamin D is clearly detrimental for muscle function, independent of alterations in calcium and phosphate levels. Observational studies also associate muscle weakness, falls and reduced muscle mass with vitamin D deficiency. However interventional trials and meta-analyses in subjects with less severe forms of vitamin D deficiency have yielded contradictory findings. The validation of physical performance measures and the definition of sarcopenia remain open questions making it difficult to standardize outcome measures of these studies (Girgis, 2015) (Girgis et al., 2014c). Regarding falls, vitamin D supplementation is effective in more vulnerable elderly people, such as the institutionalised or those who are prone to falls but not excessive, single mega-doses which appear harmful in this population (Girgis, 2014). Biological mechanisms suggests the presence of VDR in muscle, a role in intramuscular calcium handling and the modulation of muscle fiber size. Further research will help to clarify optimal vitamin D supplemental regimens for muscle health independent of its effects on calcium and phosphate levels is needed. In the meantime, people with severe vitamin D deficiency (< 25 nmol/l (10 ng/ml)) are at risk of muscle pathology and would benefit from supplementation.
