Hypovitaminosis D and frailty: Epiphenomenon or causal?

Vitamin D is not only a key component in the maintenance of calcium homeostasis and bone health, but has also been implicated in a myriad of other non-skeletal biologic systems. The frailty syndrome is an emerging and increasingly important concept in the field of aging, with the "physical" clinical phenotype being initially presented as the operational definition. The relationship between vitamin D and frailty is postulated to be largely mediated via the development of sarcopenia, a condition characterised by a combination of the reduction of muscle mass, plus either muscle strength or performance. Several molecular pathways may account for the development of muscle wasting in sarcopenia, and there is mounting epidemiological and laboratory evidence that supports a role of vitamin D on muscle cell proliferation and function. Although observational studies on vitamin D and frailty have not definitively established an independent relationship, interventional studies of the effect of supplemental vitamin D have yielded a positive influence on the frailty status, mainly via improvements in the physical performance. Further studies that are adequately powered and well-designed are warranted in an attempt to establish a causal relationship between vitamin D and frailty. In the absence of a consensus on the definition of the frailty syndrome, an appropriate and well-validated measure instrument for this health outcome would be recommended in the realm of frailty research.