Association Between Vitamin D Status and Physical Performance: The InCHIANTI Study

TL;DR: Vitamin D status was inversely associated with poor physical performance and additional studies examining the association between vitamin D status and physical function are needed.

Abstract: With a growing older population, there is an increasing need to identify potentially modifiable risk factors for the onset of disability. In the past two decades, it has become evident that the role of vitamin D extends beyond calcium homeostasis and includes modulation of skeletal and cardiac muscle function, immune cell function, and anticancer activity (1). Within the muscle cell, vitamin D plays an important role in the regulation of calcium transport and protein synthesis (2,3). In older adults, low serum vitamin D (serum 25OHD) levels have been associated with muscle weakness, poor physical performance, balance problems, and falls, although findings from different studies are somewhat inconsistent (4–11). Thus, vitamin D deficiency may not only affect the onset of chronic conditions, which are frequent causes of disability, but may also directly affect functional status through vitamin D’s role in muscle function. Consensus to define a cut-point for vitamin D insufficiency based on serum 25OHD levels is lacking. Various cut-points have been proposed based on population-based reference limits or biological indices, such as parathyroid hormone (PTH), calcium absorption, or bone mineral density, with cut-points for serum 25OHD insufficiency ranging from 50 to 80 nmol/L (12–14). Regardless, low levels of serum 25OHD are common in older populations with wide variability in prevalence depending on geographic location, season, and the cut-points used to define insufficiency and/or deficiency (15–19). Older adults are at risk for low serum 25OHD because of reduced exposure to ultraviolet B (UVB) radiation and reduced efficiency of previtamin D synthesis in the skin (20). Dietary intake of vitamin D is also often inadequate, as there are few natural food sources of vitamin D (16). Low serum 25OHD may also indirectly affect muscle function via hyperparathyroidism secondary to vitamin D deficiency (17). Primary hyperparathyroidism is characterized by muscle weakness that is improved by treatment (21,22). It has been demonstrated that administration of PTH negatively affects skeletal muscle function in animal models (2,3,17). PTH has also been shown to induce the production of serum interleukin-6 (IL-6) (23). Observational studies have shown that elevated IL-6 levels are associated with lower muscle strength and poor physical performance (24,25). However, few studies have examined the joint effect of vitamin D and PTH on physical performance (9,10). The objective of this study was to examine the associations between vitamin D status and physical performance using data from the InCHIANTI study. Comparisons between different serum 25OHD cut-points and physical performance were examined. Additionally, the association between PTH and physical performance and the role of PTH as a potential mediator in the association between serum 25OHD and physical performance was examined.