Vitamin D deficiency is widespread and may result in a vast array of health consequences including osteoporosis, falls, increased cancer risk and altered glucose and lipid metabolism – the pathogenesis of diabetes and CVD. It plays an essential role in muscle and bone health, immunity and muscle signaling and has been linked with autoimmune disorders such as multiple sclerosis (Cantorna et al. 2006; Cherniak et al. 2008; Ford et al. 2005; Mathieu et al. 2005). Obesity has been associated with decreased bioavailability of vitamin D, and percentage body fat is inversely related to vitamin D levels and directly correlated with parathyroid hormone (PTH) levels (Snijder et al. 2005; Wortsman et al. 2000).
The skeletal effects of hypovitaminosis D are evidenced in progressive stages such as calcium malabsorption with secondary elevation of PTH, increased bone remodeling and osteoporosis and further histologic changes related to continued lack of calcium and poor mineralization (Heaney 1999).
Individuals with SCI have an increased occurrence of vitamin D deficiency, resulting from a number of factors including decreased exposure to sunlight, inadequate dietary intake and the effect of medications (Hummel et al. 2012). In turn, vitamin D deficiency promotes calcium deficiency and secondary hyperparathyroidism, resulting in further bone loss and exacerbating osteoporosis. Myopathy and nonspecific musculoskeletal pain may also develop as a consequence of vitamin D deficiency (Bauman et al. 2005; Holick 2005).
Bauman et al. (1995) reported that 32 of 100 SCI subjects had 25(OH)D levels below normal range and 11 of 32 had elevated serum PTH levels. Zhou et al. (1993) measured the 25(OH)D, serum calcium, magnesium and albumin concentrations of 92 men with SCI, 38 of whom had single or multiple pressure ulcers, and compared these values with those of non-SCI controls. The SCI group had lower serum 25(OH)D, total calcium, and albumin concentrations. Individuals with tetraplegia had lower 25(OH)D levels than those with paraplegia. Additionally, the SCI subgroup with pressure ulcers demonstrated significantly lower serum 25(OH)D, calcium and magnesium levels than the SCI subjects without ulcers.
Serum 25-hydroxyvitamin D is the best test for assessing vitamin D levels in the following clinical scenarios where patient vitamin D levels may be abnormal including (Thatcher & Clark 2011):
- Significant renal or liver disease
- Osteomalacia, osteopenia or osteoporosis
- Malabsorption syndromes
- Hypo or hypercalcemia/hyperphosphatemia
- Hypo or hyperparathyroidism
- Prescribed medications that affect vitamin D metabolism such as phenobarbital, carbamazepine, phenytoin and valproate
- Prescribed medications that might interfere with vitamin D absorption such as cholestyramine, colestpiol and orlistat
- Unexplained increased levels of serum alkaline phosphatase
Intake of high dose vitamin D combined with symptoms suggesting vitamin D toxicosis, i.e., hypervitaminosis D
There is increasing support for vitamin D supplementation beyond present recommendations. Additional studies are needed to establish the best diagnostic and supplementation guidelines for different populations (Cherniak et al. 2008).