Does vitamin D deficiency contribute to post-burn bone loss?

Commentary

Children who have been severely burned develop a progressive vitamin D deficiency¹ as characterized by low circulating levels of 25-hydroxyvitamin D (25(OH)D) between 14² and 24 months post-burn with normal circulating levels of 1,25-dihydroxyvitamin D (1,25(OH)₂D). Unlike rickets there is no compensatory rise in 1,25(OH)₂D and this may be due to persistent low serum parathyroid hormone (PTH) concentrations^2,3. The hypoparathyroidism is likely due to a persistent post-burn up-regulation of the parathyroid calcium sensing receptor^3,4. By 7 years post-burn, there is a continued reduction in serum levels of 25(OH)D in all patients but now there is also a reduction in circulating concentrations of 1,25(OH)₂D¹.

There are at least two reasons for this progressive deficiency. The first explanation is that burn patients are not routinely given vitamin D supplements following hospital discharge¹ and even if they are given a standard amount of vitamin D supplementation, i.e. 400 international units per day, for six months post-burn, serum levels of 25(OH)D are found to be in the "insufficient" range, approximately 20 ng/ml⁵. Therefore, the proper dose of vitamin D to administer to these children is unknown. The second explanation is that following burn injury, not only the burn scar tissue but also the adjacent normal-appearing skin cannot convert normal quantities of the precursor 7-dehydrocholesterol (7DHC) to vitamin D₃ when subjected to UVB irradiation². Moreover, the epidermal cells both in burn scar tissue and in adjacent normal-appearing skin contain subnormal quantities of 7DHC suggesting abnormalities in cholesterol biosynthesis in skin following burn injury².

Bone loss following burn injury can be attributed to least two non-specific adaptive responses: the inflammatory response and the stress response. Both occur as a result of the destruction of skin, which is an important barrier to infection. The inflammatory response involves significant elevations of circulating cytokines, especially interleukin (IL)-1β and IL-6⁶. The stress response is characterized by a 3–8-fold rise in endogenous glucocorticoid production as measured by urine free cortisol excretion^6,7. Acutely, both processes stimulate osteoblast production of the ligand of the receptor activator of NFκB (RANKL), which then stimulates marrow stem cell differentiation into osteoclasts and results in increased bone resorption. The success of acute intravenous administration of bisphosphonates either as a single or once-repeated dose in preventing the bone loss⁸ is evidence that this is the case. Because the stress response is sustained for as long as one year post-burn, persistence of the elevated endogenous production of glucocorticoids leads to osteoblast and likely osteocyte apoptosis and reduction of marrow stromal cell differentiation into osteoblasts⁷. Thus the now adynamic bone cannot recover the deficit caused by the acute resorption. It is only after the stress response has dissipated after about one year post-burn, that remodeling resumes⁹.

Because the intravenous administration of bisphosphonates prevents the acute bone loss entirely⁸ for up to two years⁹, it is highly unlikely that the documented vitamin D deficiency plays a role in the immediate post-burn bone loss. Moreover, we have also shown that, at least acutely, there are low circulating levels of both vitamin D binding protein and albumin¹⁰, with albumin recovering by 6 months post-burn⁵. Therefore, whether there is true vitamin D deficiency during the first six months post-burn is difficult to determine. Nevertheless, what we do see in these burn patients is that by two years post-burn, cortical bone deficits, as measured by total body bone mineral content, in those not receiving acute bisphosphonate therapy recover to the level of those who did receive bisphosphosphonates⁹ while the deficit in trabecular bone, as measured by lumbar spine bone density, persisted^9,11.

A recent report by Zhou et al.¹² has shown that in adult human marrow stromal cell culture, the clinical conditions under which the bone biopsies were obtained influenced the ability of the stromal cells to differentiate into osteoblasts in response to 1,25(OH)₂D. In particular it was noted that vitamin D deficiency impaired this differentiation. This finding raises the possibility that progressive vitamin D deficiency as experienced by children post-burn may explain the persistence of their trabecular bone deficit, although why vitamin D deficiency would uniquely affect trabecular bone is not at all clear. Perhaps future studies looking at the differential effects of vitamin D on cortical and trabecular bone might determine whether this hypothesis is viable.