Corrective action was not implemented after the original clinical trial was completed because the study subjects returned to their standard of care. Until the end of this follow-up, there was no compelling reason to provide more vitamin D. Although it was reassuring that at follow-up Year 1, the serum 25(OH)D was appropriate for the group, the data at follow-up Year 3 show that 25(OH)D had not established its new plateau. Previous studies suggests that when vitamin D supplementation is stopped, the serum 25(OH)D concentrations exhibit an apparent half-life of two months . However, the decline in serum 25(OH)D observed here exceeded 1 year (i.e. the decline exceeded six of the presumptive 2-month half-life); therefore, the apparent half life can be substantially longer than what has been previously estimated. One reason for a longer apparent half-life of the decline in serum 25(OH)D, compared to a tracer 25(OH)D compound, is because firstly, unmetabolised vitamin D is released from tissue stores built up during vitamin D supplementation, and secondly, there is a background of newly acquired vitamin D from the environment or diet that continues to be generate 25(OH)D.
At one year after the end of the clinical trial, the group as a whole met the 50 nmol/L criterion for serum 25(OH)D as recently established by US Institute of Medicine. While this was apparently reassuring, the further decline in 25(OH)D shown here emphasizes the need for sustained awareness and monitoring of vitamin D status in nursing home residents. Follow-up reports after discontinuation of vitamin D and calcium supplementation are sparse. One report of a similar population that was provided lower doses of vitamin D, showed that bone turnover markers had returned to baseline by two years after discontinuation, but serum 25(OH)D concentrations were not reported .
The present report is to our knowledge the longest follow-up after discontinuation of vitamin D, and it does show a sustained biochemical benefit to the one-year point, in that the serum 25(OH)D exceeded 50 nmol/L. However, by follow-up Year 3, the benefits of vitamin D fortification, were essentially gone. At follow-up Year 1, PTH had increased significantly from the end-of-vitamin D baseline. At follow-up Year 3, there was evidence that bone loss had progressed at the hip, but since there was no reference group in this study, the bone loss may reflect the normal age-related decline. The strength of this study is in its 3-yr characterization of the progression of the decline in 25(OH)D in older adults who had before-hand, received enough vitamin D to achieve 25(OH)D concentrations that most would consider to be in the optimal range. Among the weaknesses of this work is the lack of a reference group against which to compare the variables studied.