Effects of bolus dose vitamin D3 on vitamin D metabolites, calcium metabolism, bone turnover markers, physical function and cardiovascular parameters in post-menopausal women

Several studies have shown an increase in the rate of falls and fractures after high dose vitamin D supplementation but the mechanism for the increase in falls and fracture is yet to be elucidated. One proposed mechanism is that the binding capacity of VDBP can become saturated in some situations (such as after a single large bolus dose). Therefore, the increased rate of falls and fractures that have been previously reported after a single large bolus dose of vitamin D may have been due to vitamin D toxicity in the initial period after administration due to the binding capacity of VDBP being overwhelmed by the large influx of vitamin D into circulation. It has been hypothesised that this may lead to a sharp increase in total 25(OH)D with a relatively greater increase in free 25(OH)D. In addition, there may also be a disproportionate rise in free 1, 25(OH)2D, because this metabolite has the weakest binding affinity for VDBP. There may then be several consequences of disproportionate increases in one or more of these free vitamin D metabolites that may increase the propensity for people to fall, such as hypercalcemia. Vitamin D is a potent suppressor of renin synthesis and excess free metabolites could impact upon the renin-angiotensin system, leading to postural hypotension and possibly leading to falls. Elevated free metabolites may also have direct effects on the brain (the VDR is intensely expressed in the cerebellum which is heavily involved in balanced muscular activity) and/or direct effects on muscle.
This study set out to investigate the clinically important hypothesis that a disproportionate rise in free vitamin D metabolites and hypercalcemia could explain the increase in falls and fractures after high dose vitamin D that have been previously reported.
The study was a single centre, double-blinded, randomised, controlled trial carried out at the Clinical Research Facility (Northern General Hospital, UK) to determine the effects of three different oral bolus doses of vitamin D3 (50 000IU, 150 000IU and 500 000IU) on total and free 25(OH)D and total and free 1, 25(OH)2D in vitamin D deficient, but otherwise healthy, postmenopausal women. Thirty-three vitamin D deficient (25(OH)D <30nmol/l) postmenopausal women were randomized to one of the three treatment groups. Twenty-seven vitamin D sufficient (25(OH)D >50nmol/l) postmenopausal women were recruited as a concurrent control group. Treatment participants attended four study visits (after screening) at baseline and at 5 (+/-2), 28 (+/-3), and 84 (+/-5) days after administration of the vitamin D bolus. A comprehensive range of biochemical and functional measurements were carried out to give a holistic assessment. Other biochemical measurements included, free 25(OH)D (directly measured and calculated), parathyroid hormone, FGF-23, serum calcium, ionized calcium, urinary calcium excretion and bone turnover markers. Grip strength and a Short Physical Performance Battery were used to assess muscle strength and function. Cardiovascular outcomes were also assessed, including postural changes in blood pressure and the aldosterone-renin ratio.

Despite a large dose-response effect of bolus dose supplementation on total 25(OH)D (& 25(OH)D3) and total 1, 25(OH)2D, there was no evidence of a disproportionate rise in free metabolites. The proportional increases in free 25(OH)D (either calculated or directly measured) were in line with the proportional increases in total 25(OH)D (and 25(OH)D3) across all study time points in all treatment groups. Similarly, the proportional increases in free 1, 25(OH)2D are in line with the proportional increases in total 1,25(OH)2D across all time points in all treatment groups. The percentage free 25(OH)D (derived from the ratios of calculated free 25(OH)D to total 25(OH)D and calculated free 25(OH)D3 to total 25(OH)D3) were not different between treatment groups at any time point and did not change across the study period within any treatment group. The percentage free that was derived from the ratio of measured free 25(OH)D to total 25(OH)D show a slight increase from baseline at week 1 in the 500 000IU treatment group that was significantly different to the other treatment groups. However, the percent free remained in the normal range and was comparable with percentages reported in healthy adults (0.02-0.09%). The percent free 1, 25(OH)2D did not change from baseline in any treatment group and did not differ between treatment groups at any time point. The percent free 1, 25(OH)2D in all treatment groups and at all time points was also in line with the 0.4% reported by other authors in healthy participants.

Taken together, the data presented indicates that there is little evidence to support the hypothesis of a disproportionate rise in free 25(OH)D or free 1, 25(OH)2D after a single large bolus dose in this vitamin D deficient, but otherwise healthy, older female population. It is therefore unlikely that the adverse events that have been reported after a single large bolus dose previously are caused by excess or disproportionate levels of free vitamin D metabolites.

Despite the large increases in vitamin D metabolites, there was no evidence of hypercalcemia in any treatment group. There was evidence of a fall in PTH in all treatment groups (although there was no dose-response effect) and there was some evidence of increases in FGF-23 and increases in urinary calcium excretion after bolus dosing. The increase in FGF-23 and urinary calcium excretion demonstrated in two of the treatment groups, coupled with the fall in PTH observed at different time points across all treatment groups, suggests that the catabolic pathways for vitamin D metabolites respond rapidly to the sharp increases in vitamin D metabolites in circulation after a large bolus dose. Taken together this again reinforces the theory that the homeostatic mechanisms for the vitamin D and calcium are robust and effectively prevent hypercalcemia after a single large oral dose of vitamin D3 in healthy older adults.

There was also no effect of treatment on cardiovascular outcomes, including blood pressure and ARR. It has been hypothesised that a single large bolus dose of vitamin D may lead to postural hypotension and that this may be the mechanism of the increased rate of falls reported in some studies. However, our data do not support this hypothesis.

There were no adverse effects of treatment on physical function, assessed by SPPB and grip strength. We did not see a benefit of supplementation in the largest bolus dose groups, but in the context of this study, it is important to note that we did not see any adverse effects on physical function measures in these treatment groups. As previously described, some studies have demonstrated a link between single large bolus doses of vitamin D and large repeated dose of vitamin D3 and an increase in falls and fracture. Our data would not support a decline in physical function as the explanation for these findings.

Interestingly, there was evidence of a transient increase in bone turnover markers 1-week after administration in the 500 000OIU treatment group. Osteocalcin increased by approximately 23%, PINP by 9% and CTX by 26% by 1-week after administration. Osteocalcin and CTX had fallen to baseline levels by week 4, but PINP remained elevated at week 4 by approximately 15%, before falling to baseline levels by week 12. The transient increase in CTX, OC and PINP was not observed with the lower doses. This is in line with previous findings that have also demonstrated a transient increase in bone resorption markers after a single large bolus dose. Higher levels of bone turnover are associated with bone loss and some studies have shown that higher markers of bone turnover are associated with a greater risk of fracture. It is not clear if these transient changes in increases in BTMs immediately after bolus dosing are clinically relevant, but this mechanism may help to explain the increase in fractures immediately after administration that has previously been reported and requires further investigation.

This is the first study to try and elucidate the mechanism of why people might fall and fracture more after a single large bolus dose of vitamin D3. This is the first study to show that a single oral bolus dose of up to 500 000IU does not appear to cause a disproportionate rise in free vitamin D metabolites compared to total metabolites. Total 25(OH)D has been measured using more than one method, including measurement by the gold standard LC-MS/MS method. This is the first study to report total 25(OH)D3 measured by LC-MS/MS after a 500 000IU bolus dose of vitamin D3. A variety of methods have been used to measure and calculate free 25(OH)D in this study to give a comprehensive assessment of this metabolite. This is also the first study to report the effects of a single large D3 bolus on the ARR and the first study to report the effects of a vitamin D bolus on postural changes in blood pressure.

To conclude, a single large bolus is up to 500 000IU appears to be well tolerated in healthy older adults. There was little evidence of a disproportionate rise in free vitamin D metabolites after a single large bolus dose (up to 500 000IU). Our data also does not suggest that hypercalcemia, poorer physical function and postural hypotension explain the increase in falls that had been previously reported after a large bolus. The exact mechanism of falls remains unclear. A transient increase in BTMs 1-week after a 500 000IU bolus dose may explain the increase in fractures immediately after large bolus dose administration that has previously been reported and requires further investigation.