The use of stable isotopes to measure interactions between dietary fiber, vitamin D and calcium

Date of Completion

January 1991


Health Sciences, Nutrition




An impairment in the ability of humans to absorb calcium (such as that caused by high fiber intakes) may stimulate the rate of 25-hydroxyvitamin D degradation, and lead to vitamin D deficiency. This hypothesis was examined in 7 adult male subjects. Each subject participated in 2, 23-day experimental periods. The experimental periods differed only with respect to the dietary fiber intake (6.5 vs. 31.3 g/d).^ The high fiber diet significantly reduced calcium balance from days 7-12. However, the mean calcium balance remained positive. Because calcium status was not severely impaired, no significant differences were observed in serum total or ultrafilterable calcium, 25(OH)D, 1,25(OH)$\sb2$D, or parathyroid hormone, between dietary treatments.^ Despite the lack of change in the above serum parameters, the serum clearance of an oral dose of 1,25(OH)$\sb2$D, administered on day 20, was significantly longer during the high fiber period. The stimulus for this difference was not determined.^ A thermal ionization mass spectrometry facility was established at the University of Connecticut which is capable of measuring stable calcium isotopes. Using these isotopes fractional calcium absorption was determined not to be significantly different between dietary treatments, although fractional absorption was 6 percent lower during the high fiber period.^ A new technique was developed to followed 25(OH)D kinetics using a tri-deuterated form of the vitamin $\{$6,19,19-$\sp2$H$\}$ -25-hydroxyvitamin D) and thermospray liquid chromatography mass spectrometry. The half-life of 25(OH)D using this technique was substantially faster (11 d) than previously reported values determined using tritiated 25(OH)D (21 d). This was attributed to the fact that other investigators incorporated tritium in reactive positions in the side chain and failed to discriminate between label remaining in 25(OH)D and other metabolites.^ Thus, a new method has been established which is capable of following vitamin D kinetics in humans using deuterated 25(OH)D. Further expansion of this method may allow safe measurement of vitamin D kinetics in all population groups, while avoiding potential isotope effects found in previous vitamin D kinetic studies that used tritiated tracer. ^