The effect of modifying dietary calcium intake pattern on the circadian rhythm of bone resorption

In order to assess day-to-day variations of the circadian rhythm of biochemical bone resorption markers, urinary morning (6-8 a.m.) and evening (7-10 p.m.) samples from 35 individuals were monitored during 3 subsequent days. The bone-specific deoxypyridinoline (DPD) crosslinks of type I collagen followed a circadian rhythm in all individuals. In contrast, no such pattern was observed in the urinary hydroxyproline/creatinine and calcium/creatinine measurements. The DPD crosslink measurements showed a much larger difference between the morning and evening samples collected within 1 day compared with the variation between the samples collected in the morning or evening on subsequent days, indicating the importance of adequate timing of urine sampling for clinical trials aiming to monitor effects on bone resorption. The analysis of DPD crosslinks was then used to evaluate the effects of different patterns of dietary calcium intake on the circadian rhythm of bone resorption in osteoporotic patients. No significant effect on the circadian rhythm of the DPD crosslinks was found after concentrating the normal daily calcium intake to the evening (6-10 p.m.) during 8 days (n = 7). Ingestion of a dietary calcium supplement (600 mg) at 10 p.m. during 8 days (n = 7) resulted in an increased urinary calcium excretion in the morning, and a flattening of the circadian peak and nadir concentrations of urinary DPD/creatinine. The absolute levels of DPD/creatinine in the morning and evening urine samples, respectively, were not significantly altered compared with the control day. We conclude that dietary calcium supplementation in the evening only marginally affects the circadian rhythm of urinary DPD crosslinks in established osteoporosis patients.