Acute fasting diminishes the circadian rhythm of biochemical markers of bone resorption.

OBJECTIVE Biochemical markers of bone turnover exhibit circadian rhythms with the peak during the night/early morning and the nadir in the late afternoon. The nocturnal increase in bone resorption could theoretically be caused by the absence of food consumption which brings about a decrease in net calcium absorption and an increase in parathyroid hormone (PTH), followed by increased bone resorption in response to the body's demand for calcium. The aim of the present study was to assess the influence of a 33-h fast on the circadian variation in biochemical markers of bone turnover. DESIGN Eleven healthy premenopausal women (age: 24+/-5 years) participated in a randomised, cross-over study consisting of two periods: either 33h of fasting (fasting) followed 1 week later by a 33-h period with regular meals eaten at 0800-0830h, 1130-1230h and 1800-1900h (control) or vice versa. METHODS Urinary CrossLaps (U-CL/Cr) corrected with creatinine, as a marker of bone resorption; serum osteocalcin (sOC) as a marker of bone formation; serum intact PTH (iPTH); serum phosphate; and serum calcium corrected with albumin. RESULTS Both the fasting and the control periods showed a significant circadian rhythm in U-CL/Cr (P<0.001), but the decrease was significantly less pronounced in the morning hours during the fasting period. Fasting resulted in a significant decrease in serum iPTH (throughout the study period) as compared with the control period (P<0.05-0.001). No change was observed in sOC by fasting. CONCLUSION Food consumption has a small influence on the circadian variation in bone resorption, independent of PTH. The fall in iPTH during fasting may be secondary to an increased bone resorption produced by fasting.