Time of day of exercise does not affect the beneficial effect of exercise on bone structure in older female rats

Authors: Gregoire, Brian; Cao, Jay

Submitted to: Frontiers in Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/13/2023
Publication Date: 10/26/2023
Citation: Gregoire, B.R., Cao, J.J. 2023. Time of day of exercise does not affect the beneficial effect of exercise on bone structure in older female rats. Frontiers in Physiology. 14:1-11. https://doi.org/10.3389/fphys.2023.1142057.
DOI: https://doi.org/10.3389/fphys.2023.1142057

Interpretive Summary: Many biochemical processes and physiological functions of mammals display biological or circadian rhythms. Changes of bone resorption and formation markers exhibit daily patterns and disruption of the pattern affects the balance of bone turnover leading to bone deterioration. We investigated whether the time of day of exercise alters bone metabolism in a rodent model. The study demonstrated that exercise increased femoral bone mass and there were no differences in bone structural parameters among rats exercised at different times. The findings of the study suggests that exercise benefits bone regardless of the time of day to exercise.

Technical Abstract: Background: Circadian clock genes are expressed in the musculoskeletal system and biomarkers of bone resorption and formation exhibit diurnal patterns in animals and humans. Disruption of the diurnalcircadian rhythms may affect the balance of bone turnover leading to bone deterioration. Objective: This study investigated whether the time of day offor exercise alters bone metabolism in a rodent model. We hypothesized that exercise during the active phase results in greater bone mass than exercise during the rest phase in older female rats. Methods: Fifty-five, female 12-mo-old Sprague Dawley rats were randomly assigned to four treatment groups (n = 13-14/group). Rats were subjected to no exercise or 2 hours of involuntary exercise at 9 m/min and 5 d/wk for 15 wk using motor-driven running wheels at Zeitgeber time (ZT) 4-6 (rest phase), 12-14 (early active phase), or 22-24 (late active phase). ZT 0 is defined as light on, the start of the rest phase. A red lamp was used at minimal intensity during the active, dark phase exercise period, i.e. ZT 12-14 and 22-24. Bone structure, body composition, and bone-related cytokines in serum and gene expression in bone were measured. Data were analyzed using one-way ANOVA followed by Tukey-Kramer post hoc contrasts. Results: There were no differences in body weight, fat mass, lean mass, the serum bone biomarkers, bone structural or mechanical parameters, expression of circadian genes in bone among rats exercised at different ZT. Exercise (pooled exercise data from different ZT were compared to the No-Exercise data (a priori contrast) increased serum IGF-1 and irisin concentrations, compared to No-Exercise. Exercise increased tibial bone volume/total volume, connectivity density, and decreased structural model index (P = 0.05). Exercise did not affect expression of circadian genes (P > 0.05). Conclusions: These data indicate that exercise is beneficial to bone structure and that the time of day offor exercise does not alter the beneficial effect of exercise on bone in older female rats.