The Time of Day for Exercise Does Not Affect the Beneficial Effect of Exercise on Bone Structure in Older Female Rats

Abstract

Abstract Objectives Bone resorption and formation exhibit diurnal patterns, and disruption of the circadian rhythm affects the balance of bone turnover leading to bone deterioration. This study investigated whether the time of day for 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 Sixty, female 12-month-old Sprague Dawley rats were randomly assigned to four treatment groups (n = 15/group). Rats were subjected to no exercise or 2 hours of involuntary exercise at 9 meters/minutes and 5 days/week for 15 weeks using motor-driven running wheels at Zeitgeber time (ZT) 4–6 (rest phase), ZT 12–14 (early active phase), or ZT 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. Results There were no differences in body weight, fat mass, lean mass, the serum bone resorption marker (C-terminal telopeptide of type 1 collagen), and bone structural parameters among rats exercised at different times. Compared to No-exercise, exercise (combined exercise groups) increased femoral bone volume/total volume, connectivity density, and decreased structural model index (P < 0.05). Exercise also increased trabecular thickness in 2nd lumbar vertebrae. Conclusions These data indicate that exercise is beneficial to bone structure and that the daily time of exercise does not alter the beneficial effect of exercise in older, female rats. Funding Sources Agricultural Research Service of the United States Department of Agriculture, #3062-51,000-053-00D.