Effect of fasting versus feeding on the bone metabolic response to running

Individuals often perform exercise in the fasted state, but the effects on bone metabolism are not currently known. We compared the effect of an overnight fast with feeding a mixed meal on the bone metabolic response to treadmill running. Ten, physically‐active males aged 28 ± 4 y (mean ± SD) completed two, counterbalanced, 8 d trials. After 3 d on a standardised diet, participants performed 60 min of treadmill running at 65% VO2max on Day 4 following an overnight fast (FAST) or a standardised breakfast (FED). Blood samples were collected at baseline, before and during exercise, for 3 h after exercise, and on four consecutive follow-up days (FU1–FU4). Plasma/serum were analysed for the c-terminal telopeptide region of collagen type 1 (β‐CTX), n-terminal propeptides of procollagen type 1 (P1NP), osteocalcin (OC), bone alkaline phosphatase (bone ALP), parathyroid hormone (PTH), albumin-adjusted calcium, phosphate, osteoprotegerin (OPG), cortisol, leptin and ghrelin. Only the β-CTX response was significantly affected by feeding. Pre‐exercise concentrations decreased more in FED compared with FAST (47% vs 26%, P < 0.001) but increased during exercise in both groups and were not significantly different from baseline at 1 h post‐exercise. At 3 h post‐exercise, concentrations were decreased (33%, P < 0.001) from baseline in FAST and significantly lower (P < 0.001) than in FED. P1NP and PTH increased, and OC decreased during exercise. Bone markers were not significantly different from baseline on FU1–FU4. Fasting had only a minor effect on the bone metabolic response to subsequent acute, endurance exercise, reducing the duration of the increase in β-CTX during early recovery, but having no effect on changes in bone formation markers. The reduced duration of the β-CTX response with fasting was not fully explained by changes in PTH, OPG, leptin or ghrelin.

► Prior to exercise, feeding decreased β-CTX compared with fasting, but exercise increased concentrations in both groups to a similar level.
► Fasting reduced the duration of the exercise-associated increase in β-CTX, with lower concentrations at 3 h post-exercise compared with feeding.
► P1NP increased and OC decreased during exercise, but neither response was affected by feeding.
► Changes in PTH, OPG, leptin and ghrelin with exercise were all unaffected by feeding.
► All bone markers were not significantly different from baseline at 1–4 days post-exercise.