Effects of reduced energy availability on bone metabolism in women and men

Background: The short-term effects of low energy availability (EA) on bone metabolism in physically active women and men are currently unknown. 
Purpose: We evaluated the effects of low EA on bone turnover markers (BTMs) in a cohort of women and a cohort of men, and compared effects between sexes. 
Methods: These studies were performed using a randomised, counterbalanced, crossover design. Eleven eumenorrheic women and eleven men completed two 5-day protocols of controlled (CON; 45 kcal·kg LBM− 1·d− 1) and restricted (RES; 15 kcal·kg LBM− 1·d− 1) EAs. Participants ran daily on a treadmill at 70% of their peak aerobic capacity (VO2 peak) resulting in an exercise energy expenditure of 15 kcal·kg LBM− 1·d− 1 and consumed diets providing 60 and 30 kcal·kg LBM− 1·d− 1. Blood was analysed for BTMs [β-carboxyl-terminal cross-linked telopeptide of type I collagen (β-CTX) and amino-terminal propeptide of type 1 procollagen (P1NP)], markers of calcium metabolism [parathyroid hormone (PTH), albumin-adjusted calcium (ACa), magnesium (Mg) and phosphate (PO4)] and regulatory hormones [sclerostin, insulin-like growth factor 1 (IGF-1), triiodothyronine (T3), insulin, leptin, glucagon-like-peptide-2 (GLP-2)]. 
Results: In women, β-CTX AUC was significantly higher (P = 0.03) and P1NP AUC was significantly lower (P = 0.01) in RES compared to CON. In men, neither β-CTX (P = 0.46) nor P1NP (P = 0.12) AUCs were significantly different between CON and RES. There were no significant differences between sexes for any BTM AUCs (all P values > 0.05). Insulin and leptin AUCs were significantly lower following RES in women only (for both P = 0.01). There were no differences in any AUCs of regulatory hormones or markers of calcium metabolism between men and women following RES (all P values > 0.05). 
Conclusions: When comparing within groups, five days of low EA (15 kcal·kg LBM− 1·d− 1) decreased bone formation and increased bone resorption in women, but not in men, and no sex specific differences were detected.