The investigators propose a prospective, longitudinal, department funded study protocol to evaluate whether changes in energy availability, during a period of highly intensified exercise training, will increase bone turnover markers and decrease ovarian function and exercise performance, in a dose-dependent fashion in competitive female athletes.

Female athletes participating in sports which emphasize leanness as a factor to success, while also imposing large exercise training loads, present a high prevalence of endocrine and metabolic alterations linked to low energy stores, including suppressed ovarian function and bone loss. Increased bone turnover resulting from low energy stores and suppressed estrogen production may hold the most adverse consequences for long term health, increasing the risk of premature osteopenia, while increasing the risk of stress fractures in the short term. Chronically low energy stores may also contribute to the development of overreaching and overtraining syndromes, characterized by excessive, lasting fatigue and long-term decreases in performance, which can compromise these athletes' health and athletic career. The investigators propose the first longitudinal study aiming to demonstrate the link between low energy status, impaired ovarian function, bone turnover and decreased performance in free-living, competitive female athletes. After a 4-week baseline period of habitual training, athletes will undergo 4 weeks of intensified training (IT), during which the exercise load is increased by 30% from their individual baseline load in order to sharply increase energy expenditure. As athletes will be let to freely adjust their food intake, the investigators aim to test the hypothesis that athletes will fail to adjust food intake sufficiently to match energy expenditure, and that the resulting decrease in energy availability (EA, energy intake - energy expenditure from exercise) will influence, in a dose-dependent fashion, the suppression of estrogen production, the increase in bone turnover and the severity of performance decrement. The IT period will then be followed by a 2-week taper (REC), in which exercise training volume will be reduced by 50% from baseline. The investigators will assess whether EA returns to baseline values, and characterize changes in bone turnover during this recovery period. It is hypothesized that any improvements in exercise performance resulting from IT + REC will occur in athletes who were able to remain closest to a balanced energy state during IT. Concretely linking EA to both performance and bone health may incentivize female athletes to adopt more adequate feeding behaviors for their activity level.

Inclusion Criteria:

• competitive endurance runners (racing in events ranging from 10km to ultra-marathons)
• training at least 5 days per week over the past 12 months, and more than 30 miles per week
• regular menstrual cycles (24-35 days) over the past 6 months
• maximal oxygen consumption (VO2max) > 50ml/kg/min
• English speaking

Exclusion Criteria:

• smoker
• amenorrhea
• pregnant or lactating in the past 2 years
• chronic disease that will affect bone health, metabolism or the cardiorespiratory system
• take medications that have cardiovascular or metabolic effects
• present any contra-indication to exercise testing (cardiovascular abnormalities)
• report any major illness or injury preventing training for more than 4 weeks over the past 3 months
• a history of clinical anorexia nervosa or bulimia nervosa