This study investigates how energy availability affects performance, injury risk, and training outcomes in elite triathletes. Over two competitive seasons (2026-2027), 40 elite and sub-elite triathletes will be monitored through physiological tests, blood samples, and questionnaires. The goal is to identify markers of low energy availability and understand its impact on health and athletic performance. The study is observational and does not involve experimental treatments. Participants continue their regular training and diet while undergoing periodic assessments.

Low energy availability (LEA) is a condition where the energy intake of an athlete is insufficient to support both training demands and essential physiological functions. LEA is common among endurance athletes, including triathletes, due to high training volumes and sometimes intentional dietary restrictions. Prolonged LEA can lead to hormonal imbalances, impaired bone health, reduced immune function, and increased injury risk. This study aims to explore the prevalence and consequences of LEA in elite triathletes over two seasons.

The study is a 2-year exploratory cohort study conducted at the Institute of Sports Medicine, Bispebjerg Hospital, Copenhagen. Forty elite or sub-elite triathletes (men and women, aged 18+) who train more than 15 hours per week will be enrolled. Participants will undergo five comprehensive test days spaced across the off-season, pre-season, and in-season periods of 2025-2027. These test days include blood sampling, DXA scans, MR imaging, muscle strength and stiffness assessments, VO₂ max testing, and dietary and activity monitoring.

The primary outcome is the change in the thyroid hormone triiodothyronine (T3), a known marker of LEA. Secondary outcomes include bone density, muscle mass, metabolic rate, performance metrics (e.g., running economy, VO₂ max), and injury incidence. Energy availability will be assessed via validated questionnaires (LEAF-Q for females, LEAM-Q for males), dietary logs (MyFood24), and training data from wearable devices.

Monthly injury reports and quarterly dietary assessments will be collected. Blood samples will be analyzed for hormones (e.g., cortisol, testosterone, GH, IGF-1, TSH, T3, T4), bone turnover markers (CTX, PINP, osteocalcin), immune parameters, and nutritional status (e.g., vitamin D, iron, B12). Some samples will be stored in a coded biobank until 2029 for future analysis.

Participants will receive no financial compensation but will be reimbursed for public transport. All procedures are non-invasive or minimally invasive and carry low risk. The study has been approved by the relevant ethics committee (Approval No. H-25043062), and informed consent will be obtained from all participants.

Results will be published in peer-reviewed journals and presented at scientific conferences. If not accepted for publication, results will be made publicly available via ClinicalTrials.gov and the EU Clinical Trials Register.

Inclusion Criteria:

• Age ≥18 years
• Healthy individuals
• Training ≥15 hours/week in triathlon

Exclusion Criteria:

• Smokers
• Use of prohibited substances (WADA list) without medical justification
• Existing injuries or illnesses that could worsen due to participation

Elite triathletes with a weekly training volume of more than 15 hours