Bone mineral density (BMD) relates to bone strength, and low BMD is a risk factor for fractures and osteoporosis (1-3). In athletes, the nongenetic factor most commonly causing low BMD is prolonged and/or repeated periods with significant low energy availability (LEA), which also is associated with several serious, clinical impairments (4-6). These consequences of LEA in athletes range from short-term reductions in physical performance (e.g., due to low energy stores, impaired training adaptions and recovery, and increased injury risk), to long-term or even permanent illness or functional impairments (such as gastrointestinal dysfunction, impaired immune system, disturbance in hormonal function, and osteoporosis); a syndrome spectrum called relative energy deficiency in sport (RED-S) (5, 7). Interestingly, the reference scale for BMD evaluation is based on the normal population, and as such we may theoretically overlook athletes that already have reduced their BMD and are energy deprived. This potential masking of a RED-S-related symptom might be a result from interpreting BMD on wrong assumptions. Athletes, and specifically those representing high-impact sports and experiencing high mechanical loading, are expected to have higher BMD than age-matched nonathletes (8, 9). In fact, athletes from high-impact sports are expected to have a 5%-30% higher BMD compared with nonathletes (10, 11), and as such, we should expect that they have Z-scores above the population norm (=0). Therefore, the American College of Sports Medicine (ACSM) and the International Olympic Committee (IOC) defines normal bone health in athletes from a Z-score = -1.0, as opposed to = -2.0 in the normal population (4, 5, 10). They propose that a Z-score < -1 warrants further follow-up and clinical examination of secondary risk factors. Athletes from low-impact sports, such as swimming and cycling, on the other hand, seem not to present with above normal levels of BMD (12-16). Based on current understanding on how high-impact loading positively affect BMD, a question to rise may be; is there a need for a sport-specific BMD reference values? And should such reference values depend on the type of sport/event, sex, and/or age (1)?
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