Weight Management and Body Composition

Athletes manage weight for different reasons. In weight-category sports such as combat sports, lightweight rowing and weightlifting, competitors are grouped by body weight, and many try to compete in a category below their usual weight in the belief that this confers an advantage. In endurance and aesthetic sports, athletes may seek a particular body composition. The concern is not that weight or composition is discussed, but how change is pursued, because the common practice of losing weight rapidly, often by dehydration in the days before a weigh-in, is hazardous.

The health risks of rapid weight loss are well documented. Dehydration and electrolyte disturbance impair function and can be dangerous, drinking excessive water to manipulate weight can lower blood sodium to a dangerous level, and temperature regulation is impaired, which is particularly risky when exercising in the heat. Acute kidney injury, impaired immune function and impaired performance are also described, and in the extreme, severe weight-making practices have caused deaths. Beyond the acute picture, repeated restriction and weight cycling may contribute to low energy availability, disordered eating and adverse health outcomes. It is low energy availability, rather than weight cycling alone, that drives relative energy deficiency in sport (RED-S), with its effects on bone, hormones, immunity and performance. For all these reasons, rapid weight loss is discouraged in favour of a slower, safer approach.

Body composition can be assessed with several methods, and it is useful to know that they exist while being careful about how their results are used. Dual-energy X-ray absorptiometry (DXA) is a detailed method, skinfold measurement is widely used in the field, and bioelectrical impedance is convenient but affected by hydration. Each method has strengths and limitations, none is a perfect measure, and the methods are not interchangeable, so serial measurements should use the same method, equipment and assessor, with standardised preparation including hydration and recent exercise.

Body composition is assessed only where there is a clear clinical or performance purpose, with informed consent, appropriately trained assessors, confidentiality, and access to nutritional or psychological support, and unnecessary testing is avoided, particularly in young athletes or those at risk of disordered eating. The important clinical principle is how assessment is framed: it is individualised and health-focused, used to support health and performance rather than to chase a single number, and its results are always interpreted in context. Changes in body water, which are common in athletes and are exaggerated by dehydration-based weight loss, can distort body composition measurements, so a single figure is not treated as an exact truth. Above all, numbers are not used as rigid targets to drive weight loss, particularly in young athletes or anyone at risk of disordered eating, because setting a body composition target can itself trigger harmful behaviour. Where assessment is used, it sits within professional support and a broader picture of health, training and wellbeing, and any concern that assessment is fuelling unhealthy behaviour is a reason to step back rather than to press on.

Safe weight management is gradual, adequately fuelled and professionally supported. Where a change in weight is genuinely appropriate, it is pursued slowly over time with adequate energy intake maintained, guided by a sports dietitian or registered nutrition professional rather than by anecdote, and dehydration-based cutting is avoided, with weight-category management requiring sport-specific planning, since one protocol does not fit combat sport, rowing and weightlifting alike. Sport rules can support safer practice, and the available strategies, such as minimum competition weights based on pre-season assessment, hydration testing, prohibiting the most dangerous weight-loss methods, and the timing of the weigh-in, are sport-specific regulatory tools with both advantages and limitations. A longer interval between the weigh-in and competition allows recovery but can, in some sports, encourage larger acute weight cuts, so no single rule fits every sport. Overall these measures aim to shift the culture away from extreme practices.

Alongside this, clinicians stay alert to relative energy deficiency in sport (RED-S) and to disordered eating, using a recognition-and-referral approach. The role here is to recognise warning signs and to involve the right specialist help early, within a supportive, multidisciplinary team. Where relative energy deficiency in sport is suspected, assessment is physician-led and based on the wider clinical picture, supported where appropriate by a recognised clinical assessment tool such as the International Olympic Committee relative energy deficiency in sport clinical assessment tool (second version), rather than by setting numeric thresholds or targets, and sensitive conversations are had without judgement. Safeguarding is a particular priority in young athletes, who are more vulnerable to the pressures around weight and to the harms of restriction, and in whom coaches, parents and governing bodies share responsibility for a healthy environment. The consistent message is that health comes first: performance built on unsafe weight practices is neither sustainable nor worth the risk, and the clinician's job is to protect the athlete while supporting legitimate, safe goals.

• •In weight-category sports, athletes often compete below their usual weight; rapid weight loss (weight cutting), often by dehydration, is common but hazardous.
• •Risks of rapid weight loss include dehydration and electrolyte disturbance, dangerously low blood sodium from excessive water intake, impaired temperature regulation, acute kidney injury, impaired immune function and impaired performance, with deaths in the extreme.
• •Low energy availability, which repeated restriction and weight cycling can contribute to, drives relative energy deficiency in sport, and is linked with disordered eating.
• •Body composition can be assessed with methods such as dual-energy X-ray absorptiometry, skinfold measurement and bioelectrical impedance, but assessment is individualised and health-focused, and figures are not used as rigid targets.
• •Safe weight management is gradual and adequately fuelled, guided by a sports dietitian, avoiding dehydration-based cutting; sport rules such as minimum competition weights and hydration testing can support safer practice, though such measures are sport-specific with advantages and limitations.
• •Recognise relative energy deficiency in sport and disordered eating and refer early; safeguarding is a particular priority in young athletes.