Body composition, weight and the leanness trap
Evidence: moderate
A better power-to-weight ratio helps within a healthy range, but the risk curve is U-shaped: chasing leanness past a point triggers low energy availability, bone injury and illness, and performance falls. Fuel for performance, not for the scale.
Not medical advice
This is a general knowledge base, not medical or dietary advice. If you are injured, unwell or weighing up a supplement or a change to your diet, speak to a doctor, physiotherapist or registered dietitian who knows your situation.
Distance running rewards moving a given aerobic engine over a lighter frame, so power-to-weight matters. The mistake is turning that into ‘lighter is always faster’. Below a healthy range the costs rise faster than any aerodynamic or economy gain, and the runner ends up slower, injured, or both.
The real tension
Within a sensible range, carrying less non-functional mass lowers the energy cost of running and tends to improve race times (body composition synthesis). That is the kernel of truth behind ‘racing weight’. But the relationship is not linear, and it does not keep paying out. Among elite runners, body fat clusters in a band, not at zero, and within elite groups lower skinfolds do not reliably predict faster times (body composition synthesis).
The leanness trap
The curve turns sharply downward
Pushing body fat too low, below roughly 5% in men or 10–15% in women, harms immune function, bone health and performance (body composition synthesis). There is an optimal range, and going under it costs more than it ever returns.
The mechanism is energy, not the number on the scale. Getting lean by eating too little for the training done produces low energy availability, the root cause of RED-S, which suppresses bone-building hormones, lowers bone density and raises bone stress injury rates (Gallant et al. 2024). It also weakens immunity, so illness and missed training rise. A prospective study of female runners found that injured runners ate less fat than uninjured ones, with fat intake the single best dietary predictor of future injury (Gerlach et al. 2008). The drive to be lean often shades into disordered eating, which makes the trap self-reinforcing.
So the risk is U-shaped. Too heavy carries a small economy penalty; too lean carries a steep cliff of broken bones, illness, hormonal disruption and lost training. The downside is not symmetric, which is why the safe error is to err toward eating enough.
Fuelling for performance, not the scale
Train the engine, feed the body, let composition settle
Most runners get faster by building the aerobic engine and staying healthy enough to train consistently, not by losing weight. Any weight change should come slowly and out of competition, never by restricting intake during hard training, where it sabotages adaptation and bone. If body composition is being managed at all, it is fuelled training that does it safely, not the scale.
The practical rule is to chase consistent, well-fuelled training and let a healthy body composition follow, rather than chasing a target weight and hoping performance follows. The first path is reliable. The second routinely ends in RED-S and a bone stress injury.