Body weight versus adequate fuel

Body weight and composition are topics that carry significant psychological weight in endurance sports. The benefits of lower weight in running—lower energy cost per kilometer, a better power-to-weight ratio—are real and measurable. But pursuing a lower weight at the expense of adequate energy availability is one of the most damaging mistakes an endurance athlete can make, and it is more common than many admit.

This article discusses the science of energy availability, its relationship to performance, and where the balance lies for ultra-runners.

What Energy Availability Means

Energy availability is dietary energy intake minus exercise energy expenditure, expressed per kilogram of fat-free mass. It describes the energy remaining for all physiological functions once training has been accounted for.

When energy availability is too low—due to insufficient food intake, excessive exercise, or both—the body shifts into an energy-saving mode. Non-essential physiological processes are downregulated to conserve resources for survival functions.

The threshold for significant impairment is approximately thirty kilocalories per kilogram of fat-free mass per day. This state is known as low energy availability.

What Low Energy Availability Does to an Endurance Athlete

The IOC consensus statement on relative energy deficiency in sport describes a syndrome that affects multiple physiological systems when energy availability is chronically insufficient.

Reproductive hormones are suppressed in both male and female athletes. In women, this often manifests as menstrual irregularities or amenorrhea. In men, testosterone levels drop. Both impair bone density and recovery capacity.

Bone health suffers. Low energy availability suppresses bone formation and increases bone breakdown. Stress fractures become significantly more likely. For an ultra-runner logging high weekly mileage, this is a serious injury risk.

Immune function weakens. Illness becomes more frequent, recovery from illness slows, and training consistency is disrupted.

Resting metabolic rate decreases. The body adapts to chronic restriction by reducing its energy expenditure, which is the mechanism that causes many to plateau when trying to lose weight through caloric deficit alone.

Muscle protein synthesis is impaired. Repair and adaptation slow down. The same training yields fewer benefits. The risk of overuse injuries increases.

Psychological effects also arise. Irritability, decreased motivation, impaired concentration, and disturbed sleep are documented consequences. In ultra-running, where mental fortitude is as important as physical fitness, these directly limit performance.

The Performance Paradox

A counterintuitive finding in research is that athletes with low energy availability often train more and weigh less than well-fueled peers but perform worse, get injured more frequently, and have shorter athletic careers.

Short-term weight loss through caloric deficit can momentarily improve metrics like pace per kilometer and power-to-weight ratio. Over weeks and months, however, the systematic damage of low energy availability eats away at those benefits, leading to a net-negative outcome.

A sufficiently fueled athlete, at a weight appropriate for their structure, who trains consistently over years, will perform better than a lighter, chronically underfueled one.

In a conversation with Alex Larson—a registered dietitian and endurance sports nutrition coach with fifteen years of experience working with runners, cyclists, and triathletes—Alex pointed out that many athletes entering her program are in daily deficits of eight hundred or a thousand calories, and that this is genuinely detrimental to metabolic health. The body adapts. Metabolism slows down. The drastic nutritional change, which was supposed to lead to better performance, ends up doing the opposite. The changes that truly last, she emphasizes, are gradual—small, sustainable shifts rather than aggressive restriction.

PODCAST LINK: Episode with Alex Larson — YouTube

The Ultra-Runner's Context

Ultra-running has a specific risk profile for low energy availability. The long training runs required to prepare for ultra-events create large daily energy deficits which, if not adequately replenished with food, accumulate into a chronic deficit over the training block.

Exercise-induced appetite suppression—more pronounced in women and at higher intensities—means many athletes don't genuinely feel hungry enough to meet the demands of training. This is not a willpower issue. It's a physiological response that requires proactive eating strategies.

Eating by the clock rather than by appetite, carbohydrate-rich meals and snacks around every training session, and monitoring warning signs are the most important protective practices.

Warning Signs of Low Energy Availability

Persistent fatigue disproportionate to training load. Frequent illness or slow recovery from illness. Stress fractures or persistent bone pain. Menstrual irregularities in female athletes. Declining performance despite consistent training. Loss of motivation or persistent low mood. Difficulty sleeping despite physical fatigue.

If three or more of these are present, energy availability should be assessed and increased before continuing with high training loads.

Summary

Body weight matters in running performance. But energy availability matters more. A sufficiently fueled, slightly heavier athlete will perform better, stay healthier, and train more consistently than a lighter, chronically energy-deficient one. Fuel your training. Let body composition find its naturally performance-optimal point through consistent training, not through restriction.

References

Mountjoy M, Sundgot-Borgen JK, Burke LM, Ackerman KE, Blauwet C, Constantini N, et al. (2018). IOC consensus statement on relative energy deficiency in sport (RED-S): 2018 update. British Journal of Sports Medicine. 52(11), 687–697.

Loucks AB, Kiens B, Wright HH. (2011). Energy availability in athletes. Journal of Sports Sciences. 29(Suppl 1), S7–S15.

Burke LM, Ross ML, Garvican-Lewis LA, Welvaert M, Heikura IA, Forbes SG, et al. (2017). Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. Journal of Physiology. 595(9), 2785–2807.