Undereating: why it hinders endurance progress

In the world of endurance sports, we often talk about training volume, quality sessions, and recovery. However, one factor is too often underestimated: energy intake. When energy intake is insufficient compared to expenditure, the body doesn't just "run out of fuel." It activates survival mechanisms that voluntarily reduce the physiological adaptations we strive to develop during training.

This is known as Low Energy Availability. This phenomenon is not just temporary fatigue. It's a state where the energy remaining after exercise is insufficient to cover the body's fundamental needs (hormonal function, muscle repair, immune system, bone health, etc.).

The body prioritizes survival over performance

When available energy becomes chronically low, the body enters an economy mode. It begins to reduce or suppress functions that are not essential for immediate survival. Among the first to be affected:

• Production of sex hormones (estrogen, testosterone)
• Protein synthesis (muscle repair and building)
• Regulation of basal metabolism
• Thyroid function
• Immune response

The concrete result for the endurance athlete: progress stagnates or regresses, even with a well-structured training plan. The adaptations expected from training (improvement in lactate threshold, strength gain, better running economy, faster recovery) become much harder to achieve. The body, being intelligent, chooses to preserve vital energy rather than invest in long-term performance gains.

This is precisely where the trap lies: an athlete may feel "good enough" in the short term and continue to train hard, while their body progressively reduces its capacity to adapt.

Commonly observed physiological signs

Blood tests of athletes with chronic low energy availability frequently reveal a consistent picture:

• Elevated cortisol (stress hormone)
• Often low vitamin D
• Suboptimal hemoglobin and ferritin
• Thyroid disturbances
• Decreased testosterone in men or menstrual cycle irregularities in women

These changes are not insignificant. They are generally accompanied by persistent fatigue, increased muscle soreness, slower recovery, and, most importantly, a marked decrease in training adaptations.

A 2019 scientific review published in Current Opinion in Endocrine and Metabolic Research clearly describes this phenomenon: when available energy is insufficient, the body reduces its investments in anabolic processes (building) to prioritize survival functions. Protein synthesis decreases, bone density can be compromised, and the central nervous system becomes less efficient.

Underfueling = fewer adaptations, even in motivated athletes

Many athletes still think that "eating less" will make them lighter and therefore more performant. The physiological reality is different. When low energy availability is maintained over several weeks or months:

• Resting metabolism slows down
• Appetite decreases (the body demands less food)
• Sleep quality and active recovery diminish
• Strength and endurance gains become harder to achieve

In other words, the body voluntarily reduces the extent of adaptations that training should produce. This is an ancient protective mechanism: when faced with a perceived "famine" period, the body does not waste energy on becoming stronger or more enduring.

The most insidious aspect is that this phenomenon can occur even without extreme voluntary restriction. A small, repeated daily deficit, skipped meals after long outings, or insufficient carbohydrate intake can be enough to create this state in the long term.

The importance of a sustainable approach

The message is not to "eat without limit" or consume anything. Rather, it's about adopting a non-restrictive approach to sports nutrition: eating enough to support the work done, choosing foods you enjoy, within a framework of balance and pleasure.

Carbohydrates play a particularly important role here, as they are the main fuel for endurance efforts and actively participate in hormonal regulation. Eating a proper breakfast after an overnight fast, avoiding prolonged fasted training, and filling energy deficits throughout the day are simple but powerful habits.

In the long term, athletes who maintain adequate energy availability generally notice:

• Better training adaptation
• Reduced injuries
• Faster recovery
• Better preserved overall health (hormonal, bone, immune)
• More sustained performance over several years

Conclusion: long-term performance comes through nutrition

The human body is remarkably adaptable… as long as it has enough energy. When undernourished, it prioritizes survival at the expense of performance and long-term health. That's why "always eat enough" is not an indulgence tip, but a fundamental physiological rule for any endurance athlete who wants to progress sustainably.

True performance is not that which shines for just one season. It's that which is built year after year, without sacrificing health. And this performance begins on the plate.