For many endurance athletes and highly active people, discipline is worn like a badge of honour. Early mornings, structured training blocks, meticulous planning and a willingness to push through discomfort are all part of the process. Nutrition, however, is often treated differently. Somewhere along the line, eating less became associated with being leaner, faster and more competitive.
In reality, under-fuelling is one of the most common and most damaging mistakes endurance athletes make.
It rarely announces itself loudly. There is no single moment where performance collapses overnight. Instead, the effects creep in slowly. Energy dips feel normal. Recovery takes longer. Strength fades quietly. Illness becomes more frequent. Training feels harder than it should, yet cutting calories still feels like the responsible thing to do.
The real cost of under-fuelling your training is far greater than missed personal bests. Over time, it can undermine performance, compromise health and increase injury risk, often without the athlete realising what is happening.
This is not a blog about eating more for the sake of it. It is about understanding what your body actually needs to train, recover and adapt.
What under-fuelling really means
Under-fuelling is not simply about eating too little food. It is about failing to meet the energy demands of your training, recovery and daily life.
An athlete can appear to eat well, consume nutritious foods and still be under-fuelled. The issue is often one of mismatch rather than intent. High training volumes combined with busy lifestyles make it easy to underestimate how much energy is being expended each day.
Under-fuelling can be intentional, such as deliberately restricting calories to lose weight or achieve a certain body composition. More often, it is accidental. Appetite does not always rise in proportion to training load. Busy schedules lead to skipped meals. Long sessions are followed by inadequate refuelling. Over time, the deficit becomes chronic.
The body does not distinguish between intentional and accidental under-fuelling. It simply responds to the energy available.
Energy availability and why it matters
At the heart of under-fuelling is a concept known as energy availability. This refers to the amount of energy left over for basic physiological processes after the energy cost of exercise is subtracted.
When energy availability is too low, the body begins to prioritise survival over performance. Systems that are not essential in the short term are downregulated. Hormone production changes. Bone turnover is altered. Muscle repair becomes less efficient.
For endurance athletes, this is particularly relevant. High training volumes mean high energy expenditure. If intake does not rise accordingly, energy availability drops quickly.
Low energy availability does not require extreme dieting. Even a modest daily deficit, sustained over weeks or months, can trigger adaptations that negatively affect performance and health.
Performance suffers before weight changes
One of the most frustrating aspects of under-fuelling is that performance often declines before noticeable changes in body weight occur.
Athletes may assume that if weight is stable, intake must be sufficient. In reality, the body can adapt to energy shortages by reducing energy expenditure elsewhere. Training economy may decline. Power output drops. Pace slows. Heart rate drifts higher for the same effort.
Sessions that once felt manageable begin to feel disproportionately hard. Recovery between sessions is incomplete. The athlete may push harder, believing it is a training issue, not a nutritional one.
This is often the point where more intensity is added, compounding the problem rather than solving it.
The hidden impact on muscle mass
Endurance athletes often assume muscle loss is only a concern for strength athletes. This is a misconception.
Lean mass plays a critical role in endurance performance. It supports force production, running economy, cycling efficiency and injury resilience. It also contributes to metabolic health and long-term wellbeing.
When energy intake is insufficient, the body may begin to break down muscle tissue to meet energy demands. This does not always show up clearly on the scales. Fat mass and lean mass can change in opposite directions, masking the issue.
Loss of lean mass can occur even when strength appears unchanged in the short term. Neural adaptations can temporarily maintain performance while structural tissue is slowly compromised.
Over time, this increases injury risk, reduces power output and makes it harder to maintain training intensity.
Glycogen depletion and training quality
Carbohydrate availability is central to endurance training. Muscle glycogen fuels moderate to high-intensity efforts and supports training quality.
Under-fuelling often means chronically low glycogen stores. Athletes may train repeatedly in a depleted state, believing it improves fat adaptation or mental toughness. While there may be strategic uses for low-glycogen sessions, chronic depletion comes at a cost.
Low glycogen impairs training intensity, reduces time spent at target zones and increases perceived effort. It also elevates stress hormones, which further interfere with recovery and adaptation.
When every session feels harder than planned, progression stalls. The athlete trains more but adapts less.
Hormonal consequences of under-fuelling
Hormones are highly sensitive to energy availability. In both men and women, chronic under-fuelling can disrupt hormonal balance.
In women, this may present as menstrual irregularities or loss of cycle altogether. In men, testosterone levels may decline. Thyroid hormones can be suppressed, slowing metabolic rate and contributing to fatigue and cold intolerance.
Cortisol, the body’s primary stress hormone, often increases. While cortisol plays an important role in training adaptation, chronically elevated levels impair immune function, disrupt sleep and promote muscle breakdown.
These hormonal shifts are not always obvious and are often misattributed to training stress alone.
Bone health and injury risk
One of the most serious long-term consequences of under-fuelling is its impact on bone health.
Low energy availability reduces bone formation and increases bone resorption. Over time, this weakens bone structure and increases the risk of stress fractures.
Endurance athletes, particularly runners, are already exposed to repetitive loading. When bone health is compromised, injuries become more likely and recovery becomes slower.
Bone density loss does not produce symptoms until damage has occurred. Many athletes only discover the issue after a fracture interrupts training.
This is not limited to older athletes. Bone health can be compromised at any age if energy availability is consistently low.
Immune function and illness
Frequent illness is often accepted as part of hard training. In reality, it is a red flag.
Under-fuelling compromises immune function. The body lacks the energy required to maintain an effective immune response, especially during periods of heavy training.
Athletes may experience frequent colds, lingering infections or difficulty fully recovering from minor illnesses. Training consistency suffers, and overall load becomes harder to sustain.
Illness is not a sign of weakness. It is often a signal that recovery resources are insufficient.
Sleep and recovery
Sleep quality is closely linked to energy balance. Under-fuelling can disrupt sleep through hormonal changes and elevated stress responses.
Athletes may struggle to fall asleep, wake frequently or feel unrefreshed despite adequate time in bed. Poor sleep further impairs recovery, appetite regulation and decision-making around training and nutrition.
This creates a cycle where fatigue drives poorer food choices, which worsens under-fuelling, which further disrupts sleep.
Relative energy deficiency in sport
Chronic under-fuelling can lead to a condition known as Relative Energy Deficiency in Sport, often referred to as RED-S.
RED-S affects multiple body systems including metabolic rate, menstrual function, bone health, immunity, cardiovascular health and psychological wellbeing.
It does not only affect elite athletes. Recreational endurance athletes and highly active individuals are equally at risk, particularly when training volume increases without corresponding nutritional support.
RED-S often develops gradually and can persist for years if not recognised.
Why endurance athletes are particularly vulnerable
Endurance sports reward leanness, efficiency and high training volumes. Cultural norms within these sports can normalise under-eating.
Long sessions suppress appetite. Training schedules disrupt regular meals. Body composition changes are often pursued aggressively without adequate monitoring of lean mass and energy needs.
Many athletes rely on wearable technology to estimate calorie burn, which can be inaccurate. Others base intake on outdated formulas that do not reflect their true metabolic needs.
Without objective data, under-fuelling can persist unnoticed.
Signs you may be under-fuelling
Under-fuelling does not always present dramatically. Common signs include persistent fatigue, declining performance, poor recovery, frequent illness, disrupted sleep, mood changes and stalled progress despite consistent training.
Weight stability does not rule it out. Feeling hungry is not always present. Many athletes adapt psychologically to low intake and stop recognising hunger cues.
Monitoring body composition, metabolic rate and training response provides far more insight than relying on weight alone.
Long-term consequences beyond sport
The effects of under-fuelling extend beyond athletic performance. Loss of bone density increases fracture risk later in life. Hormonal disruption can affect fertility and long-term metabolic health. Chronic muscle loss accelerates age-related decline in strength and function.
Athletic goals may change over time, but the body you build remains with you. Supporting it properly matters.
Fuel to train, fuel to adapt
Training is a stimulus. Adaptation happens during recovery, and recovery requires energy.
Eating enough does not make you slower. It allows you to train harder, recover better and sustain progress over time.
Well-fuelled athletes are more consistent, more resilient and more capable of absorbing training loads without breaking down.
Nutrition should support the work you are asking your body to do, not fight against it.
Measuring what actually matters
Many athletes rely on guesswork when it comes to nutrition and energy needs. This is where objective measurement becomes invaluable.
Understanding resting metabolic rate helps determine baseline energy requirements. Assessing body composition reveals whether weight changes reflect fat loss, muscle loss or both. Monitoring bone density provides insight into long-term skeletal health.
Without this information, it is easy to unintentionally drift into under-fuelling, especially during high training phases or weight-focused goals.
A smarter approach to endurance performance
Endurance performance is not built on suffering alone. It is built on intelligent training, adequate recovery and sufficient fuel.
Under-fuelling may feel disciplined in the short term, but it undermines the very adaptations athletes work so hard to achieve.
The most successful endurance athletes are not those who eat the least. They are those who fuel appropriately for the demands they place on their bodies.
Under-fuelling your training carries real costs. Reduced performance, impaired recovery, muscle loss, hormonal disruption, weakened bones and increased injury risk all stem from insufficient energy availability.
These effects often develop quietly and are easily misattributed to training stress or ageing. Without objective data, many athletes do not realise what is happening until progress stalls or injury occurs.
Fuel is not the enemy of performance. It is the foundation of it.
How BodyView can help
At BodyView, we help athletes and active individuals move beyond guesswork. Our services provide clear, objective insight into how your body is responding to training and nutrition.
DEXA body composition scans allow you to accurately track changes in fat mass, lean mass and bone density, ensuring performance goals are not achieved at the expense of muscle or skeletal health. Resting metabolic rate testing provides a precise measure of your baseline energy needs, removing uncertainty around calorie intake. VO2 max testing helps you understand cardiovascular fitness and training zones, so sessions are both effective and sustainable.
Together, these insights allow you to fuel with confidence, train with intent and protect your long-term health while pursuing performance goals.
Training hard is only part of the equation. Fuel properly, measure what matters and build performance that lasts.