Under-fuelling in Endurance Athletes

With all the recent exposure of the benefits of carbohydrates (and lots of them) for performance and recovery in endurance sport, you would think that under-fuelling would start to become a thing of the past. Unfortunately this does not appear to be the case, with under-fuelling or Low Energy Availability (LEA) and Relative Energy Deficiency in Sport (REDS) occurring in all levels of sport - professional, elite and amateur. 

In simple terms, REDS refers to when an athlete is not eating enough (ie. under-fuelling) to cover normal physiological processes for the amount of training being undertaken, resulting in impaired physiological function. When in a state of LEA, the body will conserve energy for essential functions, suppressing resting metabolic rate and shutting down processes not critical for short term survival such as reproduction and bone turnover.

The prevalence of LEA is high, with a systematic review in 2024 finding that 44.7% of athletes experienced LEA (44.2% in females and 49.4% in males), and that 63% were at risk of REDS. Another study of US Collegiate athletes in 2025 found LEA prevalence ranged from 15% to 100%, with most studies reporting over 50% in female athletes and 15% to 45% in males.

Whilst is is clear that cases of LEA and RED-S remain high, particularly among female athletes and certain elite groups, most studies are cross-sectional and don't compare across multiple time points, making it is difficult to conclude whether the incidence is increasing or decreasing.

WHY MIGHT WE UNDER-FUEL?

Under-fuelling and LEA is not always intentional or related to disordered eating. 

Intentional under-fuelling occurs when athletes restrict their dietary intake to control body weight and/or body composition. It can occur in the presence of disordered eating but also without. It is more likely to occur in sports where power to weight or aesthetics tend to be prioritised or there are weight classifications eg. gymnastics, ballet, rowing, running, cycling, jockeys etc.

Body image concerns, social or external pressures (eg. parents. coaches), uninformed dieting programs, and common myths or traditional beliefs and practices (such as “eating is cheating”) can compound intentional LEA. Often it is hard to break old habits and beliefs, despite the emergence of scientific evidence and expert opinion regarding adequate fuelling.

There are many contributing factors to unintentional under-fuelling, where athletes are not meeting the energy demands of their sport. It can be due to knowledge gaps regarding nutrition and eating or lack of awareness of actual energy requirements. Some people experience exercise-induced appetite suppression, and fatigue can blunt hunger cues. Additionally, many people find it difficult to eat before early morning training sessions, meaning they will go into the session under-fuelled.

There can be practical barriers related to lifestyle and work. Training in amongst busy schedules, travel, or work/study (especially for amateur athletes) can present time constraints and result in simply not eating enough. A lack of food availability in unfamiliar environments (eg. when away from home), as well as financial difficulties, can also lead to the consumption of poor quality food. 

Any diet that excludes food groups may make it harder to consume enough calories to fuel training, and require more diligence to ensure energy needs are met. Common examples in our society are vegetarian, gluten-free, lactose-free, ketogenic and plant-based diets. Depending on which diet is followed, the athlete may miss out on specific macros and nutrients such as carbohydrates, protein and calcium.

Under-fuelling can also occur when engaging in sports with high energy expenditure or during periods of increased training volume or intensity, with athletes underestimating the amount of energy they are expending. Endurance sports such as cycling and running poses a high risk for LEA and REDS as it can be both power to weight related and involve long training hours and hence high energy expenditure. 

HOW DOES IT EFFECT US?

Under-fuelling will have an effect on both performance and health, with short and longer term consequences. Reduced glycogen stores and carbohydrate availability can result in early fatigue, inability to hit high intensities, loss of explosive power, and poor endurance. Athletes can experience decreased concentration and coordination as brain function relies heavily on glucose. Recovery between sessions can be slower, with an inability to train effectively on consecutive days.

The athlete may find their performance does not improve over time due to a poor response to training and a lack of adaptation. Muscle protein synthesis drops when under-fuelled, resulting in less strength gain and reduced repair capacity. Athletes can have an increased risk of both traumatic and overuse injuries due to impaired bone turnover and soft tissue repair. Suppressed immunity can result in more frequent bouts of illness, which effects their ability to train consistently. There can also be psychological and cognitive effects such as anxiety, irritability, mood swings, poor concentration, low motivation and higher perceived effort.

Essentially, you can’t train hard, adapt fully, or stay injury-free without adequate fuel.

MALES VS FEMALES

One of the more serious health concerns of under-fuelling is a disruption to the endocrine/hormonal system. The effects are manifested differently in males and females, with females generally experiencing more profound consequences. 

In females there is significant evidence that LEA inhibits the hypothalamic–pituitary–gonadal (HPG) axis, reducing oestrogen and progesterone production. This can lead to menstrual disturbances (irregular cycles, amenorrhea), which is a clear, early marker of under-fuelling. Oestrogen plays a major role in maintaining bone health, with a lack of oestrogen resulting in low bone density, making females more vulnerable to osteopenia/osteoporosis, fractures, and bone stress injury,

Energy deficiency in males can suppress testosterone production, leading to reduced libido, impaired recovery, lower muscle protein synthesis, and possible reductions in bone health. However, these hormonal effects are often more subtle and less immediate than those experienced by females.