Energy Allocation in Sports: The Double-Edged Sword of Low Energy Availability

As someone who has personally experienced the physical and emotional toll of energy deficiency and navigated the challenging path of recovering from an eating disorder, I have developed a heightened sensitivity to the subtle cues of disordered eating and suboptimal fueling in athletes. My own journey, which included regaining my menstrual cycle alongside an increase in body weight, has prompted me to deeply reflect on the impact of energy availability on health and physical performance.

In endurance sports, particularly those emphasizing leanness and low body weight, it is unfortunately common to encounter athletes who are in a state of energy deficit. A state of low energy availability (LEA) may be present acutely or chronically, whether by choice or inadvertently. Despite symptoms of LEA – such as menstrual disturbances in women – many of these athletes still perform at a high level. This paradox raises important questions: How is the body able to maintain performance under such sub-optimal circumstances, and at what cost?

While the impact of LEA on physiological processes like reproductive function and bone formation/resorption is well documented, its effects on other aspects of physical capacity remain less clearly understood. Does the body prioritize performance over long-term health and how could that be explained? How is energy “allocated” in the body during periods of energy deficiency?

Another observation is that, especially in women, relative energy deficiency does not always go hand in hand with an extremely thin and lean body composition. This phenomenon is less commonly observed in males, which could suggest that some complex hormonal and metabolic changes occur when the (female) body is under-fueled?

After regaining my menstrual cycle and increasing body weight, I've largely benefitted from the upsides of increased energy availability, such as fewer depressive symptoms, increased bone mineral density, more training continuity, and an overall higher life quality. Nonetheless, I can’t help but wonder if there are still remnants that my body “remembers” from those extended periods of insufficient energy and especially low carbohydrate intake. For instance, long-term effects on hormones and metabolism, changes in the microbiome, brain function, or the ability to adapt to training, in addition to potentially life-long mental and emotional impacts?

These are all questions and introspections I contemplate. Coupled with the broader issue of energy deficiency among athletes, these gave me the incentive to dive into the research about the relationship between energy availability, health and physical performance. While there has been significant progress in this area of research in recent years, we still have many unanswered questions. This article, drawing on the perspectives of evolutionary biologists like Herman Pontzer^{[i],[ii],[iii]} and Daniel Lieberman^{[iv],[v],[vi]} and others, whose ideas have recently gained traction in sports science, aims to stimulate further interest and discussion on these important but complex topics.

Physical Performance in the Context of Energy Deficiency

Energy availability (EA) is a critical factor in sports and exercise nutrition, defined as the energy remaining for bodily functions after subtracting the energy expended during exercise from dietary intake^[vii]. The mathematical formula for energy availability provides the amount of energy the body has available to allocate to health functions, well-being and performance:

When EA is low – though what constitutes “low” requires further definition, which we will address shortly – a state known as low energy availability (LEA) can develop. For instance, menstrual disturbances like amenorrhea or oligomenorrhea are common among female athletes who chronically underfuel below a certain threshold, typically defined in LEA research as 30 kcal per kg of fat-free mass per day^[viii]. However, this threshold was largely derived from controlled lab settings and cannot be applied universally^[ix], hence the notion of a rigid threshold is losing ground in favor of the idea that effects of LEA exist on a continuum rather than adhering to a strict cutoff^[x],[xi]. Different physiological systems, such as reproductive, cardiovascular, and metabolic systems, respond variably to energy deficits^[xii]. Some systems may be more resilient, while others may show signs of strain earlier. In addition, other factors such as age, genetics or individual baseline health can play a role too. Furthermore, it must be considered that the formula above does not account for non-exercise activity thermogenesis (NEAT) – the energy expended during non-training activities like walking, commuting or doing daily chores. For some athletes, NEAT can significantly contribute to energy expenditure. For instance, an athlete might commute 45-60 minutes by bike each day but may not consider it “true exercise” and therefore would not plan to fuel for it accordingly. However, this energy expenditure can add up over time and lead to problematic low energy availability, making NEAT a major contributing factor for developing RED-S – Relative Energy Deficiency in Sport.

The 2023 IOC Consensus on Relative Energy Deficiency in Sport (RED-S)^[xiii] outlines the advanced understanding of LEA, describing it as existing on a spectrum. At one end, adaptable LEA refers to short-term, mild energy reductions that may lead to temporary, reversible changes – such as improved power-to-weight ratio or optimized body composition during competition peaks. These effects can be beneficial in the short term and are often part of deliberate strategies. On the other end, problematic LEA refers to prolonged or severe energy deficiency that disrupts various body systems, leading to maladaptive responses such as impaired reproductive health, bone density loss, and reduced immunity, which collectively define RED-S, a syndrome that negatively impacts health and performance^[xiv],[xv].

It is important to recognize that not all LEA leads directly to RED-S. RED-S results from problematic LEA – chronic or severe LEA exposures – that cause negative health outcomes.

A complicating and misleading issue is that the effects of LEA are not always immediately reflected in performance. Research shows that even with significant physiological disruptions, some athletes can maintain or improve their performance. A notable case study by José  documented an elite female cyclist who exhibited high aerobic capacity despite symptoms of LEA, such as low body mass and menstrual dysfunction^[xvi]. Another study of a world-tour cyclist during the Tour de France Femmes found that the athlete recorded a personal best power output (30min effort) in the final stage, despite clear signs and symptoms of energy deficiency before (oligomenorrhea and low T3) and during (negative energy balance and weight loss) the 7-day race^[xvii]. These and other studies challenge the assumption that optimal energy availability is always necessary for peak performance.

The Evolutionary Perspective on Energy Allocation

One way to reconcile these seemingly contradictory findings is to consider them from an evolutionary perspective. Humans have evolved to endure intermittent periods of food scarcity by prioritizing certain physiological functions like locomotion and physical capacity^[xviii]. This prioritization likely stems from the need to secure food and ensure survival during periods of limited energy availability. In such scenarios, the body may downregulate non-essential physiological systems – such as reproduction and growth – to conserve energy for crucial tasks to survive like finding food or escaping danger.

“It appears that physical capacity, despite being energetically expensive, is a high-priority trait for allocation of limited resources and unlikely to be affected by energy deficiency unless key energy stores (e.g., muscle glycogen) are critically low, or the deficit severe.” (Areta 2023^xviii)

This evolutionary theory, on one hand, could help to explain why some athletes can maintain or even enhance their physical performance, at least temporarily, despite being in a state of energy deficiency. However, it is important to note that evolutionary physical “performance” is quite different from modern sport performance.

Dan Lieberman, in his book Exercised^v, argues that while we evolved to sustain physical activity during periods of scarcity, this kind of activity – such as walking long distances to gather food or sprinting to catch prey – is vastly different from the demands of competitive sports. The energy required to simply forage for survival is far less than what is needed to perform at the highest levels of modern athletic competition. Lieberman points out that hunter-gatherers, for example, would only engage in physical activity when it was necessary for survival. When he visited tribes in Africa, they were baffled by the concept of "frivolous" exercise, like going for a run just for the sake of it^vi.

In fact, Lieberman suggests that the energy expenditure of some high-volume athletes today – especially in endurance sports – may be double or even triple what humans are evolutionarily adapted to handle. Our ancestors alternated between energy-intensive tasks and long periods of rest, while modern athletes often push their bodies to extremes that would have been unheard of in a hunter-gatherer society.

So, on one hand, this evolutionary perspective offers insight into why athletes may sustain physical capacity despite signs and symptoms of LEA. On the other hand, it raises more questions about the limits of our physiological adaptations. What are “critically low” energy stores or a “severe deficit” in the context of elite sport performance? And how do different forms of LEA, such as low carbohydrate availability, within-day energy deficits or an overall calorie deficit impact different body systems?

Unfortunately, to most questions there are no definitive answers yet. However, exploring these twists and turns in our understanding of LEA is valuable, especially as we continue to push the boundaries of human performance far beyond what we were evolutionarily designed to endure.

The Complex Relationship Between Energy Deficiency and Performance

The relationship between energy deficiency and physical performance is complex. We would need to look at a variety of physiological systems (down to the molecular level), consider different types of energy, localizations, severities and durations of LEA, the type of sport and the individual characteristics of athletes to illuminate this topic thoroughly (for further reading on this: Burke 2023^xii).

Therefore, within the scope of this article we will focus only on a selection of factors that should be considered and discussed:

1.    The Paradox of Performance in Energy Deficiency

The above-mentioned case studies seem to suggest that physical capacity can be maintained or even sometimes improved (depending on the level of deficit) when exercise is undertaken in the face of an energy deficit - which highlights the body’s remarkable ability to adapt. However, while we have seen considerable scientific advances on understanding the underpinning physiology and psychology of long-term problematic LEA (RED-S)^xiv,xv,[xix],  we cannot rely on many studies that explore the effect of energy deficit on muscle physiology, function and athletic performance.

Physical activity demands significant energy for muscle contraction, which remains necessary even when food intake is limited. This prioritization of physical capacity likely evolved as an adaptive mechanism, making the maintenance of physical capacity a top priority for the body when allocating energy among various functions. Although energy deficiency inhibits anabolic processes and can reduce muscle mass and downregulate muscle growth, regular muscle contraction can help maintain lean muscle and functional strength, possibly even allowing for some strength gains, at least up to a certain level of energy deficit^[xx],[xxi].

2.    Impact on Aerobic Fitness and Muscle Oxidative Capacity

Studies also indicate that aerobic fitness and muscle oxidative capacity can be maintained or even improved in the face of energy deficiency^{xvi,xxi,[xxii]}. However, we still lack enough evidence to determine when and how energy deficiency begins to negatively affect aerobic capacity. Whether these negative effects stem from disruptions in the central nervous system, cardiovascular system, skeletal muscles or a combination of these factors is unclear.

3.    The Danger of Misinterpreting LEA

Most athletes would likely perform better without being in a state of LEA. However, seeing athletes perform well and at a high level despite (sometimes problematic) LEA can be misleading, especially for young, developing athletes or women who may misinterpret the absence of their period as a sign of “hard work” rather than a health concern.

Entering a state of energy deficiency and relying on a “wait and see” mindset, hoping to improve performance or body composition without a controlled, guided approach is highly risky and can come at the cost of jeopardizing both performance and long-term health. Furthermore, relying on performance as an indicator is misleading, as signs and/or symptoms of energy deficiency can be present even while athletes perform at a high level, making it an unreliable measure for avoiding the long-term health risks associated with RED-S.

In addition, what in this context is often overlooked, by athletes as well as athlete stakeholders, is that body weight is not equivalent to body composition. If success were determined by body weight alone, junior athletes – who tend to be smaller and weigh less – would hold world records in every weight-bearing discipline. In reality, however, all those records belong to senior athletes with more developed lean muscle mass. Prolonged energy deficits reduce lean body mass, which leads to drops in power and force production^{[xxiii],[xxiv]}, ultimately harming performance. This distinction makes it clear that athletes should focus on building and maintaining lean muscle mass rather than simply reducing body weight.

4.    Risk assessment: Bear in Mind the Consequences

One should always keep in mind that adaptation occurs on a spectrum and can be positive before it turns into something negative and harmful (for more on that, read my article on Allostatic Load).

LEA occurs on a spectrum as well, with adaptable LEA potentially being short-term beneficial, but prolonged energy deficiencies being problematic and leading to detrimental effects^ix,xv,[xxv]. While LEA is generally thought to rapidly trigger a negative state and even within-day energy deficits are suggested to be avoided^{[xxvi],[xxvii]}, athletes in endurance, aesthetic, weight-class and weight-carrying disciplines, often aim to achieve a lean body composition during peak competition phases, such as championship events.

Given the potential for both positive and negative outcomes associated with LEA - depending on the person’s baseline context – a strategic approach to energy management is essential. Periodizing energy intake and body composition throughout the training cycle may help athletes take advantage of the benefits of LEA (such as improved power-to-weight ratio) while minimizing its risks^{xxii,[xxviii]}. However, for this approach to be both effective and safe, it requires expert guidance. The strategic use of short-term and subtle energy deficiency must be carefully planned, including appropriate recovery periods, and should involve athletes who are physically and mentally mature, have a low risk for eating disorders or disordered eating, and are closely monitored. This is especially important for female athletes, who may be more susceptible to developing RED-S​.

5.    A more nuanced understanding of Energy Deficiency

The relationship between energy availability and performance is complex. Energy deficiency is often discussed in broad terms without considering critical factors, such as the type of deficiency (systemic vs. localized), macronutrient composition (especially carbohydrate intake), and the timing of energy intake relative to exercise^xiii,xxviii.

Short-term forms of a low energy availability state can be induced by nutritional manipulation and periodization. Muscle glycogen is an important fuel source for muscle contraction and represents an energy storage metabolite that is tightly monitored by the cell. Training with low muscle glycogen – a form of localized energy deficiency – has sparked interest due to its potential to improve skeletal muscle adaptations, particularly oxidative metabolism^[xxix].

While strategically timed (“periodized”) low-glycogen training may enhance endurance training adaptations on a molecular level and potentially impacts energy expenditure through improvement of efficiency and economy of movement^{[xxx],[xxxi],[xxxii]}, it is not a universally beneficial strategy! Chronic carbohydrate deprivation (low-carb-high-fat diets) can impair performance^[xxxiii] and no clear evidence links nutritional (carbohydrate) periodization to long-term performance improvements^[xxxiv].

Training in a low-glycogen and/or low-energy state is therefore largely a matter of risk assessment. For athletes with high training volumes, it is important to recognize that training “low” comes at a high risk: it can significantly increase the “recovery costs”, reduce training quality, and elevate the risk of injury and illness, potentially disrupting training continuity. Moreover, for athletes who train multiple times a day, the likelihood of unintentionally training with low glycogen availability is very high anyway.

In real-world scenarios, LEA also often coincides with low carbohydrate availability (LCA), which can have an energy-independent and potentially magnifying role in RED-S development. Studies have shown that even in the absence of LEA, LCA alone can negatively affect health markers such as bone density, immunity and iron biomarkers^{ix,[xxxv],[xxxvi],[xxxvii],[xxxviii]}. These findings highlight the need to carefully manage carbohydrate intake at all times, but particularly during strategic periods of short-term energy restriction.

6.    Observational Insights and Strategic Considerations

Paradoxically, many female athletes who experience long-term energy deficiency maintain a stable body weight^[xxxix] or even higher body fat percentages^[xl]. This points to a complex interaction between the endocrine system, metabolism, and energy availability. Hormonal factors, particularly those regulating metabolism and reproductive function, likely play a significant role in how the body responds to LEA. Interestingly, within-day energy deficits – periods during the day when energy intake does not match immediate energy expenditure – were associated with clinical markers of metabolic disturbances^xxxix,[xli]. This still relatively unexplored area of LEA could further complicate the picture, potentially contributing to the observed phenomena of stable body weight despite chronic energy deficiency.

These observations not only emphasize the importance of optimal fueling, but also the timing of energy intake. Strategic energy intake in and around training could make a significant difference to enhanced training adaptation, performance in the next training session^[xlii] and more optimal body composition, while also reducing the risk of injuries, illnesses, and the potential for overreaching or overtraining. This, again, highlights the need for a more nuanced approach to managing energy intake that considers not just the total daily calories, but how those calories are composed (macronutrient composition) and distributed throughout the day in relation to training sessions.

7.    Thinking Long-Term!

A significant problem in managing problematic LEA is that humans are generally poor at assessing long-term consequences. In particular, highly ambitious athletes may continue to push their bodies, without recognizing the cumulative damage caused by a long-term energy deficit​. If we bonk in a session due to insufficient fueling, we immediately respond by grabbing the most sugary drink available. However, we tend to overlook the more insidious effects of consistently underfueling over time. Bones, for instance, don’t give immediate signals of damage unless they are on the verge of breaking. This inability to foresee long-term consequences is further exacerbated in younger athletes, who may not have the capacity to imagine how their bodies will be affected 5, 10, 20 years down the line. Trent Stellingwerff highlights this issue in his article “Patience through puberty”^[xliii]. He emphasizes that adolescence is a critical window for optimizing skeletal health, as nearly 90% of peak bone mineral density (BMD) is achieved by age 20^[xliv]. Failure to optimize BMD during this period can lead to increased fracture risk later in life, something that is further aggravated by LEA. In fact, long-term LEA has been shown to cause significant reductions in BMD, which, as studies indicate, increases the risk of bone stress injuries in female runners^{[xlv],[xlvi],[xlvii]}. Furthermore, amenorrheic athletes - often a consequence of chronic LEA - are four times more likely to suffer bone injuries and exhibit lower hemoglobin mass, both of which negatively impact performance and longevity in sport^{xxxv,[xlviii]}.

In summary, the evidence is VERY clear that long-term exposure to LEA not only reduces endurance performance but puts athletes’ careers and health at significant risk!

Therefore, Stellingwerff emphasizes the importance of patience in long-term athletic development, particularly for female athletes.

“It is normal and healthy for female athletes to undergo changes in their bodies as they hit puberty and begin menstruating. Athletes may also experience changes in athletic performance or perceived athletic performance. During this time, it is important to keep long-term goals in mind and to prioritize health and well-being over short-term performance gains. By playing the long game and embracing these changes, more girls may be able to be lifelong athletes and compete well into adulthood!” (Stellingwerff 2022^xliii)

He highlights the importance of proactive awareness and education on RED-S to improve female adolescent sport culture, female athlete retention rates and ultimately, a life-long love for sport. In addition to educating athletes – junior or senior, male or female – about the long-term consequences of chronic energy deficiency, it is crucial to equip them with the knowledge to recognize specific behaviors and symptoms of RED-S. By being aware of these symptoms, athletes can bring concerns to an expert before health and performance are significantly impacted. Therefore, it is vital to foster environments that prioritize health over immediate performance outcomes, especially for young athletes who may not fully grasp the cumulative damage they are exposing themselves to over time.

The Importance of an Optimal Body Composition in Sports and Achieving It Safely

Before concluding this article, it is important that we acknowledge that striving for an optimal body composition or a good strength/power-to-weight ratio in sports is common and not inherently wrong. In fact, it can be crucial for performance. However, the pursuit of an optimal body composition must be approached correctly. If done improperly, it is associated with massive risks, including impaired performance and, if done long enough, long-term health issues – some of which possibly being irreversible ^{xiii,xiv,[xlix]}.

Much of the current messaging in sports nutrition and education seems to focus on “being happy with the body you have” and discouraging any form of weight loss. While this is well-intentioned, it can leave athletes who wish to improve their body composition feeling lost or unsupported. The issue isn’t with the desire to change body composition but with how it’s done. Thin athletes may sometimes feel criticized or misunderstood, while heavier athletes are seen as more “normal” - or vice versa for that matter. However, an optimal strength/power-to-weight is essential in many sports, and it’s crucial that athletes understand how to achieve this safely.

This is where education plays a critical role. Athletes need to be taught not only why achieving an optimal weight should be done safely but how to do it. They need guidance on practical life skills, such as understanding the importance of key macro- and micro-nutrients in an athlete’s diet, meal preparation and timing their nutrition right. Additionally, athletes must grasp the vital role of proper fueling for recovery and adaptation to their training.

It’s also important to emphasize that for most athletes, especially females, being extremely lean year-round is almost impossible unless one is genetically predisposed to it. This shouldn’t be surprising. Just as athletes periodize their training to include periods of higher and lower training loads, the body requires “rest” periods where more energy is available^xxii.

Implications and Future Research

Further research is needed to clarify the specific mechanisms by which energy deficiency impacts physical performance and to identify the tipping point at which energy restriction becomes detrimental. Longitudinal studies exploring the interaction between energy status, reproductive function, and athletic performance would offer valuable insights. Moreover, understanding how RED-S outcomes – both health- and performance-related – are linked to factors like duration, severity and accumulated dose (duration times severity) of energy deficits, within-day energy balance or different origins of LEA, is critical. Moreover, identifying moderating factors that influence individual responses to LEA exposures will help explain the varied outcomes seen in different athletes.

Furthermore, distinguishing between different types of energy deficiency, controlling for factors like macronutrient intake, and examining energy allocation across various physiological systems will also advance our understanding of this complex relationship. Although measuring energy distribution between different body systems in humans may be technically challenging, it would provide crucial insights into how energy deficiency affects performance and physical capacity^xviii.

Conclusion

The paradox of high physical performance in the face of energy deficiency challenges traditional notions of sports nutrition and athlete health. While some athletes may appear to thrive under conditions of LEA for some time, the potential long-term consequences on health and performance warrant careful consideration. If you are an athlete aiming to improve body composition or adjust energy intake for an important competition, it is crucial to do so under the guidance of an expert team. Furthermore, if any signs of RED-S are present, consulting a specialist is vital – even if your performance remains strong. As athletes and coaches, we must recognize that peak performance is not necessarily always the most healthy, but health remains the most important foundation for long-term performance! It is essential to prioritize adequate fueling and monitor signs of energy deficiency closely, to recognize early warning signs of LEA or disordered eating behaviors and ultimately to prevent long-term detrimental consequences. By adopting an educated and nuanced approach to energy management, we can better support both the immediate and long-term well-being of athletes.

A big thank you to Trent Stellingwerff for reading my article, providing feedback and sharing some of your ideas! Also many thanks to José Areta for sharing some of your publications (with restricted access)!

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