The Real Cost of Weight Loss for Endurance Athletes: How Energy Deficiency Undermines Performance, Bone Health, Immunity, and Hormones

An Often Overlooked Equation: Losing Weight Doesn’t Always Mean Better Performance

In the cycling and running community, “power-to-weight ratio” and “pace-to-weight ratio” are frequently discussed metrics, which is why “losing weight = getting faster” has almost become an intuitive belief. On long climbs like Wuling, Fengguizui, and Beiyi, weight indeed amplifies its impact on climbing performance; in long-distance events like the Taipei Marathon or the Wan金石 Marathon, carrying an extra kilogram on your body will indeed consume more energy over an extended period. But this intuition has a fatal blind spot: it assumes that “weight loss” and “deterioration of body composition and damage to the energy system” are two unrelated things. In reality, when weight loss is achieved through chronically suppressing energy intake (eating far less than what is expended) rather than through training adaptation and dietary quality adjustments, the cost often doesn’t first appear on the scale—it first shows up in places you don’t notice: bone density, immunity, hormones, and only finally does performance itself collapse.

The general concept used in sports science to describe this phenomenon is called “Relative Energy Deficiency in Sport” (RED-S). Its core logic is simple: when a person’s energy intake is chronically lower than “training expenditure plus the energy required to maintain basic physiological functions,” the body activates a series of power-saving modes, prioritizing the sacrifice of functions “not immediately essential for survival,” such as reproductive hormone secretion, bone remodeling, immune defense, and wound repair. In the early stages, these sacrificed functions typically show no obvious symptoms—athletes often remain in a state of chronic energy deficiency without realizing it, until one day a stress fracture in a rib, a persistent cold that won’t heal, or two consecutive months of stagnant performance finally alerts them that something is wrong.

What this article wants to discuss is not “you shouldn’t lose weight,” but rather “losing weight the wrong way costs far more than you think.” This is a topic that requires especially careful discussion, because conversations about weight and diet can easily slide into extreme, coercive language, and the endurance sports culture often unintentionally reinforces the false connection of “thin = disciplined = strong.” This article will do its best to explain the mechanisms clearly, list the warning signs clearly, and also explicitly point out when to seek professional help rather than toughing it out on your own.

How Energy Deficiency Gradually Drags Down Performance

Stage One: Seemingly Effective, Actually Borrowing

In the short term, by controlling caloric intake, body weight does drop, and power-to-weight ratio or pace looks better on paper. This is why the impression that “weight loss works fast” is so deeply ingrained. But what the body is actually doing at this stage is this: temporarily diverting energy that would have been used for recovery, immunity, and hormone synthesis to maintain daily operations and basal metabolism—forming a physiological “loan.” In the early stages of borrowing, it’s hard to detect because the body’s buffering capacity is strong; you might still complete your training plan and maintain a certain pace, just recovering a bit slower and feeling a bit more fatigued. These subtle changes are easily attributed to “being busy lately” or “training volume just happens to be higher” and are overlooked.

Stage Two: Stagnation of Training Adaptation

As the energy deficit persists, the body begins to lack the raw materials needed for post-training repair and supercompensation. Muscle protein synthesis requires calories and protein as raw materials; increases in mitochondrial density, capillary growth, and strengthening of tendons and ligaments—all these physiological adaptations that make you “stronger with training”—require an energy surplus or at least energy balance to proceed smoothly. When in a chronic energy deficit, even if training volume and intensity remain the same, training effects will progressively worsen—this is one of the core reasons many people complain, “I’m training seriously, so why is my performance actually declining?” A typical phenomenon at this stage is a decline in interval training quality: paces or power zones you could previously hit consistently start to “drop a gear,” while subjective effort (RPE) doesn’t decrease—meaning the body is paying a greater cost to produce the same output.

Stage Three: Systemic Breakdown

If energy deficiency persists for months or longer, the impact spreads to multiple physiological systems—this is precisely why the RED-S concept was proposed: it emphasizes not just “losing weight” as something simple, but an entire cluster of interrelated physiological dysfunctions. Below, we explain several systems most directly affected, one by one.

Bone Health: The Hidden Weakness of Endurance Athletes

People commonly hold the misconception that “people who exercise must have better bones.” This statement roughly holds true for sports involving weight-bearing impact (such as resistance training or jumping sports), but in cycling—a low-impact, almost non-weight-bearing sport—the situation is completely different. Bone density issues in cyclists have long received attention, for a straightforward reason: spending long hours seated on the saddle means the skeleton barely experiences the mechanical stimulus of bearing body weight, yet maintaining bone strength precisely requires regular mechanical loading stimulation. If a rider is also in an energy-deficient state, it’s like having “insufficient mechanical stimulus” combined with “a hormonal environment unfavorable to bone synthesis”—two adverse factors stacking on top of each other, and over time, the risk of bone density loss is higher than in the general population.

Running, being a weight-bearing impact sport, theoretically provides greater skeletal stimulation, but if an athlete is simultaneously in chronic energy deficiency, the raw materials and hormonal support needed for bone repair remain insufficient, and the impact can instead lead to “stress fractures”—where the bone endures the next impact load before it has had a chance to complete repair and remodeling, with micro-damage accumulating until it eventually becomes a fracture. Stress fractures commonly occur in weight-bearing sites such as the tibia, metatarsals, and femoral neck. Early symptoms are often just “a dull pain at a specific spot while running that eases with rest,” which can easily be mistaken for simple muscle soreness and ignored—continuing to train only worsens the injury.

Bone-related medical warning signs (if any of the following appears, seek medical evaluation promptly; do not judge severity on your own):

• Localized pain in specific areas (anterior tibia, top of foot, hip) that hurts when pressed, worsening as training volume increases
• Pain progresses from “hurting only after exercise” to “hurting during exercise,” or even “a dull ache at rest”
• Female athletes experiencing irregular menstruation or amenorrhea for three or more consecutive months (this is an important warning signal from the body—absolutely not a “normal training side effect”)
• Recurring bone pain that briefly resolves with rest but returns upon resuming training

The Immune System: Why You Keep Getting Colds

There’s a saying in the endurance athlete community: “The two weeks before a big race are when you’re most afraid of catching a cold.” There’s actually physiological basis behind this. High-intensity, high-volume endurance training itself causes a temporary suppressive effect on the immune system—this is a general consensus in sports science; if you then add chronic energy deficiency on top, the energy and protein raw materials needed by immune cells are also squeezed out—adding insult to injury. Common clinical presentations include: recurrent upper respiratory infections, slower wound healing, minor abrasions easily becoming inflamed or infected, and unusual fatigue in response to training loads you could previously handle with ease.

This is also why many experienced coaches remind athletes that an abnormal increase in the frequency of colds should not simply be dismissed as “bad luck” or “weather changes,” but should prompt a review of whether overall energy intake and recovery are sufficient. Warning signs from the immune system typically appear earlier and more frequently than bone-related warning signs, making it a relatively easy-to-observe yet often overlooked early indicator.

The Hormonal System: Both Sexes Are Affected, but It Manifests Differently

The hormonal system is highly sensitive to energy deficiency, because the synthesis of reproductive and metabolic hormones is, at its core, a mechanism by which the body determines “whether the current environment is suitable for investing resources in non-immediate survival needs.” When the body detects a prolonged energy deficit, it automatically downregulates the secretion of these hormones. This is a protective physiological response, not simply something “breaking down.”

For female athletes, the most obvious signal is menstrual cycle irregularities, including lengthened cycles, reduced flow, and even complete cessation of menstruation (medically termed secondary amenorrhea). This phenomenon was once mistakenly regarded in the endurance sports community as a positive indicator of “low enough body fat and lean enough physique”—a myth that needs to be clearly corrected: amenorrhea is not a medal for “successful training,” but rather an alarm from the body signaling “severe energy deficiency.” Prolonged amenorrhea also accelerates bone loss, creating the vicious cycle of skeletal risk mentioned earlier.

For male athletes, energy deficiency similarly suppresses hormone levels such as testosterone, but the symptoms are less obvious and harder to detect than menstrual disruption. Common manifestations include reduced libido, decreased recovery capacity, low mood, poorer sleep quality, and accelerated muscle loss. Precisely because the hormonal warning signs in men are more “invisible,” they are more easily overlooked over the long term. This deserves more attention from the endurance sports community—do not mistakenly assume that RED-S is only an issue for female athletes.

Hormone-related medical warning signs:

• Females: three consecutive months of menstrual irregularity or amenorrhea (regardless of whether weight loss is ongoing, medical evaluation is recommended if this occurs)
• Males: markedly reduced libido, chronic fatigue combined with declining training performance, persistently low mood
• Any sex: long-term deterioration in sleep quality, abnormally elevated resting heart rate, impaired thermoregulation (easily feeling cold or hot)

The Metabolic System’s Chain Reaction: Why Basal Metabolic Rate Declines

Beyond the skeletal, immune, and hormonal systems that are frequently discussed, energy deficiency also directly impacts basal metabolic rate itself—a phenomenon sometimes referred to as “metabolic adaptation” or “metabolic downregulation.” When the body detects over the long term that energy intake is insufficient to cover expenditure, it activates a series of energy-saving mechanisms: reducing non-essential thermogenesis, decreasing the conversion of thyroid-related hormones to their active forms, and lowering unconscious daily activity levels (for example, NEAT—non-exercise activity thermogenesis—such as fidgeting and frequent movement—is unconsciously reduced). The sum of these changes causes basal metabolic rate to be lower than the figure theoretically estimated from body weight and body composition.

This creates a confusing situation for many athletes: despite eating very little and training heavily, body weight plateaus and refuses to drop further, or even creeps back up slightly. This “metabolic adaptation” is not caused by insufficient willpower or sneaky eating; it is a reasonable physiological response to prolonged energy deficiency. If the response at this point is to “restrict food even more strictly,” metabolic downregulation often worsens, creating a vicious cycle, while the skeletal, immune, and hormonal costs mentioned earlier continue to accumulate. Breaking this cycle typically requires professional assistance—through a phased, structured increase in energy intake to allow metabolic rate to gradually recover to a reasonable level. This process may take weeks to months and cannot be rushed.

The Digestive System and Iron: Two Often-Overlooked Areas

Prolonged energy deficiency combined with high training loads also frequently places strain on the digestive system. During exercise, gastrointestinal blood flow is largely shunted to the working muscles; if intestinal energy supply is simultaneously inadequate, symptoms such as indigestion, bloating, and altered bowel habits can arise. On race day, the gastrointestinal discomfort commonly known as “runner’s gut” is more likely to occur—particularly common among road runners and triathletes.

Iron is another nutrient that is easily overlooked yet has a profound impact on endurance performance. Endurance athletes inherently have higher iron requirements than the general population due to “exercise-induced hemolysis” from repeated foot strikes, iron losses through sweat and urine, and reduced intestinal absorption rates. If overall energy intake is also insufficient, iron intake often declines in tandem, and over time this can develop into iron-deficiency anemia. Early-stage iron deficiency often presents with very subtle symptoms—perhaps just “feeling more breathless than usual” or “recovering more slowly”—which can easily be mistaken for ordinary training fatigue. In reality, only a blood test checking hemoglobin and ferritin can confirm the diagnosis. This is why many sports medicine clinics recommend that endurance athletes—especially female athletes (due to menstrual losses)—schedule regular blood work, incorporating iron status into the overall assessment of energy and nutrition.

Digestive and blood-related medical warning signs:

• Chronic bloating or altered bowel habits that coincide with the timing of dietary changes
• Unusual fatigue, pale complexion, palpitations or breathlessness with minimal exertion
• Recurrent severe gastrointestinal discomfort before or during races that affects the ability to finish
• Blood tests revealing low hemoglobin or ferritin levels

The Psychological Dimension: Energy Deficiency Is Not Just a Physiological Issue

The impact of energy deficiency on psychological state is equally significant and often interacts bidirectionally with the physiological warning signs described above. Chronic insufficient caloric intake affects the function of neurotransmitters involved in emotional regulation. Common manifestations include greater mood swings, irritability, reduced concentration, diminished motivation and passion for training and racing, and even a persistently low mood resembling mild depression. These psychological changes sometimes appear earlier than the physiological warning signs, yet they are often dismissed by the athletes themselves or those around them due to the stereotype that “athletes should be tough.”

It is particularly worth emphasizing that psychological state and eating behaviors often reinforce each other: when mood is low, there is a greater tendency to gain a sense of control through strict dietary restriction, and prolonged energy deficiency further worsens mood, creating a cycle that is difficult to break through willpower alone. If you notice that your own or someone else’s mood has become noticeably unstable at the same time as training pressure increases, this combination itself deserves serious attention. Seeking professional help from a sport psychologist or general counselor is appropriate, rather than simply attributing it to “stress” and brushing it aside.

Performance Eventually Collapses Anyway

Ironically, many athletes endure all of the above costs in pursuit of performance gains, yet prolonged energy deficiency ultimately leads to an across-the-board decline in performance rather than sustained improvement. Common performance indicators that regress include: more pronounced heart rate drift at the same pace, declining metrics related to maximal oxygen uptake capacity, loss of strength and power, more severe “fading” in the latter stages of races, and longer recovery times that reduce training quality—forming a vicious cycle.

A core concept must be emphasized here: lighter does not equal faster. Power-to-weight ratio or pace-to-weight ratio is only one variable among many that influence performance. When weight loss is achieved by sacrificing muscle mass, glycogen storage capacity, and hormonal balance, even if the number on the scale is “lighter,” actual power output, muscular endurance, and recovery speed often decline in tandem—the net effect is usually worse performance, not better. Truly performance-enhancing weight management should be built on the premise of “maintaining training adaptability and recovery capacity,” rather than simply chasing a number on the scale.

A Common Warning Signs Checklist: How to Self-Detect

The following is compiled into a comprehensive self-assessment checklist, suitable for taking a few minutes to honestly review on a regular basis (e.g., monthly). If three or more items apply simultaneously, seeking professional help is strongly recommended rather than continuing to wait and observe:

This Is Not Just About “Eating More”: A Warning on Eating Disorder Risks

What requires especially careful handling in this article is the relationship between endurance sports and energy deficiency, which is sometimes not simply a matter of “training miscalculation,” but may involve more complex eating disorder tendencies—such as gaining a sense of control through dietary restriction, developing excessive anxiety and fixation about body weight and physique, or mistakenly equating “eating less” with “self-discipline.” If these conditions are not addressed early, they may develop into clinical eating disorders, causing harm to both physical and mental health that far exceeds a mere decline in performance.

Eating disorder warning signs that warrant heightened vigilance include:

• Rigid, extreme rules about food types and portions, with strong guilt or anxiety when rules are violated
• Deliberately concealing actual eating habits, or showing clearly inconsistent eating behaviors in front of others versus when alone
• Using exercise as a means of “atonement” or “burning off excess calories,” insisting on training even when injured or ill
• Obsession with body weight and body fat numbers to the point that it affects daily mood and interpersonal relationships
• Engaging in compensatory behaviors such as purging after binge eating, or overusing laxatives or diuretics
• Friends and family repeatedly expressing concern, while the individual tends to deny or defensively avoid the discussion

If you observe any of the above in yourself, a teammate, a student, or a child, this is not a matter of willpower or discipline—it is a health issue that requires professional assistance. Available avenues for seeking help include: discussing the situation with a qualified sports medicine physician or family doctor, consulting a sports dietitian with clinical background, seeking psychological counseling or psychiatric evaluation, and, in school or team settings, reporting concerns to an athletic trainer or team physician with relevant sensitivity. Most major medical centers in Taiwan have sports medicine clinics, and some counties and cities also offer subsidies for psychological counseling resources. Reaching out for help is itself a courageous act worthy of recognition—there is no need to wait until “things get really bad” to seek support.

A Reminder for Coaches and Teammates: Words Carry More Power Than You Think

In the training culture of endurance sports, “you’ve lost weight lately” is often treated as a compliment—but for certain athletes who are struggling, that phrase can be a force that reinforces harmful behaviors. When coaches, teammates, and parents interact, it is recommended to focus on specific indicators of performance and health (for example, “your interval quality has improved a lot this month”) rather than comments about physique or body weight. If you notice a teammate exhibiting the warning signs mentioned above, the appropriate approach is to express concern privately and without judgment, and to encourage the person to seek professional help—rather than discussing it publicly or offering dietary advice, because non-professional dietary advice can sometimes make the problem worse.

What Kind of Weight Management Is Relatively Safe

Although this article focuses on the risks of energy deficiency, that does not mean athletes can never consider adjustments to body weight or body composition. Relatively safe approaches typically share several common characteristics: adjustments are made slowly and progressively, rather than drastically cutting calories over a short period; the timing of adjustments avoids important training cycles or competition preparation periods; multiple indicators are continuously monitored—training performance, sleep, mood, menstruation (where applicable)—rather than fixating only on the scale; there is involvement of professionals (sports dietitians, physicians) rather than grabbing an aggressive diet plan off the internet on one’s own; and “maintaining training adaptability” is prioritized over “hitting the numbers.”

The common spirit behind these principles is: treat the body as a system that needs to be supported, not as a machine where you only watch the output without caring whether the fuel tank has enough.

Conclusion and Action Checklist

There is nothing wrong with endurance athletes pursuing better performance. What is wrong is mistaking “being lighter” as the only or most important means to that end. The bone loss, immune suppression, and hormonal disruption caused by energy deficiency are often irreversible or take a very long time to recover from—far too high a price to pay for short-term numbers on the scale.

Action Checklist:

1. Set aside time each month to review the warning signs listed in this article and honestly confront your own situation.
2. Female athletes: menstrual irregularities are not “normal”—if they occur, document them and consider seeking medical attention.
3. Male athletes: pay attention to less visible hormonal warning signs such as libido, mood, and recovery capacity.
4. For recurrent localized bone pain, especially in the shin, top of the foot, or hip, seek early medical evaluation—do not tough it out on your own.
5. If you or someone around you exhibits eating disorder warning signs, prioritize seeking help from sports medicine, sports nutrition, or psychological professionals.
6. Coaches and teammates should adjust their language habits, focusing praise on performance and health rather than physique and body weight.
7. Any weight adjustment plan should ideally be scheduled outside the competitive season and periods of high training volume, and carried out slowly and progressively.

Finally, it must be emphasized again: this article provides general knowledge and a compilation of warning signs. Individual differences are significant, and this content cannot replace professional medical and nutritional evaluation. If you are experiencing any of the warning signs described in this article, treat it as a signal to seek professional help—not as a secret you have to carry alone.

Related Reading

• Relative Energy Deficiency in Sport (RED-S): Causes, Warning Signs, Effects on Bone and Endocrine Health, and How to Seek Help
• Relative Energy Deficiency in Sport (RED-S): The Most Dangerous Dietary Pitfall for Endurance Athletes
• Bone Health in Athletes: The Risks of RED-S and Low Energy Availability
• RED-S in Female Runners: Relative Energy Deficiency Is Not as Simple as Eating Less