MALE ATHLETE HEALTH • FUELLING • PERFORMANCE

Male Athletes, Low Energy Availability & RED-S: How Under-Fuelling Can Affect Bone, Hormones, Muscle & Performance

Understanding the hidden cost of chronic energy mismatch in runners, cyclists, gym enthusiasts and competitive athletes

A male athlete can run most days, lift weights, eat plenty of protein, maintain visible muscle and look exceptionally fit while still being under-fuelled. Appearance cannot answer the question that matters most: after training is accounted for, is enough energy left to support hormones, bone remodelling, muscle repair, immunity, metabolism and everyday life?

 

1. When Lean, Muscular and Fit Can Hide Under-Fuelling

Male athletes often lack an obvious early warning sign. A woman may notice menstrual disruption; a man may continue training, competing and maintaining a relatively stable body weight while subtler changes emerge in recovery, libido, mood, injury risk or performance. This is one reason male RED-S can be missed.

Low energy availability can be deliberate—during aggressive cutting, weight-category preparation or a pursuit of extreme leanness. It can also be accidental. Training volume rises, work becomes more active, appetite is blunted after hard sessions or a fasting window removes eating opportunities, yet meals remain unchanged.

An intake that sounds large in isolation may still be too small for the person doing the work. A high-volume cyclist, runner or multi-sport athlete can eat more than a sedentary friend and remain under-fuelled. Requirements have meaning only relative to body size, training load, sport, growth where relevant and activity outside training.

 

2. What Low Energy Availability and RED-S Mean

Energy availability describes the dietary energy remaining for normal physiology after the energetic cost of exercise is considered. When too little remains repeatedly, the body may conserve resources or alter functions that are not essential for immediate survival. This is more than a weight-loss equation: the body is allocating a limited budget across training, repair, reproduction, bone, immunity and metabolism.

The Female Athlete Triad first connected low energy availability, menstrual dysfunction and impaired bone health. RED-S broadened that model to include multiple physiological and performance consequences and to recognise that male athletes can also be affected. It did not make women less important; it revealed the wider reach of the same energy problem.

Energy availability is difficult to measure precisely outside research. Food records, exercise estimates and body-composition measurements all contain error. RED-S therefore requires clinical reasoning rather than a single app calculation, body-fat threshold or laboratory result.

3. The Performance Paradox: When Lighter Stops Being Faster

Body mass can influence running economy, climbing speed and power-to-weight ratio, which is why early weight loss may appear to help. But power-to-weight has two sides. If weight falls while muscle, glycogen, training quality, hormonal function or recovery also deteriorate, the athlete may become lighter without becoming better.

The mirror can reward the problem before physiology exposes it. Greater definition may be praised while sleep worsens, soreness lingers, strength declines or injuries recur. Short, planned energy restriction is not automatically RED-S, but severe, prolonged or frequently repeated restriction can create a very different biological environment.

Performance has a fuelling floor. Below it, the athlete may be unable to produce enough high-quality work or adapt to it. The useful question is not “How lean can I get?” but “What body composition can I sustain while remaining strong, well fuelled, healthy and able to recover?”

For the broader relationship between strength and body composition, read Men, Muscle & Metabolism: How to Build Strength, Improve Body Composition & Support Healthy Ageing.

4. Testosterone, Reproductive Function and the Limits of Self-Diagnosis

Male reproductive physiology is coordinated through the hypothalamic–pituitary–gonadal axis. Signals from the brain influence pituitary hormones, which in turn communicate with the testes. Prolonged energy scarcity can interact with this network and may be associated with changes in testosterone and reproductive function.

Testosterone matters to more than libido. It is relevant to reproductive health, muscle, bone and red-blood-cell physiology. Yet fatigue does not automatically mean low testosterone, and one low result does not automatically mean RED-S. Sleep, illness, medicines, alcohol, stress, time of sampling and other endocrine conditions can affect symptoms or results.

A persistent change in libido or reproductive function can be a useful clue when it appears alongside heavy training, weight loss, restriction, poorer recovery or recurrent injury. Morning erections are influenced by neurological, vascular, sleep-related and hormonal factors; they are not a home testosterone test.

 

5. Bone Health: Load Must Be Matched by Capacity

Bone is living tissue. Mechanical loading creates microscopic strain, specialised bone cells sense that strain, and remodelling replaces and adapts tissue. This process requires time, energy, protein, minerals and an appropriate hormonal environment. Bone remodelling is not free.

Bone stress injury develops when repeated load accumulates faster than the skeleton can recover. Training progression, surfaces, biomechanics, footwear, previous injury, sleep, nutrition, hormones and bone density may all contribute. Low energy availability is an important risk factor, not the sole explanation for every stress fracture.

Running supplies weight-bearing impact but can still exceed bone capacity when volume or intensity rises quickly. Cycling and swimming can produce very high energy expenditure while providing less impact loading, so sport-specific fitness does not guarantee strong bones. Resistance training and carefully programmed impact work may provide complementary skeletal signals where appropriate—but adding more exercise to an under-recovered athlete is not automatically helpful.

 

Explore the living skeleton in Bone Biology Explained: How Your Bones Continuously Renew Themselves, the repair cycle in Bone Remodelling Explained: How Your Skeleton Constantly Renews Itself, and the training signal in Why Exercise Builds Stronger Bones: The Science of Bone Loading.

6. Muscle, Protein and the Energy Hierarchy

Muscle is not decorative weight. It produces force, protects joints, supports movement, loads bone and acts as functional reserve. Aggressive energy restriction can make lean-tissue preservation harder, especially when training demand is high and recovery is poor.

Protein matters, but high protein does not prove adequate fuelling. Protein supplies amino acids; it cannot replace carbohydrate, dietary fat or total energy. An athlete can consume shakes, lean meat and supplements while the overall diet remains too small. The body cannot run every physiological system on protein alone.

Carbohydrate can be especially important for endurance exercise, higher-intensity work and glycogen restoration. Low-carbohydrate eating and low energy availability are not the same concept, but restricting a major energy source can make adequacy harder for some high-volume athletes. Dietary fat also contributes energy and essential fatty acids and should not disappear in the pursuit of leanness.

The practical food foundation is covered in High-Protein Foods: The Foundation of Muscle, Healthy Ageing & Recovery Nutrition, while protein timing is placed in context in Protein Timing for Health, Performance & Recovery.

7. Warning Signs Are Patterns, Not Proof

RED-S can present differently between athletes. The clues below are reasons to look more closely, not a self-diagnosis.

These signs have many possible causes. Iron deficiency, thyroid conditions, infection, sleep disorders, depression, medication effects and overtraining can overlap with the picture. Proper assessment matters precisely because the symptoms are not specific.

8. Recovering Function: Eat More, Train Smarter, Reassess the Goal

Recovery is not simply “getting fat” or abandoning performance. It means restoring the resources and training environment required for normal function. Depending on severity, that may involve increasing energy intake, changing meal timing, reducing training load, pausing deliberate weight loss and addressing disordered eating or body-image pressure.

Some changes can occur quickly: hunger, mood or training energy may improve before hormones, bone density or injury risk fully recover. Bone is slow tissue. A better session next week does not prove the entire system has recovered.

Scale weight may rise when carbohydrate intake increases because muscle glycogen stores bind water. That is not equivalent to gaining the same amount of body fat. The more meaningful recovery markers are function: training quality, strength, sleep, mood, injury status, reproductive health and the ability to sustain the program.

9. A Male Athlete Fuelling Framework

Before training: fuel the work

The amount depends on the session and individual tolerance. Longer, harder or higher-intensity training often benefits from accessible carbohydrate and fluid. A banana and yoghurt, toast with nut butter, cereal with milk or a small rice-based meal can all work in the right context.

During training: consider duration and conditions

Longer sessions may require carbohydrate, fluid and electrolytes according to duration, intensity, climate and sweat losses. Fasted training can increase fat use during a session, but that does not automatically produce better long-term fat loss, performance or health.

After training: refuel, repair and rehydrate

Combine carbohydrate, complete protein, fluid and enough total energy. Protein alone addresses only one part of recovery. An athlete who drinks water, electrolytes and a protein shake but eats very little carbohydrate or total food may still leave the largest deficit untouched.

Across the day: create enough opportunities to eat

·    Use regular meals and snacks when two large meals cannot meet high training requirements.

·    Increase food on demanding days rather than asking one fixed meal plan to cover every workload.

·    Fuel rest days too; muscle repair, glycogen restoration and bone remodelling continue without a formal session.

·    Use energy-dense foods such as nuts, seeds, oils, avocado, dairy and grains when appetite or time is limited.

·    Treat hunger as information, not a failure of discipline.

For the wider performance system, read Athletic Performance Nutrition: Recovery, Collagen, Gut Health & Sports Performance, and place sweat losses in context with Functional Hydration.

10. Building a Complete Recovery Meal

A useful recovery plate covers several jobs rather than chasing one “perfect” nutrient.

One example is a post-ride beef and vegetable noodle bowl: prepare a savoury broth, add noodles, sliced beef or tofu, mushrooms, greens and carrots, finish with sesame oil and season to taste. The noodles supply carbohydrate, beef or tofu supplies complete protein, vegetables add variety, and the broth contributes fluid, flavour and protein. The complete meal—not one ingredient—is the recovery strategy.

For complete meal ideas, explore 12 Easy Recipes for Active Bodies: Protein, Collagen & Everyday Joint-Supporting Nutrition.

11. Where Bone Broth, Collagen and Supplements Fit

Bone broth can be a practical ingredient in soups, stews, noodle bowls, sauces and rice dishes. Broth & Co bone broth contributes naturally occurring protein and collagen-derived amino acids, but it is not a RED-S treatment, testosterone therapy or stress-fracture treatment. Its role is to help make nourishing meals convenient and appealing.

Collagen peptides belong to targeted connective-tissue nutrition; whey, dairy, eggs, meat, fish, soy and other complete proteins serve different purposes. Creatine also has evidence for specific strength, power and muscle applications. None can correct chronic energy inadequacy.

 

Compare these roles in Functional Proteins Explained: Why Whey, Collagen & Bone Broth All Have Different Roles, and see the food context in Bone Broth Benefits: The Complete Guide to Gut Health, Protein, Recovery & Healthy Ageing.

12. When to Seek Professional Help

Persistent focal bone pain, pain with walking or at rest, recurrent bone stress injury, substantial unintentional weight loss, marked performance decline, reproductive changes, difficulty eating enough or signs of disordered eating deserve assessment. Return-to-sport and recovery plans should be individual rather than copied from social media.

A sports physician or GP can assess symptoms and exclude other causes. An accredited sports dietitian can examine actual intake, training demands and eating patterns. Physiotherapists, coaches and psychologists may also be important. RED-S sits at the intersection of biology, behaviour, training and environment; the best support often reflects that complexity.

Frequently Asked Questions

Can men get RED-S?

Yes. Problematic low energy availability can affect male athletes and may influence endocrine and reproductive physiology, bone, recovery, wellbeing and performance.

Do you have to be underweight?

No. A male athlete can have a normal BMI, visible muscle, low body fat or stable weight and still be under-fuelled.

Does RED-S always lower testosterone?

No single hormonal pattern defines every case. Low energy availability can affect the reproductive axis, but testosterone results require clinical interpretation and other causes must be considered.

Is there a blood test for RED-S?

No. Assessment may include history, diet, training, symptoms, physical examination and selected laboratory or imaging tests, but no single result confirms or excludes it.

Can under-fuelling cause stress fractures?

Bone stress injuries are multifactorial. Low energy availability can reduce bone recovery capacity and is one important risk factor rather than the explanation for every injury.

Does eating lots of protein prevent RED-S?

No. A diet can be high in protein while too low in total energy, carbohydrate or fat. Protein cannot replace an adequate overall diet.

Can fasting contribute to RED-S?

It can if a restricted eating window removes opportunities needed to meet high energy demands. Fasting does not automatically cause RED-S; context determines whether it supports or undermines adequacy.

Should athletes eat on rest days?

Yes. Requirements may change, but repair, glycogen restoration, endocrine physiology and bone remodelling continue.

Will eating more automatically cause fat gain?

Not necessarily. The outcome depends on current intake and requirements. Restoring carbohydrate can also increase glycogen and associated water, so scale change is not equivalent to the same change in body fat.

Do creatine, collagen or bone broth treat RED-S?

No. Each may have a place in nutrition, but none treats chronic under-fuelling. Restoring adequate energy and addressing training and clinical factors are the priorities.

Continue Exploring

·    Why Protein Matters for Healthy Bones

·    Peak Bone Mass Explained: Why Your 20s Matter More Than You Think

·    Female Athletes, Low Energy Availability & Bone Health: Why Eating Enough Is Part of Injury Prevention

Final Thoughts

The central lesson of male RED-S is that appearance can conceal physiology. A man can look lean, muscular and disciplined while his body is quietly negotiating an energy shortage. Bone, hormones, muscle, immunity and performance do not respond to the image in the mirror; they respond to the resources available for the work being demanded.

The strongest athlete is not the one who can survive on the least food. It is the one who can train, recover, adapt and keep going. Eating enough is not separate from discipline. It is part of performance, injury prevention and the long-term physical capacity that sport should help build.

References & Further Reading

·    2023 International Olympic Committee consensus statement on Relative Energy Deficiency in Sport

·    Low energy availability and RED-S: systematic review and meta-analysis

·    Relative Energy Deficiency in Sport in male athletes: a commentary on its presentation and measurement

·    Male athletes and the Female Athlete Triad framework: low energy availability, reproductive function and bone health

This article provides general education and is not a diagnostic or treatment guide. RED-S, hormonal changes, bone stress injuries and disordered eating require individual assessment. Seek prompt medical care for persistent focal bone pain, inability to bear weight, recurrent injury or other concerning symptoms.