Athlete Bone Health: The Risks of RED-S and Low Energy Availability

Introduction: Lighter Isn’t Always Faster

In the world of endurance sports, “light” is often equated with “fast.” Keep the watts the same, drop 2 kg of body weight, and you climb faster. This simple physics drives countless cyclists and runners to chase lower body fat percentages and lighter body weights.

However, when energy intake chronically fails to match training expenditure, the body enters a kind of “energy-saving mode”—not as simple as your phone’s battery saver, but a serious health issue that can lead to osteoporosis, hormonal disruption, immune suppression, and increased cardiovascular risk.

This is RED-S (Relative Energy Deficiency in Sport).

From the “Female Athlete Triad” to RED-S

Historical Evolution

In the 1990s, the medical community proposed the “Female Athlete Triad”:

1. Eating disorders or energy deficiency
2. Menstrual dysfunction
3. Osteoporosis

But this framework had a major blind spot—it implied that only women were affected.

In 2014, the International Olympic Committee introduced the new concept of RED-S, expanding the scope to all sexes and recognizing that low energy availability affects not just the skeletal and reproductive systems, but the entire body.

What Is “Energy Availability”?

The formula for Energy Availability (EA):

EA = (Energy Intake - Exercise Energy Expenditure) / Fat-Free Mass (kg)

• Healthy threshold: ≥45 kcal/kg FFM/day
• Low energy availability: <30 kcal/kg FFM/day
• Clinically significant deficiency: <25 kcal/kg FFM/day

Example: A male cyclist with 55 kg of fat-free mass consuming 2,800 kcal daily and expending 1,500 kcal in training:

EA = (2800 - 1500) / 55 = 23.6 kcal/kg FFM/day → Below the threshold!

This situation is extremely common during high training volume periods, and many athletes aren’t even deliberately dieting—they’re simply “too busy to eat” or “not hungry after training.”

RED-S Effects on the Whole Body

Skeletal System

Bone is the most concerning and severely affected system:

• Low EA leads to decreased estrogen (in women) and testosterone (in men)
• These sex hormones are critical factors in maintaining bone density
• IGF-1 (Insulin-like Growth Factor-1) declines, reducing bone formation
• Cortisol rises, increasing bone resorption
• Result: Decreased bone density, greatly increased risk of stress fractures

The fracture risk for endurance athletes deserves special attention. A study of elite long-distance runners showed that female runners with a history of menstrual irregularities had a 4.5-fold higher incidence of stress fractures compared to those with normal menstruation.

Even more concerning: some bone loss may be irreversible. The young adult years (ages 15-25) are the golden period for bone mass accumulation, and RED-S during this time has the most profound impact on lifelong bone health.

Special Risks for Cyclists

Cycling is a non-weight-bearing sport, meaning riding itself provides minimal mechanical stimulus to bone (unlike running, which involves impact). Research shows:

• Bone density in pure cyclists is generally lower than in runners and the general population
• Athletes who train exclusively in cycling for years without other activities may have lumbar spine and hip bone density below age-matched averages
• Add RED-S on top of that, and the risk of osteoporosis is compounded

Hormonal System

Women:

• Functional hypothalamic amenorrhea (FHA): irregular or completely absent menstruation
• Luteal phase deficiency
• Low estrogen state

Men (affected too!):

• Decreased testosterone (may drop below the lower limit of normal)
• Reduced libido
• Declining semen quality
• Mood changes (irritability, low motivation)

A 2021 study published in the British Journal of Sports Medicine noted that the prevalence of low testosterone among male endurance athletes may be as high as 20-40%, and it often goes undetected because “fatigue” is attributed to training.

Effects on Other Systems

• Immune system: Increased frequency of upper respiratory infections
• Cardiovascular: Dyslipidemia, endothelial dysfunction
• Metabolic: Reduced resting metabolic rate, hypothyroidism
• Psychological: Depression, anxiety, difficulty concentrating
• Athletic performance: Decreased strength, delayed recovery, increased injury rate, reduced aerobic capacity

How to Identify RED-S?

Warning Signs

Women:

• Irregular or absent menstruation (after ruling out pregnancy and other causes)
• Recurrent stress fractures
• Unexplained performance plateau or decline

Men:

• Markedly reduced frequency of morning erections
• Decreased libido
• Unexplained low mood
• Recurrent injuries or illnesses

General:

• Chronic fatigue, poor recovery despite adequate sleep
• Frequent illness (more than 3-4 upper respiratory infections per year)
• Ongoing weight loss beyond expectations
• Changed attitude toward food (excessive preoccupation, restriction, anxiety)

Medical Evaluation

Recommended tests:

• DXA bone density scan: Assesses lumbar spine and hip bone density (limited coverage under Taiwan’s National Health Insurance; out-of-pocket cost approximately NT$1,500-3,000)
• Blood hormones: FSH, LH, estradiol (women); total testosterone, free testosterone (men)
• Thyroid function: TSH, free T3, free T4
• Metabolic markers: Fasting glucose, insulin, IGF-1, cortisol
• Bone turnover markers: P1NP (bone formation), CTX (bone resorption)

Prevention and Management

Core Strategy: Increase Energy Intake

This sounds simple, but it’s difficult to execute because it involves the dietary beliefs and weight anxiety that athletes have built up over years.

Practical recommendations:

1. Ensure carbohydrate intake 1-2 hours before and after training
2. Don’t train fasted for sessions exceeding 60 minutes
3. Consume 40-90g of carbohydrates per hour during long rides/runs
4. Don’t rely on “train more, eat less” to lose weight
5. Work with a sports dietitian to develop a personalized nutrition plan

Bone Protection Strategies

• Cyclists: Add 2-3 weight-bearing sessions per week (running, jump rope, resistance training)
• Ensure adequate calcium intake: 1,000-1,500mg daily (food first)
• Vitamin D: Maintain serum 25(OH)D >30 ng/mL (see the dedicated vitamin D article)
• Avoid excessive dietary restriction

When to Seek Professional Help

If any of the following occur, it is recommended to seek assistance from a sports medicine team (including physicians, dietitians, and psychologists):

• Menstrual irregularities lasting more than 3 months
• DXA Z-score <-1.0
• Recurrent stress fractures
• Inability to adjust eating behaviors on your own
• Suspected eating disorder

Resources in Taiwan

• Sports medicine clinics: Available at all major medical centers
• Sports dietitians: Search for certified dietitians with “sports nutrition” credentials
• Bone density testing: DXA can be arranged through orthopedics or endocrinology departments at hospitals
• Health Promotion Administration, Ministry of Health and Welfare: Provides osteoporosis-related health education resources

Conclusion

Pursuing better results is the nature of every athlete, but never at the cost of your health. The danger of RED-S lies in the fact that it is progressive and insidious, and in its early stages it may even come with performance improvements (due to weight loss). But in the long run, the price you pay could be lifelong bone problems.

Remember: Your body is a lifetime piece of equipment, not a consumable for a single season.

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References

• Mountjoy M, et al. IOC consensus statement on relative energy deficiency in sport (RED-S): 2018 update. Br J Sports Med. 2018;52(11):687-697.
• Nattiv A, et al. American College of Sports Medicine position stand: The Female Athlete Triad. Med Sci Sports Exerc. 2007;39(10):1867-1882.
• Keay N, et al. Relative Energy Deficiency in Sport (RED-S) in male athletes. Br J Sports Med. 2023.
• Barry DW, Kohrt WM. BMD decreases over the course of a year in competitive male cyclists. J Bone Miner Res. 2008;23(4):484-491.

Related Reading

• RED-S Relative Energy Deficiency: The Most Overlooked Health Breakdown in Endurance Athletes
• RED-S Relative Energy Deficiency: An Energy Supply Warning for Female Athletes
• RED-S Syndrome in Runners: The Harm of Energy Deficiency to Menstruation and Bone Health in Women
• Relative Energy Deficiency (RED-S): The Most Dangerous Dietary Trap for Endurance Athletes