Runner’s RED-S Syndrome: The Dangers of Energy Deficiency on Women’s Menstruation and Bone Health

Introduction

For many female runners, the long-term pursuit of a “lighter running physique” can be a path toward injury. RED-S (Relative Energy Deficiency in Sport) is a concept formally introduced by the International Olympic Committee (IOC) in 2014, replacing the earlier “Female Athlete Triad” that focused only on women. It more comprehensively describes the impact of energy deficiency on an athlete’s entire body systems. This article focuses on the two most common and severe aspects affecting female runners: menstrual dysfunction and bone damage.

What is RED-S?

The core mechanism of RED-S is low Energy Availability (EA):

EA = Total daily energy intake - Exercise energy expenditure

Calculated per kilogram of body weight, when EA falls below 30 kcal/kg of fat-free mass (FFM), the body begins a cascade of physiological protective mechanisms—curtailing “non-essential” physiological functions to prioritize brain and heart function.

Normal EA should be maintained above 45 kcal/kg FFM, with a suboptimal range of 30–45 kcal/kg FFM.

Systems Affected by RED-S

Menstrual Dysfunction: The Warning Sign Runners Most Often Overlook

Functional Hypothalamic Amenorrhea (FHA)

When EA is insufficient, the hypothalamus stops the normal pulsatile secretion of GnRH (gonadotropin-releasing hormone), leading to:

• Persistently low estrogen levels
• Disrupted progesterone secretion
• Lengthened, irregular menstrual cycles, eventually ceasing (no menstruation for 3 consecutive months or more = secondary amenorrhea)

Common Misconceptions

• “Irregular periods with heavy training are normal”: Incorrect. Healthy female athletes can maintain regular menstruation with adequate energy supply
• “Stopped periods mean low body fat and a great physique”: Incorrect. Amenorrhea is an alarm for an energy crisis, not an achievement
• “Missing a few months isn’t serious; I can make up for it later”: Incorrect. For each month of missed menstruation, the loss in bone density is difficult to fully recover

Bone Damage: The Silent Killer of a Runner’s Career

Why Does Low Estrogen Harm Bones?

Estrogen is one of the key hormones for maintaining bone density; it inhibits osteoclast activity and promotes calcium absorption. Under prolonged low estrogen states:

• Bone resorption rate exceeds bone formation rate
• Bone mineral density (BMD) can decline 1–3% per year (equivalent to the rate of bone loss after entering menopause prematurely)
• Lumbar spine and hip bone density are typically affected first

Stress Fractures: The Most Direct Cost for Runners

• Female runners with irregular menstruation have a 2–4 times higher risk of stress fractures compared to those with normal menstruation
• Common sites: tibia, metatarsals, femoral neck (the latter is the most dangerous and may require surgery)
• When bone density T-score falls below -1.0 (osteopenia), one has already entered the high-risk zone

Identifying RED-S: Do You or Your Running Buddies Have These Conditions?

Direct danger signals (seek medical attention immediately):

• Menstruation has stopped for 3 or more consecutive months
• A stress fracture occurred within the past 6 months
• BMI persistently below 17.5

Warning signs to note:

• Weekly training volume increases without a corresponding increase in caloric intake
• Long-term avoidance of high-calorie foods (fats, starches)
• Excessive anxiety about and need to control body weight or physique
• Very fatigued after training, yet deliberately not eating more

Treatment and Recovery: How to Overcome RED-S?

1. Increase energy intake: Target EA ≥ 45 kcal/kg FFM, typically requiring an additional 300–600 kcal per day
2. Temporarily reduce training volume: Give the body room to recover; sometimes a 20–30% reduction in training volume is needed
3. Supplement calcium and vitamin D: Recommended daily intake of calcium 1000–1500 mg and vitamin D 1000–2000 IU
4. Psychological support: Many RED-S cases involve underlying disordered eating tendencies and require assistance from a psychologist
5. Medical evaluation: Sports medicine or gynecology assessment of hormonal status to determine whether medical intervention is needed

Practical Recommendations

• Coaches and training partners should all understand RED-S and work together to build a healthy training culture
• Female runners should record their menstrual cycle each season as a monitoring indicator of energy sufficiency
• When training volume increases significantly, proactively increase food intake rather than worrying about “eating too much and gaining weight”
• Bone density testing (DXA) is recommended every 1–2 years, especially for those with a history of menstrual interruption

Conclusion

RED-S is not the price of “pursuing the perfect running physique”; it is a preventable medical condition. Menstruation is the most direct indicator of energy sufficiency in a woman’s body—once it is interrupted, it is the body saying, “I need more energy.” Running can make you stronger and healthier, but only if you support it with proper energy supply. Protecting your bone and hormonal health is what allows running to accompany you for the long haul.

Related Reading

• RED-S in Female Runners: Relative Energy Deficiency Is More Than Just Eating Less
• Energy Deficiency in Female Runners (RED-S): Identifying the Risks and Prevention
• RED-S Relative Energy Deficiency: Energy Supply Warning Signs for Female Athletes
• Athletes’ Bone Health: The Risks of RED-S and Low Energy Availability