RED-S in Female Runners: Relative Energy Deficiency Is Not as Simple as Eating Less

Introduction

Have you ever strictly restricted your diet to improve your running performance? Believing that being lighter makes you faster? This “lighter is better” myth is precisely the breeding ground for RED-S (Relative Energy Deficiency in Sport). RED-S is not simply “eating too little”—it is a complex syndrome affecting the entire body’s systems. In 2014, BJSM officially replaced the old term “Female Athlete Triad” with this terminology, emphasizing that both men and women can be affected, though female runners are at particularly high risk.

What is RED-S

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

Energy Availability = Total Energy Intake − Exercise Energy Expenditure

This number is then divided by fat-free mass (kg), and the ideal value should be above 45 kcal/kg fat-free mass/day.

When EA remains below 30 kcal/kg/day for an extended period, the body activates energy-saving mechanisms, prioritizing vital organs such as the heart and brain while sacrificing the reproductive system, bone metabolism, and immune function.

Systems Affected by RED-S

• Menstrual system: Functional hypothalamic amenorrhea (FHA), irregular or absent menstruation
• Skeletal system: Decreased bone mineral density, significantly increased risk of stress fractures
• Metabolic system: Reduced resting metabolic rate, weight plateau or even gain (energy-saving effect)
• Psychological state: Low mood, heightened fixation on food
• Immune function: Recurrent upper respiratory tract infections
• Cardiovascular system: Decreased heart rate variability (HRV), limited maximal oxygen uptake

High-Risk Scenarios for Female Runners in Taiwan

How to Recognize RED-S Warning Signs

Early Signals (may not be noticed)

• Menstrual cycles becoming longer or irregular (changing from 28 days to 35–40 days)
• Speed plateauing or regressing despite training
• Taking longer to recover after training than before
• Thoughts about food occupying significant mental space

Mid-Stage Warning Signs

• Menstruation completely absent for more than 3 months
• Stress fractures (feet, tibia, lumbar spine)
• Frequent colds or colds that linger for a long time
• Irritability, decreased concentration

Seek Immediate Medical Attention

• Amenorrhea lasting more than 6 months
• History of stress fractures
• Bone mineral density T-score below -1.0 (osteopenia)

RED-S Recovery Strategies

Step One: Increase Energy Availability

There is no need to stop training—start by increasing intake:

• Add 300–500 kcal per day, especially carbohydrate supplementation after training
• Do not skip breakfast; consume foods containing both carbohydrates and protein within 30 minutes after training
• Calculate your TDEE (Total Daily Energy Expenditure) to ensure you are not underestimating

Step Two: Work with a Medical Team

• Gynecology: Assess hormonal status and menstrual recovery plans
• Orthopedics or Sports Medicine: DEXA scan for bone mineral density
• Nutritionist: Develop a personalized dietary plan
• Sports psychologist: Psychological support is essential if there are signs of disordered eating

Practical Recommendations

• Let go of the “marathon body” myth: Elite female marathon runners come in diverse body types; there is no single standard
• Set training goals based on “improving running performance” rather than “losing weight”
• Record your diet in your training log: Not to restrict, but to ensure adequate intake
• Tell your doctor you are a runner: General physicians may not be familiar with athletes’ energy needs
• If menstruation stops, do not assume it is normal: “Amenorrhea is common in runners” is a dangerous misconception

Conclusion

RED-S is an issue where prevention matters more than treatment. As you begin running and improving, increase your attention to caring for your body rather than reducing it. A healthy body with normally functioning hormones is the foundation for running long-term. Taiwan’s running community is gradually placing greater emphasis on this issue—let us work together to break the false myth that “lighter means faster.”

Related Reading

• Relative Energy Deficiency in Sport (RED-S) in Female Runners: Identifying the Risks and Prevention
• RED-S Syndrome in Runners: The Impact of Energy Deficiency on Women’s Menstruation and Bone Health
• RED-S Relative Energy Deficiency Syndrome: Energy Supply Warning Signs for Female Athletes
• Relative Energy Deficiency (RED-S): Causes, Warning Signs, Effects on Bone and Endocrine Health, and How to Seek Help