Relative Energy Deficiency in Sport (RED-S) in Female Runners: Identifying the Risks and Prevention

Introduction

A common misconception circulates in the running community: “The more you run and the less you eat, the leaner and faster you’ll become.” However, this logic is a dangerous physiological trap, especially for female runners. When the body is in a prolonged state where energy intake is insufficient to support exercise expenditure, it triggers “Relative Energy Deficiency in Sport” (RED-S), causing a cascade of health issues affecting multiple body systems.

The precursor concept to RED-S, the “Female Athlete Triad,” specifically referred to women and encompassed three interrelated problems: low energy availability, menstrual dysfunction, and decreased bone mineral density. In 2014, the International Olympic Committee expanded this concept to all genders, but women remain the highest-risk group, particularly those in weight-conscious endurance sports (marathon, triathlon).

The Core Concept of RED-S: Energy Availability

Energy Availability (EA) is the key metric for assessing RED-S:

EA = Daily energy intake (kcal) - Exercise energy expenditure (kcal) / Fat-free mass (kg)

To put this into a practical example: A female runner weighing 55 kg with a fat-free mass of 45 kg who burns 500 kcal per day running needs to consume at least 500 + 45 × 45 = 2525 kcal daily to maintain optimal energy availability. If she eats only 1500 kcal to control her weight, her EA drops to approximately 22 kcal/kg FFM, placing her in the danger zone.

Body Systems Affected by RED-S

When energy is insufficient, the body prioritizes vital organs like the brain and heart for self-preservation, sacrificing “non-essential” systems such as reproduction, bone metabolism, and immune function:

• Reproductive system: The hypothalamus stops secreting GnRH, causing oligomenorrhea (fewer than 6 periods per year) or complete cessation of menstruation (functional hypothalamic amenorrhea, FHA)
• Skeletal system: Decreased estrogen increases osteoclast activity and reduces osteoblast activity, leading to bone mineral density loss and a significantly higher risk of stress fractures
• Cardiovascular system: Decreased resting heart rate, altered cardiac structure, and dyslipidemia (decreased HDL)
• Metabolic system: Reduced basal metabolic rate and lower thyroid hormone (T3) levels, impairing running economy
• Immune system: Reduced immunoglobulin secretion, leading to recurrent colds or infections
• Psychological system: Low mood, poor concentration, and loss of motivation for training

Common High-Risk Scenarios for Female Runners in Taiwan

The following common scenarios can easily lead to RED-S, and Taiwanese runners should pay special attention:

1. Intentional weight loss during race preparation: Rapidly losing weight in the 2–3 months before a marathon may temporarily improve pace, but the body has already entered an energy deficit
2. High training volume without adequate calorie awareness: Running more than 80 km per week while still aiming for a low-calorie diet based on “low oil, low sugar” principles
3. Vegetarian/vegan diets: Without careful attention to protein and micronutrient supplementation, energy density can easily become insufficient
4. Over-reliance on heart rate monitors or smartwatches to calculate expenditure: These devices have error rates of up to 20–40%, making them insufficiently accurate as a basis for dietary planning
5. Social media pressure: The lean “runner body” image on Instagram may lead women to deliberately suppress their weight

Early Warning Signs of RED-S

If the following symptoms appear, energy intake should be reassessed immediately:

• Menstrual cycles become irregular (more than 35 days apart) or disappear for more than 3 months
• Persistent fatigue that does not resolve even with adequate sleep
• Running pace plateaus or declines without any identifiable cause
• Recurrent stress fractures (tibia, metatarsals, hip)
• Low mood, irritability, and loss of enthusiasm for running
• Frequent illness or slow recovery from injuries

Practical Recommendations

1. Menstruation is a health indicator, not a luxury to be sacrificed: If your period has disappeared for more than 3 months, seek medical attention immediately to rule out RED-S and undergo hormonal evaluation
2. Do not “diet” during training periods: Race preparation requires more calories, not fewer; moderate weight management can be done during the post-race recovery period
3. Increase dietary energy density: Supplement calories with healthy, energy-dense foods such as nuts, avocados, and olive oil, rather than relying solely on large volumes of vegetables
4. Seek help from a sports dietitian: If you are unsure about your calorie needs, consult a dietitian familiar with endurance sports to help calculate them
5. Schedule regular bone density scans (DEXA): Especially for runners with a history of amenorrhea, check bone density every 1–2 years
6. Psychological support: If you notice compulsive calorie restriction or anxiety around food, consider seeking psychological counseling; RED-S often coexists with eating disorders

Conclusion

RED-S is not a matter of “weak willpower” — it is a scientific question of energy management. The prerequisite for running fast is a body that can function, recover, and adapt properly. Female runners in Taiwan are growing rapidly in numbers, and race performances continue to improve, but only with adequate energy support can this passion be sustained long-term. Eat enough, run well, and stay healthy for the long haul — that is the true training philosophy for female runners.

Related Reading

• RED-S in Female Runners: Relative Energy Deficiency Is More Than Just Eating Less
• RED-S: The Energy Supply Warning Sign for Female Athletes
• RED-S Syndrome in Runners: The Impact of Energy Deficiency on Menstruation and Bone Health in Women
• Relative Energy Deficiency (RED-S): The Most Dangerous Dietary Trap for Endurance Athletes