What is Relative Energy Deficiency in Sport (RED-S) and how does it affect weight in a 39-year-old woman with iron deficiency, low cortisol, and normal ferritin levels?

What is RED-S and Its Effect on Weight

RED-S (Relative Energy Deficiency in Sport) is a syndrome caused by inadequate energy intake relative to energy expenditure that affects multiple body systems, and yes, it directly affects weight through metabolic suppression, though weight changes alone are poor indicators of the condition. 1

Definition and Core Mechanism

RED-S represents an evolution beyond the "Female Athlete Triad" concept, recognizing that energy deficiency affects both male and female athletes across multiple physiological systems, not just the traditional triad of menstrual dysfunction, bone health, and energy availability. 1

The fundamental problem is low energy availability (EA), calculated as energy intake minus exercise energy expenditure relative to fat-free mass. 1 Healthy adults require approximately 45 kcal/kg fat-free mass/day to maintain energy balance, and values below this threshold trigger compensatory metabolic suppression. 1

Weight and Metabolic Effects

Low energy availability causes the body to reduce energy expenditure through metabolic rate suppression, making weight maintenance or even weight gain possible despite chronic energy deficiency. 1 This creates a critical clinical pitfall: athletes with RED-S may maintain normal body weight or body composition while experiencing severe physiological dysfunction. 2

Key metabolic consequences include:

• Reduced resting metabolic rate (RMR) as an adaptive response to conserve energy 1, 2
• Decreased glucose utilization and mobilization of fat stores 1
• Slowed metabolic rate and decreased growth hormone production 1
• Reduced muscle protein synthesis even at EA levels of 30 kcal/kg fat-free mass/day 1

Critical Clinical Insight:

In a case series of Division I female runners, most athletes maintained body mass and body composition despite meeting criteria for low EA (mean 31.6 kcal/kg FFM/day) and elevated Triad risk scores. 2 One athlete experienced severe performance decline with reduced RMR but was the only one with adequate ferritin levels, highlighting the complex interplay between nutritional status and metabolic adaptation. 2

Relevance to Your Patient Context

For a 39-year-old woman with iron deficiency, low cortisol, and normal ferritin, these findings are particularly relevant:

The combination of low cortisol with iron deficiency strongly suggests chronic energy deficiency affecting multiple endocrine axes. 1 The IOC consensus specifically identifies that stress hormones (catecholamines and cortisol) are altered with low EA and have negative effects on multiple systems. 1

Normal ferritin does not exclude RED-S or iron deficiency anemia, as ferritin can be maintained while other iron parameters deteriorate. 2 In the Division I runner case series, ferritin showed a trend toward decrease during competitive season (24.2 to 15.7 ng/mL) despite maintained body weight. 2

Broader Health Consequences Beyond Weight

RED-S affects multiple physiological systems simultaneously: 1

• Endocrine dysfunction: Menstrual irregularities, reduced testosterone in males, thyroid hormone alterations 1, 3
• Bone health: Irreversible bone loss, increased stress fracture risk, low BMD 1
• Cardiovascular: Impaired cardiovascular health, bradycardia 1
• Immune function: Increased viral illnesses and infections 1
• Hematological: Anemia and nutrient deficiencies 1
• Psychological: Depression, anxiety, irritability 1, 4
• Performance: Decreased training response, reduced endurance, impaired coordination 1, 4

Diagnostic Approach

Do not rely on body weight or body composition as screening tools for RED-S. 2 Instead, assess:

• Detailed dietary intake versus exercise energy expenditure to calculate EA 1, 2
• Resting metabolic rate measurement (suppressed RMR indicates metabolic adaptation) 2
• Menstrual history in women (primary or secondary amenorrhea) 1
• Hormonal profile: Including reproductive hormones, thyroid function, cortisol, IGF-1 1, 3
• Bone health markers and DEXA scanning 1
• Nutritional biomarkers: Vitamin D, ferritin, hemoglobin, T3 2
• Psychological screening for disordered eating (20% prevalence in female elite athletes) 1

A large cross-sectional study of 1,000 female athletes demonstrated that low EA measured by self-report questionnaires strongly correlates with RED-S health and performance consequences. 4

Management Priority

The primary treatment is non-pharmacological: increasing energy intake and optimizing training load to achieve adequate EA (≥45 kcal/kg FFM/day). 1 Hormone therapy should only be considered after nutritional intervention fails, using transdermal estrogen with short-term progestin in women. 5

Weight gain or maintenance during treatment does not indicate resolution—monitor RMR recovery, hormonal normalization, and performance restoration as primary endpoints. 2