RED-S Relative Energy Deficiency: The Most Overlooked Health Breakdown in Endurance Athletes

From the Female Athlete Triad to RED-S

The concept of low energy availability in sport has evolved from the early “Female Athlete Triad” (disordered eating—menstrual dysfunction—low bone density) into Relative Energy Deficiency in Sport (RED-S), which applies to both men and women and describes the systemic harm caused when “energy intake is insufficient to simultaneously support exercise expenditure and the body’s basic physiological functions.” Recent reviews indicate that low energy availability (often driven by deliberate dietary restriction or unintentional under-eating) is the core driver, affecting a remarkably high proportion of elite female endurance athletes.

Energy Availability and the Threshold of 30

Energy Availability (EA) = (Daily energy intake − Exercise energy expenditure) ÷ Fat-free mass (kg). When EA falls below approximately 30 kcal/kg fat-free mass/day, the body switches into “energy-saving survival” mode: suppressing reproductive, metabolic, skeletal, immune, and other “non-essential” functions to stay alive, resulting in a cascade of impairments.

Men Are Affected Too

RED-S is not exclusive to women. Low energy availability in male endurance athletes can lead to decreased testosterone, reduced bone density, and declines in libido and performance—and because there is no obvious “amenorrhea” warning sign, it is more often missed. Red flags in men are often recurrent injuries, performance plateaus, low morning gonadal markers, and mood changes.

Identifying the Red Flags

• Menstrual irregularities or amenorrhea (the most critical warning sign in women—absolutely not “normal from training hard”)
• Recurrent stress fractures or low bone density
• Performance plateau/decline, easy fatigue, loss of training adaptation
• Abnormally low resting heart rate, feeling cold, frequent infections
• Tendency toward dietary restriction, rapid weight loss, excessive obsession with being light (high risk in climbers/weight-sensitive disciplines)
• Low mood, irritability, poor sleep

Why Endurance Sports Are High Risk

Marathon, long-distance triathlon, climbing-focused cycling, race walking, and similar disciplines emphasize watts/kg and being lightweight; combined with high training expenditure, athletes easily fall into an “unintentional or intentional” energy deficit. Carbohydrate periodization, fasted training, and weight-loss phases—if executed improperly—amplify RED-S risk (see related articles), which is why these strategies require safeguards.

Management Principles

The core is increasing energy availability: increase total energy intake (especially carbohydrates and overall food intake), reduce training expenditure when necessary, and aim to bring EA back to adequate levels (generally ≥45 kcal/kg fat-free mass/day is considered a healthy reference; those deficient require professional nutrition and medical team intervention). Menstrual recovery, bone density improvement, and performance rebound often take months and require a multidisciplinary team (sports medicine, nutrition, psychology). Early identification and intervention yield the best prognosis.

The most dangerous thing about RED-S is that it disguises itself as “the price of hard work”—amenorrhea, fractures, getting leaner and faster look like proof of dedication, but in reality the body is turning off the lights to survive. Remember this: a stopped period, a cracked bone, a collapsed performance—these are not medals, they are alarms.

Related Reading

• RED-S Relative Energy Deficiency: Energy Supply Warning Signs for Female Athletes
• Athlete Bone Health: Risks of RED-S and Insufficient Energy Availability
• Relative Energy Deficiency (RED-S): The Most Dangerous Dietary Trap for Endurance Athletes
• Energy Deficiency in Female Runners (RED-S): Identifying the Hazards and Prevention