RED-S: The Hidden Consequence of Chronic Underfueling

Relative Energy Deficiency in Sport (RED-S) is, simply put, a mismatch between energy intake and energy output. When the gap between energy needs and actual intake persists, the body is left short of what it needs to support normal biological processes after exercise. Over time, the body begins to make “budget cuts.”

This phenomenon was originally described as the Female Athlete Triad, after clinicians noticed a recurring pattern in female athletes: disordered eating behaviors, amenorrhea (loss of the menstrual cycle), and low bone mineral density. While this framework helped identify a real problem, it was inherently gendered and incomplete. Men experience many of the same physiological consequences when energy needs are not met, even if the outward signs differ.

For this reason, the language has shifted toward Low Energy Availability (LEA) and Relative Energy Deficiency in Sport (RED-S). LEA describes the cause—insufficient energy remaining to support normal physiology after accounting for exercise. RED-S describes the syndrome—the downstream impairments in physiological function that result from LEA.

Put simply: LEA is the problem; RED-S is how the problem shows up.

RED-S can present as a constellation of signs and symptoms, including but not limited to:

• Reduced metabolic rate
• Poor bone health
• Loss of menstruation
• Blood abnormalities (low iron, B12, folate)
• Low libido and erectile dysfunction
• Poor concentration and performance
• Chronic fatigue
• Poor sleep
• Constipation

LEA and RED-S exist on a spectrum, and athletes may move in and out of different stages over time. This is not an “on/off” or “yes/no” diagnosis like an infection or illness. It is a physiological state that can worsen or improve depending on training load, recovery, and fueling.

RED-S may appear suddenly following a large stressor or slowly accumulate over time.

When an athlete consistently pushes training demands without adequate energy to support recovery, the body adapts by downregulating non-essential functions. This is a survival response. Processes unrelated to immediate performance—hormone production, bone remodeling, immune function—are reduced to conserve energy. Over time, this leads to declining health and performance. In contrast, when energy and recovery are sufficient, we expect supercompensation and adaptation to occur.

Short-Term Effects

In the short term, RED-S can be deceptive. Some endurance athletes may initially see performance improvements, particularly through weight loss. A higher power-to-weight ratio can create the illusion that underfueling is “working.”

These gains are often short-lived.

As recovery falters, athletes struggle to hit consistent training quality, experience increased illness, and become more injury-prone. By this point, however, the athlete has often already been positively reinforced by early performance improvements. When performance later declines, the response is frequently to restrict intake further in an attempt to “fix” the issue—driving the cycle deeper.

Short-Term Red Flags (Days to Weeks)

Short-term RED-S often presents as:

• Unexplained poor training sessions or races
• Decreased recovery capacity
• Inability to gain muscle, weight, or strength
• Mood and energy swings
• Poor sleep
• Constipation

Hormonal disruptions can also occur quickly. In female athletes, measurable changes in reproductive hormones have been observed within five days of low energy availability. These early warning signs are not gender-specific and should not be ignored.

Medium-Term Effects (Weeks to Months)

With more prolonged LEA, risks escalate. One of the most common and concerning outcomes is amenorrhea, defined as the loss of three consecutive menstrual cycles. Some athletes experience intermittent cycle disruption (oligomenorrhea or luteal phase deficiency), while younger athletes may experience delayed menarche (failure to begin menstruation by age 15).

This is particularly dangerous because estrogen plays a critical protective role in bone development and protection. When estrogen levels are suppressed—combined with insufficient energy, protein, calcium, and micronutrient intake—bone accrual is compromised at a time when bone development is most rapid and most important. Much of the bone mass that is lost is difficult to recover, and it is unlikely that an athlete will fully regain what was lost. More on RED-S impact on bone below.

RED-S does not present uniformly. The wide range of possible symptoms—and the fact that each athlete may express them differently—underscores the need for a qualified care team. Regular evaluation by a Sports Medicine Physician and a Sports Dietitian (CSSD or CISSN) should be considered standard care for competitive athletes.

Additional warning signs may include:

• Reduced body weight and lean mass
• Low sex drive
• Decreased nocturnal erections
• Increased illness and infection
• Poor training adaptation
• Low blood pressure and heart rate
• Abnormal blood markers

Common lab abnormalities may include:

Low:
Iron, folate, B12, testosterone, estrogen, luteinizing hormone, T3, IGF-1, blood glucose, insulin

Elevated:
A1C, LDL cholesterol, cortisol

Long-Term Consequences

Some of the most challenging and irreversible consequences of prolonged LEA and RED-S involve bone health. While long-term issues like depression and gastrointestinal dysfunction are serious, bone loss is often the hardest to recover from.

In professional sport, we’ve seen both extremes. In rare cases, recovery occurs early enough to allow meaningful reversal. Tour de France stage winner Jonas Abrahamsen reportedly experienced a second growth spurt—gaining several inches in height and over 30 pounds—after adequately fueling following years of underfueling.

More often, recovery comes too late. Athletes such as Mary Cain experienced severe bone demineralization after prolonged pressure to maintain a specific physique. Similarly, Veronica Ewers struggled with RED-S-related health issues and was ultimately unable to fully reverse the damage despite medical and nutritional support.

This risk applies to both men and women. Bone mass is difficult to accrue, and approximately 95% of peak bone mass is achieved by age 18. While improvements are possible later in life, the opportunity for large gains is greatest earlier.

This is especially concerning given early sport specialization. While weight-bearing exercise supports bone development, adequate energy availability is required for this adaptation. Many athletes only discover poor bone health after repeated stress injuries or fractures—often once imaging reveals already elevated fracture risk.

Cyclists face a unique challenge. Unlike runners, they are less likely to develop stress fractures, which can delay awareness of compromised bone health. Bone density may go unassessed until a major crash or injury occurs.

Female athletes who do not menstruate may lose 2–6% of bone mass per year, with total losses reaching up to 25%, much of which is irreversible. Male cyclists are also at increased risk, with research showing three times the prevalence of osteoporosis and more than double the incidence of osteopenia compared to the general population.

Men are not protected from RED-S. While they may appear more resilient—and often lack obvious warning signs—this can lead to underdiagnosis and delayed intervention, especially as men are often less likely to seek help.

Time Course Matters

RED-S and LEA can develop:

• Acutely, from large energy deficits (race days, bikepacking, ultras)
• Periodically, through repeated moderate deficits (training blocks, stage races)
• Chronically, through daily underfueling

This complicated syndrome of RED-S is nuanced and no one size fits all method will be able to cover all athletes. For these reasons it’s imperative that athletes go beyond the standard once a year health check and find a team of sports medicine doctors, dietitians, and psychologists to assist them in screening and preventing issues before they manifest and grow.

References 

Mountjoy, M., Ackerman, K. E., Bailey, D. M., Burke, L. M., Constantini, N., Hackney, A. C., Heikura, I. A., Melin, A., Pensgaard, A. M., Stellingwerff, T., Sundgot-Borgen, J. K., Torstveit, M. K., Jacobsen, A. U., Verhagen, E., Budgett, R., Engebretsen, L., & Erdener, U. (2023). 2023 International Olympic Committee’s (IOC) consensus statement on Relative Energy Deficiency in Sport (REDs). British journal of sports medicine, 57(17), 1073–1097. https://doi.org/10.1136/bjsports-2023-106994.

Torstveit MK, Ackerman KE, Constantini N, et al Primary, secondary and tertiary prevention of Relative Energy Deficiency in Sport (REDs): a narrative review by a subgroup of the IOC consensus on REDs British Journal of Sports Medicine 2023;57:1119-1128.

The post RED-S: The Hidden Consequence of Chronic Underfueling appeared first on PezCycling News.