RED-S in Triathlon: The Under-Fueling Syndrome That's Quietly Limiting Your Performance

Lionel Sanders — arguably the most consistently competitive male age-group-turned-pro IRONMAN athlete of the last decade — went public with a RED-S diagnosis. Not an eating disorder. Not burnout. Not overtraining. RED-S: Relative Energy Deficiency in Sport. And his candid disclosure did something meaningful for the sport: it put a clinical name on something that has been quietly limiting the performance of age-group triathletes — men and women — for years.

This article is not about recognizing subtle signs of under-fueling. That ground has been covered. This is about the clinical syndrome RED-S: what it actually is, what it does to your body at a mechanistic level, why it's easy to develop in triathlon without intending to, and what you do about it.

What RED-S Is — and What It Isn't

RED-S stands for Relative Energy Deficiency in Sport. It was formally defined by the International Olympic Committee in 2014 and updated in 2023. The core concept is low energy availability (LEA): a state in which the energy you consume from food is insufficient to cover both your training demands AND the physiological processes your body requires to function normally — reproduction, bone maintenance, immune function, hormonal regulation, protein synthesis.

RED-S is not synonymous with disordered eating. That's the most important distinction to make. You can develop RED-S while eating what feels like plenty of food — if your training load increases faster than your fueling keeps pace, if you're unintentionally skipping pre-workout calories, if your post-workout nutrition window is consistently missed. In triathlon, where three-discipline training volumes create enormous and rapidly fluctuating energy demands, this happens far more often than coaches and athletes want to acknowledge.

RED-S affects both sexes. Research published in PMC estimates 36–58% of female endurance athletes and 25–45% of male endurance athletes experience symptoms consistent with LEA or RED-S. In a 2025 study published in Nutrients (MDPI) examining top-performing female triathletes competing at Quarter to Double IRONMAN distance, investigators found RED-S risk in a substantial proportion of athletes training over 11 hours per week — despite the participants being experienced, high-performing competitors who presumably knew how to eat.

The expression of RED-S differs between sexes. In female athletes, the most recognized marker is menstrual dysfunction — irregular or absent cycles. In male athletes, the primary hormonal signal is reduced testosterone and elevated cortisol. Both represent the body's rational response to chronic energy deficit: deprioritize costly physiological processes to preserve survival.

Athlete Takeaway: RED-S is not extreme. It exists on a spectrum, and mild LEA can impair performance without triggering obvious symptoms. If your training load is high and your fueling is not keeping deliberate pace with it, you may be operating in LEA without knowing.

Why Triathletes Develop RED-S Without Trying

Triathlon creates a specific set of conditions that make unintentional LEA more likely than in single-sport athletes.

Training volume accumulates fast. A typical IRONMAN build peaks at 12–18 hours per week. The combined caloric demand of swimming, cycling, and running at that volume is enormous — and unlike a runner who might develop a clear appetite feedback loop, triathletes often rotate between disciplines in ways that suppress normal hunger cues. A 4-hour Saturday ride followed by a 1-hour run may leave you too fatigued to eat adequately before glycogen replenishment windows close.

The training-to-eating lag. Many athletes fail to preemptively increase caloric intake as training volume rises during a build phase. Instead, they eat reactively — when hunger signals arrive, which lag actual energy needs. When training volume surges in weeks 6–10 of a build, the athlete may be running a meaningful energy deficit for several weeks before symptoms become obvious.

Body composition culture. Triathlon, like most endurance sports, has a cultural tendency toward associating leanness with speed. Athletes sometimes deliberately reduce caloric intake during a build to arrive at race day lighter, without fully accounting for the performance cost of the LEA they're creating. This is a false economy. The marginal speed gained from reduced body mass does not offset the performance loss from suppressed hormonal function, reduced glycogen storage capacity, and impaired protein synthesis.

The long training day problem. At the IRONMAN build level, athletes regularly complete 5+ hour training days. These sessions create acute energy deficits that require significant post-session caloric intake to close — and many athletes under-fuel these recovery windows because they're not hungry immediately after long-duration aerobic work.

Athlete Takeaway: If you're in a high-volume training phase and you're not deliberately and proactively increasing your daily caloric intake — not just eating when you're hungry, but eating because you know the training demands it — you are likely in some degree of LEA.

What RED-S Does to Your Performance

The performance consequences of RED-S operate through multiple mechanisms simultaneously, which is why the effect compounds and why athletes can't train their way through it.

Hormonal Disruption

In male athletes, chronic LEA suppresses luteinizing hormone (LH) pulsatility, which in turn reduces testosterone production. Lower testosterone impairs muscle protein synthesis, meaning your body cannot adequately repair and rebuild from training stress. Recovery suffers. You adapt more slowly. Over time, training load creates damage that doesn't fully rebuild before the next session.

In female athletes, the equivalent disruption is hypothalamic-pituitary-ovarian axis suppression — the same energy-conservation response, expressed through menstrual dysfunction rather than testosterone reduction.

Simultaneously, cortisol — a catabolic stress hormone — tends to be elevated in LEA states. Elevated cortisol accelerates muscle breakdown, suppresses immune function, and impairs sleep quality. You're breaking down and not building back up.

Bone Density

Both male and female athletes with chronic LEA show accelerated bone mineral density loss. In practical terms, this increases stress fracture risk — one of the most common overuse injuries in endurance sport. Research consistently shows that RED-S athletes sustain more stress fractures and bone injuries than adequately fueled athletes at equivalent training loads. If you've had a stress fracture that seemed disproportionate to your training volume, it's worth examining your energy availability history.

Immune Suppression

The immune system is metabolically expensive to operate. In LEA states, the body deprioritizes immune defense. This manifests as increased frequency and duration of respiratory illness, slower recovery from minor infections, and a generally depressed capacity to fight off the repeated minor immune challenges that high training volumes create.

Athletes who seem to catch every cold during a build — who spend two weeks out of every eight fighting illness — often have a fueling deficit, not a rest deficit.

Glycogen and Aerobic Capacity

Chronic LEA impairs the body's ability to fully replenish muscle and liver glycogen. This limits sustainable training intensity — your high-end power output drops, your ability to complete quality sessions erodes — and it directly limits race-day fuel availability. You can't race fast on chronically depleted glycogen stores.

Athlete Takeaway: The irony of RED-S is that it makes you work harder and get less. Training stress accumulates without adequate adaptation. You may feel like you're training hard and making no progress — because you're not recovering. The solution isn't more training. It's more food.

How to Identify RED-S in Your Own Training Data

You don't need a blood panel to suspect RED-S. Your training data already contains the signals — you just need to know what you're looking for.

Power or pace stagnation. If your FTP or threshold pace hasn't meaningfully changed over an 8–12 week block despite consistent training and no change in volume, you may not be recovering adequately to supercompensate.

HRV trend decline. If your morning HRV is trending downward across a multi-week period without a corresponding recovery week response (i.e., even easy weeks aren't bouncing HRV back), suppressed parasympathetic activity may reflect chronic physiological stress consistent with LEA.

Increasing illness frequency. Track this. If you're getting sick more than once per 8-week training block, that's a signal — not a coincidence.

Sleep quality decline. Elevated cortisol from LEA disrupts normal sleep architecture — athletes report waking in the early hours, difficulty returning to sleep, and non-restorative sleep despite adequate duration. If your sleep tracker is showing declines in sleep quality during high-volume training blocks, examine your fueling first.

Female athletes specifically: irregular or absent menstrual cycles during a training build are a clear clinical signal and should be evaluated promptly — both for performance reasons and long-term bone health.

Athlete Takeaway: Bring these data points to your coach, not just your own assessment. RED-S is a clinical condition, and if you suspect you're experiencing it, working with both your coach and a qualified sports medicine physician or registered dietitian with endurance sport experience is the appropriate step.

The Bottom Line

RED-S is not a fringe issue. Research suggests it affects a meaningful proportion of endurance athletes at every level of competition, including experienced IRONMAN age-groupers who think they know how to eat. The condition exists on a spectrum, develops without intention, and impairs performance through mechanisms — hormonal, immune, musculoskeletal — that training alone cannot overcome.

The fix is not complicated. It's uncomfortable for athletes conditioned to believe that less is more, but it's not complicated: eat enough to support your training, specifically and proactively, not reactively. Fuel the training you're doing, not the training that fits your comfort zone.

If you suspect RED-S, don't guess your way through it. Talk to your coach. Seek appropriate medical evaluation. And stop treating your nutrition as an afterthought to your training plan — it is the training plan.

NVDM coaches work with athletes to build fueling strategies matched to their actual training loads. If you've been spinning your wheels in a build and suspect your nutrition might be the limiting factor, let's talk.