What Actually Causes RED-S in Athletes: Low Energy or Overtraining?

Stacy emphasizes that the first step into RED-S is low energy availability. The most actionable defense is consistent fueling timed with exercise. If an athlete finishes a training session without a post-exercise meal, or skips pre-workout fuel, the brain interprets the gap as starvation that day. Over repeated days, this cumulative deficit leads to menstrual disruptions, altered blood lipids, and eventually full RED-S. The protocol is not just about total daily intake but about nutrient timing to keep the brain's energy sensors calm. She mentions that society's demonization of carbs leads many women to under-eat carbohydrates specifically, which is another path to low energy availability, so they must focus on including carbs.

Stacy explains that many women assume they are healthy if they’re still menstruating, but the quality of the bleed matters more. A typical healthy bleed lasts about 5-7 days with a moderate flow. When energy intake is too low, the cycle may lengthen and the bleed become shorter and lighter—just 2-3 days. This indicates the body is under too much stress and is already in the early stages of RED-S. By tracking this simple metric, athletes can catch the problem before it progresses to full amenorrhea or bone density loss. She highlights that this is a simple, zero-cost tool every female athlete can use.

Stacy points out that when physicians see a young athlete with high cholesterol, they often prescribe a low-fat diet or statins—completely missing the underlying RED-S. Similarly, high CK is frequently dismissed as a side effect of creatine use or tough training. She wants practitioners and athletes to see these lab results in the context of energy deficiency. If an athlete also has menstrual changes or poor recovery, these blood markers can help confirm that the body is in a low energy state. Catching this early can prevent the full-blown systemic shutdown, which may take years to reverse.

Stacy contrasts this approach with the common practice of trying to be at race weight year-round or losing weight immediately before a season. Athletes who are chronically lean or calorie-restricted suffer from RED-S symptoms, impaired recovery, and more illness. By starting a few kilos heavier, they have metabolic flexibility and resilience. She acknowledges that for the top 1% of athletes who professionally manipulate weight cyclically (with careful monitoring), being lighter can improve performance, but even they bring weight back up in the off-season. For everyone else, constant restriction doesn't work. The set point is where the body performs best when fully fueled, and that may not match the number in the athlete's head.

Stacy mentions that under-recovery—both in terms of sleep and emotional stress—is a key promoter of the symptomology formerly called 'overtraining.' It's not just about physical rest. The brain's interpretation of overall load includes psychological stressors. Without managing these, an athlete can enter a low-energy state even if they believe they are eating enough. Integrating good sleep hygiene and stress reduction is part of the 'under-recovery' side of the energy availability equation.

The demonization of carbs since the 1990s and the rise of diets like Atkins has led to ingrained fear in many female athletes. Stacy notes that women often under-eat carbohydrates relative to their training, thinking it will help them be leaner and faster. In reality, low carbohydrate availability causes significant endocrine disruption and is a major contributor to RED-S. Even if an athlete is eating enough calories from fat and protein, the brain may still perceive low glucose as a shortage, triggering the same protective shutdown. The protocol is therefore to make carbohydrates a non-negotiable part of sports nutrition.

Stacy references the top 1% of athletes who successfully use low body weight—they do it by cycling their weight up in the off-season and down during competition. This contrasts with an amateur’s chronic restriction. She advises that for the vast majority, the approach should be to have intentional cyclical changes, not permanent leanness. This strategy allows the body to recover from the stress of a season and enter the next one with full hormonal function, robust bone density, and better immune function.

People may brag about a 20-second faster 1K rep after dropping weight, but Stacy asks the question: How does that translate to eight 1K reps two weeks later? It doesn't. Because the body is already in a deficit, recovery is impaired, and the adaptation needed to sustain that pace across multiple reps is absent. Athletes need to think about repeat performance, not one-off metrics.

Stacy Sims explains that the original concept of overtraining syndrome focused on training stress exceeding recovery capacity. However, research now shows that the cascade of symptoms—from menstrual dysfunction to lipid abnormalities—is triggered when the brain perceives food scarcity because the athlete isn't eating enough before, during, and after training. When energy intake is consistently below what the body needs for exercise and basic physiological functions, the brain down-regulates all non-essential systems. This is a protective response that mimics starvation. The more an athlete tries to push through, the deeper the shutdown becomes, eventually leading to a syndrome that can take years to reverse. Importantly, the emphasis is now on 'under-recovery' (sleep, emotional stress management) and 'low carbohydrate availability' as key contributors, rather than solely training load.

Stacy describes how the menstrual cycle is one of the first systems to be affected by low energy availability. The initial change is not always amenorrhea but can be a lengthening of the cycle or a change in the bleed pattern, such as going from a normal 5–7 day menses to only 2–3 days of light bleeding. This lighter, shorter bleed occurs because the body is not producing enough estrogen to build a thick uterine lining, often due to lack of ovulation. Most women incorrectly assume that any bleeding means they ovulated and are healthy. By tracking the number of days and the flow, they can catch the early warning of RED-S before it progresses to full-blown amenorrhea and bone loss.

Stacy explains that when athletes start a race season heavier—specifically a couple of kilograms above what they think they should weigh—they have a fat buffer that makes them more robust. During the season, training and racing naturally increase fitness and lead to some weight loss, so they end up at their ideal weight without the dangerous effects of chronic under-fueling. Starting heavier prevents early-season illness, upper respiratory infections, stress reactions, and bone issues. It also allows the athlete to monitor signs of RED-S if they start losing too much weight too quickly. She notes that the ideal approach is to have cyclical weight: letting weight come up in the off-season and dropping it gradually over a long period during competition. This contrasts with constant calorie restriction, which impairs recovery, adaptation, and sustainable performance.

Stacy explains that one of the metabolic disturbances in RED-S is a change in blood lipids: total cholesterol and low-density lipoproteins (LDL) can rise. This happens because the body, perceiving starvation, alters fat metabolism. Concurrently, a high CK level is often blamed on creatine supplementation or hard training, but it can be a sign of muscle tissue stress due to insufficient energy. These markers, when taken together with other symptoms like menstrual changes, should prompt a RED-S evaluation rather than isolated treatments. Early detection via routine blood work can prevent progression to full RED-S.

Stacy notes that while RED-S also occurs in men—especially in aesthetic sports like hybrid training—it is more dire for women. The female brain's appetite and energy-sensing centers are more sensitive to low glucose and low energy availability. When a woman doesn't eat enough carbohydrates, the hypothalamus rapidly down-regulates reproductive and metabolic hormones as a survival mechanism. This is compounded by the fact that many women fear carbohydrates due to decades of demonization in diet culture, leading them to under-fuel for their training. The result is a faster path into endocrine dysfunction and RED-S.

Stacy clarifies that while the physiologic consequences are often more severe for women, men are not immune. In sports where body image, leanness, or weight categories are important—such as bodybuilding, cycling, or hybrid events—men can experience low energy availability, leading to reduced testosterone, poor recovery, bone loss, and performance decline. This broadens the scope of RED-S screening and highlights that under-fueling is a problem across genders.