Overtraining or Energy Deficiency? How OTS and RED-S are Connected

Summary for you on the go: Performance development requires a balance between training and recovery. When that balance is upset, the result can be a lack of progress, prolonged fatigue, and in the worst case, illness. This article clarifies the difference between overtraining and relative energy deficiency—two conditions that are often confused and share many symptoms. The main question is whether what is sometimes called overtraining is actually more often due to low energy availability. The conclusion from current research is clear: true overtraining is rare, while energy deficiency is much more common and often overlooked. Preventing problems requires a better understanding of the relationship between training load, recovery, and adequate energy intake.

Overtraining or energy deficiency?

Performance improvement is often seen as a combination of the right amount of volume, intensity, rest, and continuity. These factors, along with other psychological aspects of life, affect the total load we experience. If the goal is to truly perform at your best—whether you're an elite athlete or an avid enthusiast—you need to balance on the edge of what your body can manage. Exceeding that limit can lead to overtraining. Sometimes this "overtraining" is planned, known as Functional Overreaching (FO), and can lead to supercompensation and a temporary performance peak. If the load continues for too long without adequate recovery, it can develop into Overtraining Syndrome (OTS).

What is overtraining?

In short, overtraining means that your workouts are delivering less effect than before, and recovery takes significantly longer. But, as always, there are shades of gray. During a training block, it's normal to feel tired — that's often part of the plan. Weekly volume and consistency are key factors for performance. Here, you need to differentiate between FO (Functional Overreaching) and NFO (Non-Functional Overreaching). It's common to need a few days to bounce back, but when it takes up to two weeks, you're on thin ice. Anything over 14 days is termed overtraining, OTS. At this point, it's no longer easy to turn things around, and the recovery time can take several months or, in really unfortunate cases, even longer. Read more about overtraining here.

A commonly cited definition describes OTS as: “an accumulation of training and/or non-training stress resulting in long-term decrement in performance capacity.”

Symptoms of OTS can vary but may include:

• Fatigue
• Performance deterioration
• Sleep difficulties
• Weight loss
• Osteoporosis
• Concentration difficulties
• Mood swings
• Increased risk of injury and illness

Many of these symptoms also align well with the diagnosis Relative Energy Deficiency in Sport (RED-S), known in Swedish as relatif energibrist. This is a relatively new diagnosis first introduced by the IOC in 2014. But what exactly is relative energy deficiency?

What is relative energy deficiency?

Relative energy deficiency is defined as a complex syndrome caused by low energy availability in the body, which leads to impaired function for both performance and health. Relative energy deficiency is also closely associated with the female athlete triad — something we've written about before, read more here.

When comparing definitions and symptoms of overtraining and relative energy deficiency, there is significant overlap. This has raised the question of whether a common and often-cited explanation for overtraining is actually energy deficiency. 

What does the research say?

In May 2021, a review article was published that explores the research landscape and attempts to clarify what really applies: Overtraining Syndrome (OTS) and Relative Energy Deficiency in Sport (RED-S): Shared Pathways, Symptoms, and Complexities. The study included research on overtraining and OTS where data on energy intake, carbohydrate intake, energy expenditure during training, and/or energy availability were also measured or collected. The aim was to shed light on how often these diagnoses overlap and to help athletes, coaches, and medical personnel prevent and manage the problem.

The image above clearly illustrates the similarities between the symptoms of OTS and RED-S. A criterion for diagnosing OTS is to rule out low energy availability or relative energy deficiency as the cause. However, measuring energy expenditure is not always as precise as needed, and many cases of low energy availability may mistakenly be identified as overtraining — potentially leading to late detection of the problem. Another challenge is the ethical dilemmas in research: it's neither fair nor easy to subject athletes to prolonged overtraining or low energy availability. The studies on OTS are often shorter, and participants are steered towards NFO instead of actual OTS.

Researchers suggest that many OTS diagnoses are incorrect and are actually due to low energy availability, i.e., RED-S. To investigate this, they reviewed published OTS studies and found 21 that met the criteria. Of these, 14 indicated that energy availability decreased over time for those with OTS compared to the control group. Overall, the analysis shows that 84% of the studies point to low energy availability and/or low carbohydrate availability, leading to symptoms that could be interpreted as OTS/RED-S — clearly illustrating the difficulty in making the correct diagnosis.

“It is important to note that preventing under-recovery is multifactorial, but many aspects revolve around EA and CHO availability. Herein we have demonstrated that OTS and RED-S share many pathways, symptoms, and diagnostic complexities.”

Conclusion

For those who wish to explore further, the complete results can be found in the study. But what can athletes take away from this? Researchers highlight that OTS as a diagnosis is quite rare, and most endurance athletes are adept at balancing sleep, recovery, and energy intake during shorter periods of overload. Symptoms of relative energy deficiency, however, are significantly more common, and many experience this without always being aware of it — during both shorter and longer periods. Data shows there are significant knowledge gaps among athletes, coaches, and medical staff regarding RED-S. Therefore, efforts should focus on more education to prevent the issue.

So next time you're heading to a training camp or embarking on a volume block at home, remember to compensate adequately for the increased volume. The figure below highlights key factors that coaches and athletes can use to guide their actions when symptoms arise.

Listen to your body, but also have tools in your arsenal (and in your gut) to determine if it's training or under-eating that's causing issues. If you're unsure, it's better to consult a knowledgeable coach or medical professional than to guess. We'd much rather see you achieve a personal best than experience a personal collapse.