The Causes

Stripping back the science

I’m not usually someone who reads scientific research papers for fun. I prefer hearing personal accounts of peoples' experiences, or talking to someone who can break it down for someone with a ‘creative’ mind...But when it comes to the complex mechanisms involved in hormonal regulation and the evolutionary biology behind energy conservation, I have to admit I find it fascinating.

I could talk for days about the endocrine system and the effects of low energy availability upon the hypothalamic-pituitary-adrenal axis that governs your hormones. I could attempt to explain why the release of vital neurotransmitters (chemical messages) that regulate your mood and appetite aren't working in the way that they should; why you might be feeling restless, yet exhausted during exercise; or why exogenous hormones are not the answer to your period problems. Yet while all of this knowledge is important, it’s not necessarily more so than understanding a few basic principles of energy balance when it comes to getting your head around your RED-S problem. So, in case you’re not wildly enthusiastic about scrutinising the science, I've shared a very basic summary of what I've learnt below. (Though if you are, you can find plenty of research here).

Rewind 12,000 years

Way back when humans hunted and foraged for food, stressful situations like being chased by predators or food scarcity provoked a clever bodily response, shunting it into ‘survival mode’. Today, the same physiological response is produced by circumstances surrounding exercise, nutrition and life stress, which I've broken down below. While you can tell the difference between being on the hunt for food, intentionally restricting calories, training for a triathlon, or running away from predators, your body cannot. It can only sense a stressful situation and will quite rightly respond by entering into 'survival mode' until it’s returned to a ‘safe’ environment. This concept of holds true regardless of body size or weight when the body’s nutritional needs are not met over a period of time, even by as little as 300 calories a day.

Although ‘survival mode’ may not sound dangerous, there’s a reason it’s reserved for high-threat scenarios. As I touched upon here, the body responds to this situation by directing energy towards the most critical physiological processes for short-term survival and away from other important processes which help the body function optimally. In the long-term, suppression of these other important functions leads to hormonal dysregulation, which has an adverse impact on almost every other system in the body. Find out more about those here.

So, what are the potential causes behind this processes?


The fundamental principle of RED-S surrounds a lack of ‘energy availability’, which simply describes the amount of calories we consume, versus the amount of calories we expend via exercise, daily activity and rest.

When this is nicely balanced, our hormonal systems are supplied with the energy required to drive beneficial adaptations to training, supporting: muscle growth and repair; changes in body composition; injury prevention; immune, cognitive and reproductive function; mental and emotional well being and so on.

When the balanced is tipped towards insufficient energy consumption, we disrupt the network of hormones working away to deliver energy to the right places. When this imbalance develops into a relative energy deficit (RED-S), the important physiological processes that rely upon this energy and these hormones become impaired.

Most athletes are aware that a balance of carbohydrates, fats and proteins are essential to replenishing our energy stores, yet the principle of energy balance is commonly overlooked. Sometimes, this is entirely unintentional, but often this occurs for reasons more complicated than simply just failing to eating enough.

Disordered eating

‘Disordered eating’ among athletes can mean a lot of things, caused by a lot of factors (I've suggested just a few of these here). Essentially, it involves intentionally restricting caloric intake in an attempt to cope with insecurities surrounding body image, a desire to attain a performance advantage, or a way to gain control within a pressurised environment. In many cases, disordered eating is a pre-cursor for a clinical eating disorder, but any level of disordered eating can have a significant impact on your overall health and performance. This can be a complicated but essential issue to identify.

Common indicators include:

  • Sticking to a rigid routine surrounding food (and exercise)
  • Avoiding certain food groups or attaching guilt and/or shame to eating certain foods
  • Feeling anxious about eating around others or about going to new places to eat
  • A general preoccupation with food, weight and body image in a way that negatively impacts quality of life
  • Feeling out of control around food
  • Using exercise, food restriction or fasting to make up for eating ‘bad’ foods
  • Exercising to 'earn' food
  • Restricting food intake on rest days

A quick word on eating disorders: Eating disorders, such as anorexia nervosa or bulimia nervosa, are diagnosed according to specific and narrow criteria surrounding body weight and eating behaviours. The difference between disordered eating and clinical eating disorders surround how much the disordered eating behaviours disrupt your daily life and impair your ability to function. Many athletes with RED-S experience some degree of disordered eating which falls short of an official diagnosis for an eating disorder. It is still crucial to identify and monitor these with help from a medical professional BEFORE a full blown eating disorder develops, if it hasn't already. It’s incredibly hard to get an accurate idea of prevalence but current best estimates suggest eating disorders are twice as likely to develop among athletes than non-athletes.

If you're worried about any form of disordered eating behaviours, please, please, seek support here or visit Train Brave, a resource dedicated to helping athletes talk more openly about their experiences with food and RED-S.

Unintentional under-eating

Some cases of RED-S occur simply as a result of under-eating in line with an increased training load or intensity (see below). Although it can be hard for some athletes to believe when they already seem to eat a great deal, it is incredibly easy to underestimate how much nutrition is required to fulfill the requirements of training, especially when an athlete's intake is based on misguided information from others or the internet. Unfortunately, there are lot of social media icons, so called 'health' bloggers or just general Joe Bloggs' who promote dangerous eating behaviours and inaccurate dietary information. Among the worst offenders are those who offer advice to women based upon research solely carried out on men...This topic deserves an entire page in itself but it's summarised brilliantly by Stacey Simms here.

The good news is that 'unintentional RED-S' should be relatively easily resolved once an athlete is armed with the right knowledge of how to sufficiently fuel for their training load. If you think this could be the cause of your energy deficit, I would highly recommend seeking the advice of an expert dietitian.

A word of caution: Even unintentional under-eating can develop into something more intentional overtime. If, for example, the negative energy balance results in weight loss which boosts performance in the short term, it can be easy to develop a desire to lose more weight or maintain the inadvertent lower weight. Or, if you’re under-eating because you’re simply afraid of over-eating then it can be equally harmful. Either way, I’d definitely suggest talking it through with a dietitian who's familiar with RED-S.



As any athlete knows, longevity and success in sport requires a balance of extremes. Exercise, particularly high intensity exercise, increases stress on the body. When this is balanced with adequate recovery and a sufficient supply of energy, the body is able to adapt and improve. When the balance is continually tipped towards too much exercise and too little recovery, it will likely lead to a more stressed and energy-depleted system. This isn't something that happens overnight, or after a few tough workouts. It's more of a slow accumulation of physiological, biochemical, mental and emotional stress that progressively builds in the absence of adequate rest. One key component of recovery (alongside rest) is nutrition, alongside a myriad of other stressors outlined under 'life stressors' below.

Increase in volume/intensity

One common cause of RED-S surrounds an increase in training volume or intensity, when it isn’t matched with an increase in caloric intake and adequate recovery. For some, it’s not just the overall increase in terms of volume (e.g. running mileage) that contributes to my negative energy balance, but the intensity of the sessions themselves. When athletes find themselves in a high pressure environment or suddenly surrounded by competitive teammates it can be easy to underestimate the amount of extra energy expended in this transition alone.

For others, an increase in overall training volume, in terms of total time spent exercising, can easily lead to the same result. Even adding in extra drill sessions, strength and conditioning work, extended warm-ups or cool-downs can very quickly contribute to decreased energy availability – even if these activities do seem fun or easy!

Exercise dependence/excessive or compulsive exercise

As with many factors that surround RED-S, exercise dependence/compulsion/addiction is a tricky one to define. It covers a broad continuum of behaviours surrounding a craving for physical training and a tendency to exercise excessively. Other examples can include resisting rest days, training through illness or injury and exercising to avoid feelings of guilt or other negative emotions.

Since a commitment to training and drive to push yourself is an encouraged and expected behavior in sport, it can be hard to tell when the line is crossed. In sports where a low body weight is advantageous, it can be especially easy to blur the margins between training to enhance performance and training to control body composition. On the severe end of the spectrum, exercise addicts continue to train regardless of any injury or illness that results. British Athlete Anna Boniface talks about this candidly here. 'The Truth About Exercise Addiction' by Katherine Schreiber and Heather Hausenblas also explains this in depth here.

Lifestyle Factors

When it comes to the overall 'stress' that causes RED-S, it's not just those surrounding training and nutrition.

Sleep quality and quantity is a a commonly recognised component of effective recovery, as summarised by this excellent article, but the stress that comes from a whole host of other psychological and environmental factors is often overlooked. These can include: a high pressure environment (created by ourselves, coaches or an institution); being on the go all day; never saying ‘no’ to social opportunities; constantly travelling from one training session or competition to the next, and so on.

Each of these potentially stressful situations stimulate the same stress response, which you can read about in more detail here.

So, to helps identify where your other stresses might be coming from, you could try asking yourself:

  • Do I sleep for less than 8 hours per night on average?
  • Is my sleep broken or disturbed?
  • Do I feel stressed quite a lot of the time?
  • Am I often nervous, anxious or worried about things?
  • Do I find myself rushing from one activity to the next, most days?
  • Do I find it hard to say ‘no’ to things and end up packing out my diary?
  • Do I find it hard to sit still and put my feet up?
  • Do I set extremely high expectations for myself?
  • Am I dealing with any 'general' life stressors like exams, moving, going through a divorce, bereavement, getting married etc?


Angeli A, et al. (2004). The overtraining syndrome in athletes: A stress-related disorder. J Endocrinol Invest. 27(6):603-12.

Torstveit, M. K., Rosenvinge, J. H., & Sundgot-Borgen, J. J. (2008). Prevalence of eating disorders and the predictive power of risk models in female elite athletes: A controlled study. Scandinavian Journal of Medicine & Science in Sports, 18, 108–118. doi:10.1111/ j.1600-0838.2007.00657.x

Currie A. (2010). Sport and eating disorders - understanding and managing the risks. Asian journal of sports medicine, 1 (2), 63–68.