Food for thought: Why under-eating could be putting you at risk

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We all know that cycling is an energy-intensive activity. Long rides can leave you feeling flat and empty, with a big meal needed to replace all the energy you’ve expended. But for so many riders (racers in particular), the pursuit of leanness can make it tempting to eat less than you need to, to keep your weight down.

As dietitian Alan McCubbin writes, creating an energy deficit might land you with you a range of unwanted symptoms, starting with lingering fatigue and getting more serious from there.

Quite soon after I started working with cyclists, I started getting a bunch of them coming to me for advice. They’d dug themselves into a hole of fatigue, and they couldn’t get out. Many of these riders most likely got there due to a syndrome known as Relative Energy Deficiency in Sport (RED-S).

While unexplained fatigue is the most common reason that cyclists seek help for this issue, there are far more serious consequences of long-term energy deficiency, including poor bone health, mental health issues, infertility and others.

Some of the athletes I saw had elements of disordered eating, but for others it was simply a lack of awareness of how much they should be eating for their training and racing.

Story Highlights

  • Relative Energy Deficiency in Sport (RED-S) describes a lack of energy coming in from food, relative to the amount the body needs due to training demands.
  • RED-S can lead to a range of issues including a weakened immune system, poor physical and cognitive performance and cardiovascular, gastrointestinal and psychological issues.
  • Concerned riders should consult a sports dietitian, nutritionist or a sport and exercise physician.

Relative what?

You may not have heard of RED-S before, but you may be familiar with an older term: the Female Athlete Triad. The triad, which was first described in the 1990s and updated in the mid-2000s, describes a relationship between:

1. Low energy availability (basically a lack of calories eaten relative to the amount of exercise being done), with or without disordered eating
2. Irregularities in the menstrual cycle
3. Lowered bone density.

By 2014, a group of experts in the field got together and agreed there was enough evidence to suggest that this phenomenon was not just seen in females, and was far more than a triad. They coined the term RED-S, which expanded on the triad model to cover other health and performance consequences of under-eating relative to training demands.

A key of both the original triad model and now RED-S is the concept of energy availability.

You’re probably familiar with energy balance — the difference between the number of calories eaten and the amount your body expends in a day. Energy availability looks at the same concept through a different lens. In this case, it’s the energy available for the body’s normal physiological functions, once the calorie cost of exercise has been taken into account.

In other words, when you go for a ride you consciously choose for your muscles to expend energy. Energy availability is what’s left from the food you’ve eaten for your vital organs to do their normal thing.

When energy availability is low, we are evolved to conserve energy. Rather than risking starvation when food is scarce, our body instead begins to temporarily turn down physiological systems that are not essential for day-to-day survival. This allows us to conserve some calories, keeping us alive longer until we can find some more food and increase energy availability once more.

The first and most obvious system to be turned down is the reproductive system. No point getting pregnant when there’s not enough food around to sustain yourself, let alone a baby. Besides, things like brain function, keeping your heart and lungs working, and so forth, are quite rightly the priority.

Low energy availability is a relative lack of energy to fuel normal body functions. This can be caused by either an increase in training without compensating through increased calories eaten, from restrictive dieting relative to training load, or a combination of both.

We see reduced reproductive function in both women and men during periods of low energy availability. For women, the most obvious sign is irregular (oligomenorrhoea) or completely absent menstrual cycles (amenorrhoea), although identifying this can be complicated by use of the contraceptive pill.

Many female athletes have often viewed ammenorrhoea as normal or even a positive sign that they’re working hard in training. Unfortunately, the cause of menstrual cycle irregularities in this case is a reduction in reproductive hormones, which has a flow-on effect elsewhere in the body. Some of the female cyclists that have come to see me haven’t had a period for several years and the same is true for many professional cyclists.

The effects of low energy availability are harder to spot in males, as there’s nothing as obvious to look out for. But it is now generally agreed that a similar phenomenon occurs in men, which can be indicated in a blood test by reductions in testosterone to the low end of what is considered clinically normal. And like females, reductions in reproductive hormone function in males can also have flow-on health effects, besides infertility or lack of libido.

It’s important to emphasise that the problem here is low energy availability, not weight loss and not being too lean. They can certainly occur together, but not necessarily. Because of the reduction in calories expended during periods of low energy availability, it is possible to experience RED-S without any loss of body weight.

Over-training and RED-S: one and the same?

With similar symptoms and potential causes, it’s tempting to think that the phenomenon defined as over-training syndrome and the concept of RED-S are in fact two sides of the same coin. In this case over-training syndrome is viewed in terms of excessive training load, while RED-S is seen as an inadequate nutritional focus. Some in the field of sports science and sports medicine speculate that they are indeed the same, but I think there’s a lot more water to go under the bridge of science before we can draw that conclusion.

Consequences of RED-S

Besides the reduction in reproductive function due to altered hormone levels, it’s now recognised that there are a lot more health and performance implications of low energy availability. The Female Athlete Triad recognised the impact on bone health, as the same hormones effect the maintenance of bone mineral density. This is one of the reasons that cyclists (and other endurance athletes) are thought to be at particularly high risk of low bone density, and in many cases osteopaenia or full-blown osteoporosis at a relatively young age.

Many of the cyclists with RED-S I’ve worked with have been identified as osteopaenic (having low bone density) in their early 20s, and recently one British runner shared her story of being diagnosed with osteoporosis at age 20. It is not surprising also that athletes with RED-S are far more likely to suffer from stress fractures compared to their colleagues with normal energy availability.

But the concept of RED-S expanded this concept further, recognising a whole variety of potential health and performance concerns from under-fuelling relative to training needs. These include a reduced metabolic rate, a weakened immune system, poor physical and cognitive performance, impaired recovery from training, cardiovascular, gastrointestinal and psychological issues (poor mental health can contribute to RED-S, but it can also be a consequence), and delayed growth or development in teenagers.

Whilst there was some initial controversy of the inclusion of these extra facets beyond the original triad, what the RED-S model has achieved is a renewed focus on research on all of these areas, in both males and females. Some initial results have started coming through in this area, with more expected to follow as interest within the scientific community grows.

Potential health consequences of Relative Energy Deficiency in Sport (RED-S). Note that the Female Athlete Triad is indicated, and that psychological problems can be both a factor contributing to RED-S, or a consequence.

Measuring RED-S

Measuring energy availability isn’t as easy as it sounds. Estimates of how many calories someone eats are notoriously inaccurate, and there is debate on what to include when measuring exercise energy expenditure. Do you only include what happens on the bike, or does a walk to the train station count? Do you subtract the calories your body still would have used if you were sitting on the couch instead of riding?

Indeed a recent study suggested that attempts to measure energy availability (by experienced researchers in this field no less) were less accurate in predicting the athletes at higher risk of bone injuries compared to simply assessing the menstrual cycle of females, testosterone level of males, and their current bone mineral density using a DEXA scan. Even a standardised questionnaire was more predictive than attempts to compare calories in and out.

Another surrogate measure that has been used to predict low energy availability is the measurement of Resting Metabolic Rate (RMR). An RMR of less than 90% of what’s predicted is suggestive of an issue, because one of the consequences of low energy availability is reduced resting metabolism.

Measurement of hormones probably provides the most objective marker of what’s happening in the body. However this requires a panel of multiple hormones to be tested, and they require expert interpretation by a sports physician with specific expertise in this area, since hormone levels can vary for other reasons, or even throughout the day.

Energy Availability – is it all in the timing?

Another reason that measuring energy availability mightn’t be a good predictor of bone injuries is that it’s being done over the wrong timeframe. Whilst measuring things over a 24-hour period is easy for us to get our heads around, new research has suggested that in fact it may be the fluctuations over much shorter time periods that’s important for our health.

Two studies (one in males, one in females) have suggested that even with the same 24-hour energy availability, athletes that more closely matched their calories coming in from food to the time of the day they were expending the most, were less likely to have reduced RMR (males) or menstrual dysfunction (females).

This may also be part of the explanation of why endurance athletes tend to have low bone density – because most of our daily calories are consumed hours before and after a ride, not during those few hours a day when we’re actually expending them.

Potential performance consequences of Relative Energy Deficiency in Sport (RED-S). Note that reduced endurance performance refers to both aerobic and anaerobic performance.

What to do if you’re concerned?

As the saying goes, prevention is better than a cure. Avoiding periods of very low energy availability is a great first step to staying healthy, happy and riding your bike. This means planning to eat more during hard training blocks, avoiding dramatic crash diets to try and lean up for a race, and being aware of what your body’s telling you.

If you’re trying to get as lean as possible for racing, it’s crucial to have a good idea of what’s realistic for you and to have a careful plan of how to achieve that whilst still fuelling your training. I’ve written previously on this topic, and on the culture and psychology of eating and cyclists.

There are a few approaches you can take if you’ve identified with one or more parts of the RED-S model and are concerned. Some athletes go straight to a sports dietitian or nutritionist to address the eating side of things – after all, low energy availability is the primary cause of RED-S, so to address these problems an athlete needs to either eat more or train less (or a combination of both).

Others will go to a sport and exercise physician with expertise in this area for a full examination and testing first, especially if an injury or illness has triggered the alarm bells. Most of the health concerns associated with RED-S could be caused by other medical conditions that need to be excluded too.

Ultimately it probably doesn’t matter which approach you take, as you’ll probably benefit from the expertise of both professions, and the two regularly refer between each other in these cases. Personally, I’ve worked with riders in both scenarios, and I haven’t noticed a difference in outcome between the two.


Relative Energy Deficiency in Sport is a relatively new term that describes a lack of energy coming in from food, relative to the amount the body needs due to training demands. It can be caused by under-eating relative to training load, increased training without an increase in food to compensate, or a combination of the two.

It can occur to any level of rider, male or female, with any body size or shape, and the consequences for health and performance are far-reaching. Cyclists who are concerned or are struggling with these issues are advised to seek help from suitably qualified health professionals that have specific knowledge and experience in this area.

About the author

Alan McCubbin is an Accredited Sports Dietitian, Accredited Practicing Dietitian and past president of Sports Dietitians Australia. He is currently studying his PhD in sports nutrition at Monash University. He is also the founder of Next Level Nutrition, an online sports nutrition consultancy through which he works with a range of athletes from recreational to Olympians.

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