Energy Availability for Athletes - RED-S

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The Female Athlete Triad

Many people get into sport and exercise to either lose weight or to maintain a healthy weight. Often, a person has been led to believe that the only way to lose weight is to do lots of exercise and this leads them to do go to extremes; doing lots of high intensity and high volume exercise while eating a very low-Calorie diet. This isn’t sustainable because a high energy output coupled with a low energy intake is a recipe for disaster. The graphic below highlights the areas that become affected by low energy availability. Note the red triangle. The 'female athlete triad' was an evidence-based type of overtraining syndrome that only affected biological women.

Mountjoy. M et al (2018)

In times gone by female athletes with concerns over body composition, whether that was due to considerations over power to weight ratio, such as in endurance sports or aesthetic reasons, such as in physique competitions and even ballet were often seen to develop these symptoms. This was where a low energy availability resulted in physiological complications such as low bone mineral density and amenorrhea. However, researchers re-categorised this because low energy availability doesn’t just affect female athletes. It’s easy to see why the condition wasn’t considered a problem for men because men don’t have a menstrual cycle and losing that is a really obvious sign that something is badly amiss. Therefore, the female athlete triad was out and RED-S (Relative Energy Deficiency Syndrome) was in. 

Relative Energy Deficiency Syndrome

RED-S was first defined by the International Olympic Committee in about 2014 and was revised in 2018. Research was done on a number of sporting populations, with sports where body weight plays a big role, either for optimising power to weight ratio or making weight for competition. Burke et al showed that RED-S shows up in male road cyclists, rowers, combat sports competitors, East African runners (not because of their ethnicity but because of their approaches to training and nutrition) and jockeys, all sporting disciplines not exclusive to female athletes. I'll discuss the stratification of these populations in a little more detail in a coming section, read on.

Looking at the graphic above you can see a number of physiological aspects that can lead to RED-S. While, below, you can see the typical signs and symptoms that athletes should be mindful of. Any or all of these could be a sign of RED-S.

Mountjoy. M et al (2018)

Just because men don’t experience a loss of menstruation, they are vulnerable to the other symptoms. If you regularly experience any of these symptoms as a result of a high training volume you need to start taking your diet and recovery more seriously. It might just be that your post-training DOMS become more severe, or hang around longer. That could be the start of it.

Anecdotally speaking, I often hear friends or new clients saying how they have low energy, increased fatigue, increased joint pain, reductions in performance, disruptions in sleep and when I ask them what their Calorie intake is, they often look at me blankly. Let me explain why this is important to get right.

Low Energy Availability

Relative Energy Deficiency Syndrome (RED-S) is caused by a low energy availability (LEA). In other words, the athlete simply isn’t consuming enough Calories to fuel their body during exercise. This, as mentioned above, doesn’t just affect exercise performance. When Calorie intake is too low your body cannot perform its normal biological functions, quite literally, you are using so much energy from exercise that your body cannot function efficiently and starts to break down. Think of it like a car that is being driven with very little petrol in the tank, sooner or later the engine will cut out and, if you do this on a regular basis, the catalytic converter, carburettor or starter motor might burn out and stop working. The same things happen to your body and, over long periods of time, can cause considerable harm to your health that could lead to severe illness. For example, low bone mineral density could lead you to develop fractures or osteoporosis. Or there are the psychological considerations too, eating disorders and depression are common and mental health scars often run deep.

Stratification of populations is important too. Female athletes are more likely than male athletes to develop disordered eating or clinical eating disorders. African American and black African athletes are less likely to experience bone fractures than Caucasian athletes according to the data. That’s not all, eating disorders like bulimia, for example, are known to be common among female fashion models and Ballerinas, but it’s also common among male Jockeys who have similar body composition concerns. If you or your athletes start to display obsessive attitudes towards food or food anxiety this could be a red flag that their mind is not in the right place, or that their body is under considerable stress. All athletes are at risk and not all athletes will display the same symptoms but if they have an energy availability below 30kcal per kg of fat free mass (FFM) they are in a LEA state and at risk.

Calculating Energy Availability

How do you avoid low energy availability and the problems associated with this? Well, you need to do a bit of maths. Because energy balance is an aspect of the laws of thermodynamics and physics is all about maths you have to run the numbers. The equation for energy availability is as follows:

EA: Energy Intake (EI) – Exercise Energy Expenditure (EEE) / FFM.

Less than 30 Calories per kg of FFM is a problem. According to Mountjoy et al (2018) 45kcal/kg FFM is ideal for healthy physiological function, the cut-off point for LEA of EA <30/kg FFM still might not be high enough to protect against amenorrhea in some females. Therefore, I would urge you to err on the side of caution, if you do this calculation and your score is 30 or less push it up closer to 35 or more. Especially if you are a female with a very low body fat percentage. This is really common among female bikini competitors - I have seen women in peak week eating around 800kcals per day, which is ludicrously low. This is why I don't work with bikini competitors, because I don't consider the pursuit of dangerous levels of leanness to be a healthy performance goal. If you are a female who has lost her menstrual cycle you will need to bring your kcals up perhaps as high as 45kcals per kg for weeks or months before your period returns.

It is important to understand that, although you ought to factor exercise into your predictive calorie equations to estimate your energy requirement it’s the number of Calories burned during high effort sessions that could be the issue, and this means upping your intake on those days. Therefore, when doing Calorie calculations, you should separate activity and exercise. To do this you have to work out what your energy expenditure from general activity is. Here’s how:

First estimate your resting metabolic rate: kg x 22 (female) or kg x 24 (male)

Then decide your physical activity Ratio (PAR) from non-exercise activity. Unless you are extremely active in your work this is likely to be somewhere between 1.1-1.3.

Let me illustrate this using myself as an example:

  • 75kg x 24 = 1,800kcal (RMR) x 1.2 = 2,160kcals (EE).

Now I need to estimate the Calories I have burned during my training. Using the Metabolic Equivalents, or METs, as outlined in Ainsworth’s Compendium of Physical Activities, an example of which is shown in the graphic here.

Böhmer et al (2014)

Having estimated my energy expenditure, I need to work out how many Calories I burn in a training session. Let’s say I go for a 3-hour cross country bike ride, I can look that up on the compendium (here) and see that ‘mountain biking’ is 8.5 METs. This is where things get a bit more complex.

Bear in mind that METs are only an estimate based on averages and don’t specifically stratify populations. METs is based on kcals/kg/hour and doesn't allow for rest periods, that’s not an issue with an activity like cycling but for team sports or gym activities you have to take into account how much of the time spent on that activity is rest and how much is exercise. Also, and this is the bit that a lot of people fail to understand, you have a basal metabolic rate and if you don’t take that into account you could possibly over estimate Calorie expenditure. But, this is simple, your basal metabolic rate is roughly 1kcal per kg of body weight per hour.

Continuing the example, I gave above, based on my own body weight. The energy expenditure for my ride will look like this:

  • 75kg x 8.5METs (638) x 3 hrs = 1,914kcals
  • But, my basal metabolic rate over those three hours would be: 75 x 3 = 225kcals
  • 1,914 – 225 = 1,689kcals

Remember my initial calculation for Energy Expenditure? I need to combine these two figures together.

  • 2,160 + 1,689 = 3,849kcals. Remember, this is total energy expenditure for the day.

Putting this all together I need to work out what my energy availability is for this day, using the energy availability equation (EI - EEE / FFM). My average Calorie intake is 2,600kcal per day. With a BW of 75kg and a BF% of 16% my fat free mass (FFM) is (75 x 0.84) = 63. The exercise energy expenditure, as already calculated is 1,689kcals, so the energy availability equation looks like this:

(2,600 – 1,689) 911 / 63 = 14.5. This is well below the recommended EA<30 recommendation set by the IOC. If I were to consistently under eat by that much I would initially struggle to recover in time for the next session and, as stress on the body continues, I would start to see manifestation of some of those symptoms listed above. But, if I was to consume more Calories, let’s say 4,000kcals on that day my energy availability ratio would be 34.

That's all super complicated. So, if you are an athlete with a high training volume whose days vary from low to medium to high activity you might try a simpler method like this; simply work out your Calories on a day to day basis using an appropriate PAR. For example a rest day might be BMR x 1.1 a medium  training day might be BMR x 1.4 and a heavy training day might be BMR x 1.9. Again, these are estimates and it will take a little trial and error to get this right.  Don't just crunch your numbers into a calculator and assume that is 100% accurate. It isn't. Calculating fat free mass is difficult without access to the right equipment so let's simplify it. If you are a relatively lean athlete, just go with your current bodyweight. If you are an overweight athlete this might be as much of an issue for you due to the extra stored fat, as long as your Calories are above your resting metabolic rate and you are hitting your daily protein targets.

But - and this is important - most people work out the Calories based on average total daily activity. This means that on low activity days they probably eat more than they need and on training days they eat a bit less than they need, and over the week it probably balances out. It’s also worth pointing out that a person with a few extra kilos of body fat to spare has more energy stored in their body and is probably at a much lower risk than a very lean athlete with a very low BMI. So, in conclusion, this is worth knowing but unless you have dangerously low body fat percentages and very low body mass for your height and, possibly, have a somewhat disordered relationship with food or a potential body dysmorphic disorder you’re probably fine. Just make sure you eat to fuel your activity and manage your recovery correctly. Also, if you track Calories on an app that also records your heart rate or training, it will tell you how many Calories to eat back. These are always inaccurate, if you have accounted for activity in the initial equation you have already accounted for that activity and could end up eating the Calories twice, which brings other problems for the physique or power conscious athlete. Use the app to track your intake, but trust your maths to predict what those numbers ought to be on any given day. 

But to circle back to my initial point, energy balance matters, knowing how many Calories you eat and how many you burn are important considerations for even semi-serious athletes and don't let anyone tell you otherwise. I appreciate that this is all very complicated but it's worth knowing if you are a high performing individual. I tend to favour simpler approaches because the simpler an approach, the easier it is to understand and the easier it is to get it right. That's why you should outsource your nutrition to an evidence-based coach.

Coach Troy


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  1. The IOC consensus statement: beyond the Female Athlete Triad--Relative Energy Deficiency in Sport (RED-S). Mountjoy M, Sundgot-Borgen J, Burke L, Carter S, Constantini N, Lebrun C, Meyer N, Sherman R, Steffen K, Budgett R, et al.Br J Sports Med. 2014 Apr; 48(7):491-7
  2. MountjoyM, Sundgot-Borgen JK, Burke LM, et al. IOC consensus statement on relative energy deficiency in sport (RED-S): 2018 update Br J Sports Med 2018;52:687–697.
  3. Dr Nicky Keay. 2018 UPDATE: Relative Energy Deficiency in Sport (RED-S) BJSM March 2018
  4. Statuta, S. M., Asif, I. M., & Drezner, J. A. (2017). Relative energy deficiency in sport (RED-S). British Journal of Sports Medicine, 51(21), 1570–1571.doi:10.1136/bjsports-2017-097700 
  5. Burke, L. M., Close, G. L., Lundy, B., Mooses, M., Morton, J. P., & Tenforde, A. S. (2018). Relative Energy Deficiency in Sport in Male Athletes: A Commentary on Its Presentation Among Selected Groups of Male Athletes. International Journal of Sport Nutrition and Exercise Metabolism, 28(4), 364–374.doi:10.1123/ijsnem.2018-0182 
  6. Ainsworth BE, Haskell WL, Herrmann SD, Meckes N, Bassett Jr DR, Tudor-Locke C, Greer JL, Vezina J, Whitt-Glover MC, Leon AS. The Compendium of Physical Activities Tracking Guide. Healthy Lifestyles Research Center, College of Nursing & Health Innovation, Arizona State University. Retrieved [date] from the World Wide Web.
  7. Böhmer, Andreas & Wappler, Frank & Zwissler, Bernd. (2014). Preoperative Risk Assessment—From Routine Tests to Individualized Investigation. Dtsch Arztebl Int.



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