RED-(S) Alert

Managing your energy levels is a careful balancing act that can affect every part of a runner’s life. Here’s what you should know…


Relative Energy Deficiency in sport – RED-s – is the evolution of what used to be called the Female Athletic Triad, a syndrome observed in women which included low energy availability, menstrual dysfunction, and low bone mineral density.

RED-s still features those three outcomes; but it has since been recognised that it also occurs in men, and that it affects far more than just reproductive function and your bones.

What hasn’t changed is that RED-s develops when our energy intake (the food we consume) falls far below our energy demand (what we spend ‘doing life’, and running) for a long period of time. In other words, when we are chronically under-nourished. 

It’s so important to pay attention to the general principle of matching your energy intake to your energy demand.

Obviously our bodies require energy from food to perform everything, from the most basic functions to powering us to half-marathon PBs. The more you train, the greater the energy demand; and once you get up to say, 10 hours a week of running, your daily energy requirements are very large indeed.

Failing to meet these demands causes the equivalent of a financial crisis in your body – you’re now spending more than you earn. Expenses exceed income, and something has to give. 

What follows, though undesirable, is actually a clever series of events. Your body is ‘smart’ enough to adapt to the self-inflicted energy crisis. It systematically shuts off, or dials down, functions that are not totally essential to your survival.

In a physiological sense, you’re being ‘load shed’ (thanks Eskom); your (voluntarily) limited energy supply is redirected towards crucial functions. But this can only be done by sacrificing some non-essentials. First among them is often the extremely energy-costly reproductive function; it makes sense to put this ‘luxury’ on hold while energy is scarce. 

During times of starvation in our hunter-gatherer days, this enabled our survival. In the modern age, it leads first to affected menstrual function, which has numerous knock-on effects for hormone levels and bone health. Eventually, osteoporosis may occur. 

In men, RED-s may affect libido, and potentially bone density; which is why distance runners – already at risk, because of the repeated impacts of running – often develop stress fractures as the first sign of a Relative Energy Deficit.


Immune function is another sacrificed function. We can become chronically sick – mildly at first, more severely later; and persistent infections are a classic sign of energy insufficiencies. 

If this situation persists, the hormone or endocrine system is affected, which has all manner of knock-on consequences – including negative effects on growth and development, a particular problem for young adults. Gastro-intestinal function, the cardiovascular system and metabolism are also affected; the end result is a system in chaos – and ultimately, disrepair.

This can be very difficult to remedy. It is reversible; but it often takes a long time. So as is often the case, prevention is better than cure. Given that RED-s is the result of energy deficiency, the key to prevention is to maintain energy balance as far as possible.

This is simultaneously simple and complex. It’s simple in the sense that RED-s is caused by chronically low energy availability. So provided your diet keeps pace with your training, and fuels you to recover and rebuild, you stay in balance and avoid the issue.

It gets complex because a lot of people are able to cut energy intake significantly, for quite long periods at a time, and still avoid these harmful consequences. Through a combination of the miracle of physiology and our remarkable stubbornness, we can keep training and performing – often quite well, despite the energy crisis we impose on ourselves! 

But eventually, again, something must give. When this point will arrive is impossible to know, and it varies from person to person and circumstance to circumstance. 

That’s why it’s so important to pay attention to the general principle of matching your energy intake to your energy demand. If you ramp up training volume, you must eat more. Or you must eat differently, to keep pace with the ‘cost’ of that increased training. 

If you’re trying to lose weight by restricting kilojoules, fine; but be very careful that you do this intelligently. Type of food, timing and volume need to be just right, and don’t do it for too long at once. And not in combination with aggressive training increases; pick your battles. Because your body won’t. All it sees is the energy crisis. 

It’s also so important to be mindful of the signs: menstrual dysfunction, fatigue, persistent colds and mild infections are the earliest warning signs. 

Pay attention to those; and if they happen, act promptly in two ways – cut your training volume and intensity by at least 30%, and make smart dietary changes to increase energy intake, and to keep that scale in balance.  


Prof. Ross Tucker is one of the world’s top sports scientists, and the co-host of the Real Science of Sport podcast.
Follow Ross: @scienceofsport (Twitter) RossTucker10 (Instagram)

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