Reach Your Peak!

Can running with nothing in the tank give you the edge when it comes to endurance? RW tells the fuel story of running on empty

The new way to unlock more endurance. You may have to empty your tank…

Any runner familiar with early-morning alarm calls knows there’s a unique joy to be had from rolling out of bed and hitting your stride. Once you overcome the initial bleary-eyed struggle with consciousness and the intense urge to dive back under the duvet, a serene world of crisp air, birdsong and solitude awaits. When a run is the first thing you do, it has the power to de-stress and set you up for the day. Do it before breakfast and it may also make you a better runner.

A growing body of research suggests that running after an extended period without food – ‘fasted running’ – can increase our ability to burn body fat as fuel. For the typical carb-craving runner – who can be overly reliant on a range of sugar-loaded products to maintain peak performance – becoming fat-adapted could be a particularly attractive proposition.

The fasting element isn’t quite the hardcore exercise in self-denial it may sound like, either. ‘Fasted running is usually what we talk about when people go running after an overnight fast,’ explains performance dietitian Renee Mcgregor (reneemcgregor.com). For most people, then, ‘overnight fast’ simply means our usual sleep.

It’s a relatively painless and effective way to reduce your body’s available glycogen – the stored sugars (usually provided by carbohydrates) that are converted into energy to fuel our muscles. Depleting that carb availability forces the body to tap into your body’s alternative – and much larger – energy reserve: fat. Free fatty acids in our blood and liver can be converted to usable energy. This process is known as gluconeogenesis, which, as Mcgregor explains, ‘quite simply translates as the “production of new glucose” – it is the body’s ability to produce glucose from non-carbohydrate sources such as lactate, glycerol and amino acids.’ In short, by training in a fasted state you can increase your body’s options when it comes to finding energy.

FUE L E F F ICIENT BEFORE WE GO on, a quick word on carbohydrates and why, for many runners, they reign supreme. Once eaten, carbs are quickly and easily broken down into the aforementioned sugars – one of which is glucose (aka blood sugar) – which provide your body with a readily available form of energy. ‘The body requires less oxygen to burn carbohydrate as compared to protein or fat,’ says Mcgregor, ‘so carbohydrate is considered the body’s most efficient fuel source. Carbohydrate is increasingly vital during high-intensity exercise when the body cannot process enough oxygen to meet its needs.’

Glucose, however, is burned quickly and must be topped up regularly. Failure to do so can result in a sugar crash and, eventually, hitting the dreaded ‘wall’. Equally, ‘in a high-carb regime, favoured by most runners, the body will get used to preferentially using carbs as an energy source’, explains Don Maclaren, emeritus professor of sports nutrition at Liverpool John Moores University. In other words, carbohydrate becomes indispensable. And there lies the great lure of fasted running: by training your

For most people, then, ‘ overnight ’ fast simply means our usual sleep

body to become fat-adapted – to tap into its natural fat reserves – an internal energy store is always close at hand; gels no longer have to be the lifeblood of long runs and longdistance races.

‘Some people are confused about what fat-adapted running actually does,’ says Mcgregor. ‘They think that becoming fat-adapted means you will

have better body composition and you lose weight, but that’s not really what it is. Put simply, to be a good fat-adapted athlete means that you’re more efficient at burning fat as fuel.’ She emphasises that you will still rely on carbohydrate for energy in your shorter races and in your hard training sessions. ‘When you are working at speeds and intensities that require an exertion of around 8/10 or more (80 per cent of maximum heart rate or higher), carbohydrate is the only fuel source that is readily available to provide energy to the working muscles.

‘While our bodies are perfectly capable of using fat stores to produce fuel, it is a much slower process, which means the energy is delivered at a slower rate. This, of course, limits how fast you can run. In order to run fast, muscles require a quick and easy source of energy, which is only available through carbohydrate.’

The point is backed up by Maclaren, who says ‘glycogen is essential’ for running well above a certain intensity – ‘even for a sprint finish at the end of a marathon’.

Therefore, fasted running to train the body to become more fat-adapted is best suited to the long-distance runner. ‘I would be cautious about recommending it [fasted running] for 5Ks, 10Ks and half marathons,’ says Mcgregor, ‘because they tend to require an intensity that is just above your threshold effort, and to maintain such an intensity your muscles will rely purely on carbohydrate for fuel. When carbohydrate becomes limited, you will need to slow down.’ For marathon and ultramarathon runners, however, who tend to run at a low-enough intensity for the body to take its time in using the less accessible fat reserves, fasted running can be very beneficial.

‘If you were to do an exercisetolerance test – one in which you incrementally increase your speed – someone who isn’t fat-adapted is going to burn more carbohydrate as they get closer to their lactate threshold (the intensity of exercise at which fatigue-inducing lactic acid begins to accumulate in the blood at a faster rate than it can be removed). With someone who is fat-adapted, they will still follow that same pattern, but they will be able to burn fat for a little bit longer.’

There’s even evidence to suggest fasted training can improve the capacity of your muscle-glycogen stores, therefore improving energy supply during high-intensity, nonfasted exercise. Research from New Zealand showed that cyclists who completed early-morning fasted exercise increased their glycogen storage capacity by up to 50 per cent.

One fasted run, however, does not make a fat-adapted runner. ‘Ultrarunners and marathon runners are naturally fat burners as an adaptation to their training,’ says Maclaren. ‘They have greater fat stores within their muscles, which they use preferentially and so spare the limited glucose. But fat-adaptation requires training in a fasted state over weeks, months or longer – in these instances the body adapts to greater fat burning and very low carb burning.’

Fasted training is best reserved for low- intensity sessions

who uses a lot of fasted running still relies on carbohydrates as fuel for harder efforts.’

Britton’s point is important: fasted training is best reserved for lowintensity sessions – slow runs of anything up to 90 minutes, depending on your fitness – and should be limited to one or two runs a week. Any more and the quality of your training is going to suffer, because you will struggle to hit the intensities needed for continued improvement.

Some research has even suggested fasting can impair performance. One study, in the Clinical journal of sport Medicine, followed 18 well-trained middle-distance runners during the Islamic holy month of Ramadan, during which they fasted in daylight hours. The participants completed two sessions, one before Ramadan and one during the final week of fasting. The sessions involved several cardiovascular and muscular tests that culminated in a 5000m time trial. At the end of Ramadan fasting, a decrease in maximal oxygen consumption was observed, as was a five per cent decrease in performance over 5000m.

Maclaren argues that this shouldn’t put you off they idea of fasted training. ‘The literature regarding Ramadan is divided and many more recent publications show either no effect or a small negative effect,’ he says. ‘It depends on two factors: the quality of the athletes (good ones don’t tend to have problems), and the type and amount of food consumed during the night period. This latter point really is crucial, since eating too little or too much carbohydrate or fat will significantly affect subsequent performance over a 28-day Ramadan period.’ And there’s also a significant difference between simply skipping breakfast, running and refuelling afterwards, and fasting from sunrise to sunset. The main issue for the runners in this particular study, says Maclaren, was the fact they were eating just one large meal a day.

Performance aside, there are also some concerns around increased injury risk when running on empty. Again, the key point here is that fasted runs are best reserved for low intensities. ‘I would be wary of fasted training for athletes engaged in short-distance and power events, in which repeated sprint bouts could be compromised with lower muscle glycogen and a greater likelihood of strains and muscle pulls from fatigue,’ he says.

Mcgregor also warns of the possibility of fasted running inhibiting recovery. ‘If you start doing all your sessions in a carb-depleted state, you will put a lot more pressure on your body and that can affect your biomechanical control and your hormonal control, which can affect your ability to recover after each run,’ she says.

That ability to recover, even after an easy fasted run, is largely dependent on ensuring your body has the tools it needs – protein being prime among them – to begin the repair work. As Weir says, ‘Even a gentle run breaks down muscle fibres and you need to provide your body with the protein it needs to kick-start the recovery process.’ (See Re:fuel, right) for five excellent postrun food options.)

Along with the expert consensus that runners competing over shorter distances should focus on running fully fuelled, there’s also a prevailing view that running on empty isn’t for beginners. As a new runner, the simple act of running can be hard enough, without the added complication – and increased difficulty – of trying to do so with limited energy.

carbohydrate availability (the amount of glucose in your system at any one time), you maintain the ability to use both fat and glycogen for fuel when needed. Over longer distances, such as the marathon, that is exactly what you want: you want to be able to use both.’

Lively debate continues over claims that fasted running can aid with fat loss. Maclaren believes ‘fasted running will result in lower body fat’, because ‘after an overnight fast and no breakfast, the body is already in fat-burning mode, so when exercise is undertaken the fat burning is preferential.’ It’s a point backed up by a study published in the British journal of nutrition that showed exercising in a fasted state can burn up to 20 per cent more fat than exercising after eating.

Mcgregor, though, argues that weight loss is not the result of fatadaptation, but of creating a calorie deficit. ‘There’s a real misconception that going out fasted is somehow going to make you lose weight, because of the idea that you’re burning fat,’ she says. ‘But the reality is you only lose weight if you under-consume calories.’

And, as with attempting any kind of high-intensity training in a fasted state, under-consuming calories is eventually going to result in a dip in performance, as well as the increased injury risk associated with inadequate recovery. With time, fasted running will help you burn more fat – that, after all, is the aim of it. But if you’re running on empty with the goal of becoming a better endurance athlete, any calorierelated weight-loss ambitions should probably be put on hold.

EMPTY PROMISE AS WITH ANY NEW training strategy, fasted running should be adopted gradually and with care if you want to get the best results and stay healthy. Some runners may be able to run for an hour or more the first time they brave a fuel-free outing, but others will struggle to get half a mile before lethargy and ‘hanger’ (hunger + anger) set in and put an end to the session.

If your performances have plateaued, or you’re a long-distance runner looking to hit some better times, fasted running can help you improve endurance and reduce reliance on midrun energy products. To avoid stalled progress, though, fasted runs should supplement, rather than dominate, your training. The full benefits of running on empty are only realised when, for the most part, there’s fuel in the tank.