Fasted training: Does fast equal fast?

Hit racing weight and become a fat-burning machine? That’s the calling card of glycogen-depleted training. But, says a new study, it may not be all it’s cracked up to be. Rouleur slips into its rather fetching lab coat to dig deep into the science…

One of Team Sky’s marginal gains pruned the pasta to forge peak performance; in other words, cut carbohydrate intake to fire up the fatty furnace in search of leaner, faster riders – even Sir Dave Brailsford was getting in on the act, regaling to Strava that his recreational-riding regimen in 2020 included morning fasting, a primarily protein- and fat-rich diet, plus lots of low-intensity rides. “I’ve only eaten carbs from veg and had two bananas the rest of the time, so I’ve stuck to fat and protein, and only eaten between 11am and 8pm,” he said. “I’ve lost 3kg.”

Glycogen-depleted and/or fasted training has been a staple of many elite and amateur riders’ training plans for years, but recent anecdotal and empirical evidence questions its benefits. This is from a recent The Times interview with Geraint Thomas: “Having a crazy diet back then definitely affected your mood, but that’s a big change. We still have a few low-carb rides, but now I tend to fuel the rides a lot more. Then I’m not as hungry off the bike, so I eat less. When you’re riding, that’s when your metabolism’s working, and you’re just burning [weight]. That’s the new thinking; it has been working.” 

Then, a piece of research in the journal Nutrients ran wildfire through online socials that disputed recent dogma: “The results of the present study show that periodisation of carbohydrate versus a high-carbohydrate diet during five weeks of supervised exercise training in well-trained athletes does not influence maximal lactate steady-state (MLSS) riding and does not change substrate oxidation during a time-to-exhaustion test at MLSS intensity.” In short, there is no point in training fasted when fuelling ‘normally’ delivers the same benefits without any of the drawbacks. Is it time to power up with porridge before every ride? Rouleur digs into the paper while tapping into the expert input of an Ineos Grenadier plus an experienced cycling coach. 

Fanning the fatty furnace

“It’s simple – zero-calorie sessions boost your ability to burn fat,” triathlete Torbjorn Sindballe told me many years ago after finishing third at Ironman Hawaii 2007. The Dane then revealed that many of his sessions were fuelled on nothing more than water.

But why? Well, on a rudimentary level, the physiology is simple. As road cyclists, we’re looking to crank up fat metabolism – the amount of energy we derive from fat – because this enables us to burn more fat when carbohydrate stores are limited, which in long, brutal events like L'Étape du Tour or the Fred Whitton Challenge is often. At a deeper level, one of the key adaptations to a water-only session is greater mitochondrial volume in your muscles. That means all the enzymes and sites of aerobic metabolism are upregulated to a greater extent.

This phenomenon is known as mitochondrial biogenesis, and as a result of these changes, you become more efficient at using fat for fuel at a given exercise intensity, which means you produce less lactate – and less fatiguing hydrogen ions – so reserve glycogen for the harder parts of the race, like a pitchy climb or sprint to the next lamppost.

If you’re not enjoying the adaptations that are mooted, this phenomenon can be elevated even further, according to experienced coach Joe Beer. “There’s some great data by [Luc] van Loon [professor of exercise physiology at Maastricht University] and others looking at the intra-muscular triglyceride (IMTG) use in such sessions,” he says. “These droplets of fat that sit next to the muscle really get used and restocked to significant levels when you manipulate the duration and recovery afterwards. Leave the body with nothing for a while after a fasted session, and it will neatly restock IMTG around muscle fibres. I think that’s the missed part about maximising the benefits of this type of session. It's definitely not a case of ride fasted, get stupidly hungry, and then hit the recovery drink and massive amounts of carbs in the first one to two hours of recovery.”

To that end, Beer says that most of the cyclists and triathletes he coaches employ fasted training to some degree. “Typically, it’s a 45-minute to two-hour session before breakfast with nothing but coffee, maybe protein, beforehand. Then again, some do multiple sessions where they might start fasted and, in some cases, take nothing, but on other days, start fuelling once they’re 15 to 25 minutes into the session. It’d never extend so far that the output in calories starts to affect the second session of the day or the mood, so generally not over 90-120 minutes and always low intensity.”

The key word here is ‘intensity’, according to Javier Gonzalez, professor of nutrition and metabolism at Bath University and formerly lead performance nutritionist at Ineos Grenadiers. “If you’re doing low-intensity exercise, you’ll be burning more fat and less carbohydrate. As you increase the exercise intensity, you’ll be switching to more carbohydrates and less fat. How hard you can ride is influenced by muscle and liver glycogen levels. If they’re low, so too will blood glucose levels, which will make the session feel harder. That’s why it’s arguably more appropriate to pencil in glycogen-depleted efforts during the off-season when you’re looking to increase aerobic capacity and fat-burning capabilities by training at a moderate intensity.”

How many times a week? For elites, this could be several during the close season. For amateurs who are balancing life, once a week is the norm, and it’s worth it from the results Beer’s witnessed. “With the cohort I coach, this form of training actually makes some early morning sessions easier to focus on the muscle use, not their jostling breakfast contents. Secondly, it keeps effort low, and thus, the benefits of the base session are accrued instead of breakfast making the athlete feel they should train hard. Third, a small calorie deficit at the start of the day can make the fugal genes and the athlete’s sensation of fullness reach a proper baseline. Too many amateurs say they cannot lose body weight, yet they never reset this ‘training-empty’ sensation.”

All the pain for no more gain?

So, Beer is convinced of fasted training’s merits. Unlike the likes of Team Emirates’ nutritionist Gorka Prieto-Bellver, who led the recent study that compared the physiological and performance results of training on a periodised carbohydrate diet (PCHO) compared to a carbohydrate-heavy diet (HCHO). 

The researchers randomly assigned 17 trained male cyclists to two groups. The PCHO comprised nine cyclists with an average age of just under 25 years and a peak power output of around 379 watts. The eight remaining cyclists were in the HCHO group. They averaged just over 28 years old and generated a peak power output of nearly 391 watts.

Each group, whose normal training clocked up 15 to 20 hours each week, then followed the same five-week training programme, albeit fuelled by different nutritional strategies. Here’s a sample week:

  • Monday – gym for an hour and 20 minutes that included a 15-minute warm-up followed by 20 minutes working the core followed by weights of four sets of four reps at around 55-60% of one rep max.
  • Tuesday – three-hour ride at zones one to two.
  • Wednesday – two-and-a-half-hour ride that comprises a 30-minute warm-up plus four efforts of 10 minutes at zone three, recovery for 10 minutes in zones one to two, all finished off with 40 minutes at zones one to two.
  • Thursday – another two-and-a-half-hour ride, but this time at zones one to two.
  • Friday – gym for an hour and 20 minutes that included a 15-minute warm-up followed by 20 minutes working the core followed by weights of four sets of four reps at around 55-60% of one rep max.
  • Saturday – three-hour Saturday sojourn at zones one to two.
  • Sunday – another ‘easy’ two-and-a-half ride at zones one to two.

For feeding, when the low group hit the rides, they did so fuelled by a breakfast containing no carbohydrates, 0.4g per kg of protein and 0.2g per kg fat. This compared to the carb guzzlers’ 2g per kg of carbohydrate, 0.2g per kg of protein and 0.1g per kg of fat. Both groups undertook 13 low-intensity sessions, plus the other harder efforts and gym work.

The results? Both groups realised similar improvements in the maximal lactate steady-state (MLSS) test and had similar patterns of carbohydrate and fat utilisation during exercise, indicating that the type of diet didn’t really affect how their bodies used fuel. All 17, regardless of the diet, showed reductions in body fat and increases in muscle mass, and so questioned the use of glycogen-depleted training that, by its nature, the paper continued, “may cause adverse effects that will eventually compromise sports performance and health”.

Not immune to illness

How compromised was elaborated on by co-author Inigo San Millan, who worked with Prieto-Bellver at UAE Team Emirates in the position ‘head of performance’. San Millan’s recently taken up that role at Athletic Bilbao but is still an advisor with Tadej Pogacar’s outfit. He took to Twitter soon after the research was published to lament glycogen-depleted training. “I have seen enough athletic careers ruined and finished by carbohydrate restriction and related low-energy intake disorders as well as mental health issues,” San Millan Tweeted. “Truly heart-breaking.”

Beer concurs with the immunity repercussions of fasted training, though only if not closely monitored. “Some athletes pick up sore throats so we might then have them start fasted, but then fuel for most of the session. Some, probably due to low liver glycogen, can have visual impairment or even dizziness and concentration dips. The key is that you must slowly extend the duration and see an athlete’s best-case scenario to complete the sessions with low perceived exertion. It’s definitely not suggesting the ridiculous ‘eating is cheating’ runner’s mantra. Instead, ‘eating-is-timing’ must be the lesson we learn.”

Another lesson learnt stems from the population Prieto-Bellver and San Millan shone the physiological lens on – in other words, the elites. “Elites are so aerobically trained that they’re incredible fat-burners already and have little room to grow,” says Gonzalez. “They’re highly trained and genetically gifted, too. And that might be because they, for example, have more mitochondria. Or it might be that their genes respond very effectively to endurance training.”

San Millan often commented on Pogacar's genetic gifts, especially his ability to recycle lactate for energy and his capacity to burn fat for energy at high intensities. This highlights that while fasted training might have limited benefits to near-perfect elites, recreational riders could benefit more as they’re starting from a lower base. As long as Beer reaffirms, it’s ticked off with pragmatism.

“I’ve read that paper and I’d certainly not advocate training several times a week in a ‘train low’ scenario like that paper did,” says Beer. “It can just be mentally and physically hard. Ultimately, carbohydrates are king for the majority of sessions but let’s not forget that ‘fasted training’ is not only employed by amateur riders in search of physiological adaptations, but it’s also a matter of convenience. We often have to get the training done before our real jobs and responsibilities railroad the rest of our day. Not pausing to eat your Sugar Puffs or four croissants means you crack on with your training. You should have carbs to hand if you may need them but try the odd zero-carb ride (under 90 minutes). You’ll really love your delayed breakfast and a few genes may get triggered to be more thrifty.”

The elite study also lasted just five weeks and featured only 13 sessions on low carbohydrates, so conclusions aren’t set in stone. Women were also precluded from the study and, as women react differently to fasted sessions, again conclusions can’t really be drawn. The study itself also highlighted that high-carbohydrate and periodised carbohydrate feeding both have a place when planning a diet as long as they are carried out according to prescribed training.

So, where does that leave us? We’ll leave the last word to Beer. “If used judiciously, fasted training can be an effective tool. But keep tabs on how you feel. As a coach, ultimately I want to build athletes, not break them.”

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