‘Bonking’. ‘Hunger flats’. ‘Hunger knocks’. ‘Hitting the wall’. Whatever you call them, they suck, so I’ve pulled together a guide to what your body goes through when it’s depleted of glycogen, with some practical advice on the different things you can do to avoid it ever happening again...

What actually happens when you ‘bonk’?

How much would you pay for this can of Coke, Webber? £10, £20, £50?

During a training camp for racing drivers in the late ‘90s, I remember lightly teasing soon-to-be Formula 1 driver Mark Webber at the summit of one of the mountains that punctuate the Lanzarote IRONMAN bike route. I was dangling a can of the world’s most famous fizzy drink in front of him as we took a brief break during a hard 112-mile training ride.

Poor Mark was, at the time, suffering with a severe ‘hunger flat’ - sometimes called a ‘bonk’ or ‘hitting the wall’ - because we’d been riding for several hours and his body had started to run critically low on stored energy to supply his working muscles.

I was implying that he might have been tempted to shell out up to 50x the RRP of the soda because of its high sugar content (~39g in a regular 12oz/330ml can), which was exactly what his body and brain were screaming out for in order to correct the crisis that they were undergoing in the moment.

In actual fact he just told me to “F-off”, but he didn’t waste a minute in downing the whole can once he got his hands on it!

I’m sure this is a scene many experienced endurance athletes can relate to. Most of us have been there at one time or another. Often at the side of a road in the middle of nowhere feeling like we’ve had our metaphorical plug pulled out and, in that very moment at least, willing to part with a significant portion of our net worth for a simple bag of Haribo or a Snickers bar to get us home.

Image Credit: Pixabay (copyright free)

When ‘the knock’ comes-a-knocking, your legs turn to jelly, your stomach feels like it’s been sucked inwards, cognitive function declines and your sense of humour disappears altogether (as demonstrated by Mark’s response to my ‘joke’ about selling him the drink!)

In all, the experience sucks and, if it happens in a race, it’s usually ‘game over’ at that point.

The science of 'bonking'

Physiologically speaking, a bonk occurs when your glycogen stores are depleted to the point where they can no longer adequately supply your working muscles with the fuel needed to produce energy, or maintain your blood glucose levels. As a result, your body has to take drastic measures to slow you down.

Glycogen is the storage form of carbohydrate in your body and it’s composed of thousands of glucose molecules tightly packed together (‘a highly branched glucose polymer’). A good quantity of it is found in your liver, where it can be broken up to provide glucose for the bloodstream. But the lion’s share is stored in your muscles, where it can be made available to power muscle contraction.

Glucose is the body’s preferred fuel source when the intensity of exercise gets above about 65% of VO₂ max; an intensity above which most serious endurance endeavours are undertaken. That’s because carbohydrates can be burned without large amounts of oxygen, which is in shorter supply when you’re working hard.

But, when the carbs start to run out (and if they’re not adequately replaced by regularly ingesting them), fatty acids need to be burned to produce energy instead. The burning of fat can release a hell of a lot of energy, but it requires way more oxygen, so a ‘bonk’ is really just nature’s highly persuasive way of slowing you down to the point where sufficient oxygen can be provided for you to use fat as your primary fuel source, so you can limp home and collapse on the sofa.

How much glycogen can you store?

The glycogen storage capacity for the ‘average’ human body is generally quoted as being around 600g. This is highly variable from person to person based on size, fitness level, eating habits and any recent exercise undertaken. About 100g of glycogen is found in a fully stocked liver and the rest is almost all found in the muscles, which makes sense as that’s where it’ll ultimately get used to power movement.

According to a recognised expert in the field of carbohydrate fuelling, Bob Murray, a trained athlete can expect to store up to 50% more glycogen in their muscles than an sedentary individual. And a ‘super compensated’ athlete (more on that later) can store another 50% more again.

How low can your glycogen stores go before you bonk?

In all, glycogen generally provides about 90 to 120 minutes' worth of endogenous (stored) fuel for pretty high intensity exercise before levels drop to the extent that performance is substantially impaired.

Interestingly, this is why ‘hitting the wall’ is so common in the later stages of a marathon - that 18-22 mile zone is right when many people run out of gas.

Whilst there doesn’t seem to be a total consensus on exactly how low your glycogen stores can drop before a bonk takes full effect (what constitutes the exact point at which you blow-up is, at the end of the day, something of a subjective call), Bob Murray suggests that performance is certainly going to be impaired when you get down below 50% of stores and massively compromised at 25% in this excellent podcast on the topic.

How to avoid bonking

When it comes to keeping ‘the knock’ away from the door, there are 2, possibly 3, main strategies you can deploy...

  1. Starting exercise with more glycogen on board
  2. Taking in some carbs via energy gels, chews and drink mixes during exercise, to slow the rate of glycogen depletion
  3. Training your body to burn more fat to spare your glycogen stores
Starting with more glycogen on board

Why carb-loading before a race is a good idea...

It stands to reason that, if you can increase your glycogen storage capacity, then your fuel tank will effectively be bigger and therefore (all other factors being equal) it will take longer to run out.

Training has a strong influence on how much glycogen your muscles can store and endurance athletes naturally develop a larger capacity to store glycogen than couch surfers.

Beyond that, the idea of ‘carb-loading’ has been heavily promoted since the 1960s, when some pioneering Swedish physiologists first proved that eating more carbs could result in extra glycogen storage.

Whilst it was initially thought that you needed to seriously deplete your carbohydrate stores before loading them up in order to stimulate this ‘super compensation effect’, it has since been proven that the depletion phase is largely unnecessary and that simply tapering your training and increasing your carb intake for a couple of days can have the same effect.

‘Super compensation’ can result in as much as 50% more glycogen being stored in the muscles of fit athletes, along with additional water, that may also have beneficial effects for your hydration status. So, it’s unsurprising that this is a relatively common practice for athletes to undertake before longer and harder efforts. It’s certainly something that I’ve done before most long races, with great success.

How to carb-load before a race

The specifics of carbohydrate loading protocols are hotly debated (and should be considered somewhat individual), but basically boil down to significantly increasing the total amount and % contribution of carbs to your dietary intake in the 2-3 days before competing, whilst observing a drop in your training load.

Most athletes favour loading up on carb sources that are lower in fibre, fat and other ‘stuff’ to avoid GI issues, so the traditional simple pastas, potatoes, white breads and processed snack foods are all solid options. You could even use the Carb Only Drink Mix for an easy option practically and digestively.

My own approach is usually based on having an extra portion of carbs with all my main meals during the last 2 days before a race.

For example, toast and honey as well as porridge or cereal for breakfast and an extra serving of rice, pasta or potatoes with lunch or dinner. Plus, I’ll eat plenty of high carbohydrate snacks like cereal bars, flapjacks or cakes between meals. You basically get to unleash your inner ‘fat kid’ for a short period of time!

Whilst carb loading can result in feeling a bit sluggish and bloated if taken to extremes, I’ve always favoured feeling a bit ‘full’ at the start of ultra long races over going in a bit too light. There may be a finer balance to strike with shorter endurance events, such as those lasting less than a couple of hours, where the trade off with feeling ‘stodgy’ at the start may not be worth it later on. All that being said, some individual experimentation is basically what you need to do here to learn what works for you based on the sport and event you’re doing.

The dangers of starting low on glycogen

It’s worth emphasising that it’s very possible to start races or (more likely) training sessions with significantly less than a full glycogen payload. This is probably a more common scenario than most athletes realise and happens if you’re somewhat depleted from prior training and/or you don’t get enough carbohydrates from your day-to-day diet.

Whilst starting somewhat ‘low’ on glycogen is inevitable some of the time, for anyone doing a high volume of training, the stepwise depletion of your glycogen stores over a handful of days can lead to a bonk happening surprisingly early on in a session or race, as this anecdote from pro-tour cyclist Nicolas Roche handily illustrates:

Taking care to refuel adequately on carbs after any long or intense workout is an essential part of the recovery process that should not be ignored if you’re in a big phase of training, or have races coming up.

Taking in some carbs during exercise

What does your body do with the carbs you take in?

Since the 1920s, it’s been understood that there’s a link between exogenous (supplementary) carbohydrate intake and the preservation of muscle glycogen and blood sugar levels during exercise.

Put simply, when taking in carbs (in the form of a drink, energy gel, bar or whatever), your body can absorb the sugars directly into your bloodstream and use it to stabilise your blood glucose levels, sparing your liver glycogen. It can also transport it to your working muscles to be immediately converted into energy, sparing your muscle glycogen at the same time. It’s like putting fuel into your car whilst you’re still driving.

How much carbohydrate should you consume during exercise?

There’s a whole lot of myth, legend, controversy and marketing BS around exactly what to eat and drink during exercise to elicit peak performance, but suffice to say the fundamentals are what really matters.

For shorter, or less intense, efforts

Most experts tend to agree that ingesting around 30g of simple carbohydrates per hour (what you'll find in a PF 30 Gel) is sufficient for activities of up to 2 hours, or for longer sessions when the intensity is low to moderate.

Whether this carbohydrate is better coming from a liquid, gel, chew, bar or some real food very much depends on the type of activity being undertaken, the relative intensity of it and, of course, personal preference. But, the bottom line is that getting roughly the right amount of carbs ‘down the hatch’ is what really matters in the first instance. I’ve written about the practicalities of getting your nutrition strategy right before here.

For longer, or more intense, efforts

For longer events, or when you’re working really hard, rates of 60g (or even 90+g) of carbs per hour have been shown to be beneficial.

Experimenting with different types of carbohydrates and rates of ingestion is something that most elite athletes spend a decent amount of time on in a bid to become better at sustaining high energy outputs for hours on end.

Can you train your gut to handle more carbs?

Recently, there’s been an increased amount of interest in the ability to ‘train the gut’ to uptake more carbohydrate during exercise, with reports of some elite athletes taking in over 100g of carbs per hour.

Practitioner and researcher Aitor Viribay Morales is at the forefront of this in his work with pro-tour cyclists and elite ultra marathon runners, which he discusses in detail with Mikael Erikson in this interesting podcast. The intakes he talks about in that chat certainly fit with the kind of intakes we’re used to seeing from the pro-tour riders and elite IRONMAN athletes we work with too.

There’s no solid scientific consensus on exactly how trainable the gut is as yet, but it seems like many experts are leaning towards the idea that uptake can be increased significantly, potentially as much as doubled, in a period of 4-6 weeks of training.

The protocol for achieving this is usually as simple as taking in gradually more carbs during key training sessions, over a period of weeks, whilst monitoring performance outcomes and GI issues. This article by Asker Jeukendrup provides a more comprehensive overview of the topic.

Image Credit: Emma Pallant-Browne ©

Training the body to burn more fat

We’ve covered the ‘carbs vs fat’ debate that’s been raging in sports nutrition for a while now, but it’s fair to say that increasing your body’s ability to burn fat at higher exercise intensities does present the possibility of sparing muscle glycogen and helping to avoid a bonk.

This approach involves training in a glycogen depleted (or ‘fasted’) state some of the time and potentially eating a lower carb, high fat (LCHF) diet. It’s an approach that has it’s hard-core fans and detractors, but is ultimately most likely to suit activities that are very long and low intensity and that don’t require sudden bursts of high intensity work.

The science seems to be clear that by undertaking training in the relative absence of carbs (and with increased fat availability), your body can massively up-regulate its ability to burn fat and to spare glycogen.

But this comes at the expense of being able to perform really high intensity work so your ability to sprint, or climb hills is compromised. Basically, there’s no such thing as a free lunch, even if it’s a LCHF meal!

So, whilst it’s something to bring into the conversation around avoiding bonking, it’s not a tactic that can work in all circumstances. If your activity demands periods of high intensity work, you’ll burn carbs whatever you do, and therefore you’ll be at risk of bonking sooner or later, if you don’t replace them.

How long does it take to recover from a bonk?

The literature suggests that, even with aggressive carbohydrate replenishment, it can take 24 hours to restore your glycogen stores to optimal levels. And practical experience suggests up to 48 hours or slightly longer in extreme cases, or when carbohydrate ingestion is anything other than massive.

If you’ve ever seriously hit the wall, you’ll know that it really is game over for that particular day. There’s no quick fix. Even if you ingest a load of carbs and slow your pace to try to recuperate, the damage is usually done and you’re not going to bounce back to pre-bonk levels of output very quickly. Rest and a serious commitment to eating is required.

The relatively lengthy recovery period from a bonk is one of the main reasons why it’s a good idea to avoid it happening too often in the first place! It puts a lot of strain on your body from a metabolic perspective and can reduce the overall effectiveness of your training regime quite significantly if you lose 1-2 days of decent output each time it happens whilst you get your energy levels back.

If it does happen to you then prioritising a large carb intake in the first few hours is critical and you then need to keep training intensity low, whilst snacking between meals, for a day or two until you start to feel like you’re back to where you need to be energy wise.

I hope this helps you avoid those dreaded ‘hunger flats’ going forwards. Oh and, if you’re wondering what happened to Mark Webber, he went on to have a brilliant career in Formula 1, only running out of fuel once in 2013 as far as I remember.

So, he obviously learned something from his experiences with that very expensive can of Coke in Lanzarote all those years ago...

Further reading