As an athlete, you quickly realise that if you stress a muscle (say for example your quadriceps while cycling) often enough, it will change over time. It might grow bigger, change shape and perhaps feel firmer to touch. Muscles are an obvious example of the relationship between stress and adaptation given they’re on display and easily observable.

The truth is, our entire physiology is ever-evolving and adapting to its environment all the time, including our gut. Amazing then that ‘gastric distress’ is one of the most common issues reported by endurance athletes on race day.

Why do we allow a problem like GI distress to develop when we can train our gut long before race day arrives?

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Learning from experience

As a budding young multisport athlete in 2006, I remember racing duathlons on nothing more than bananas, simple glucose gels and sports drink. I was generally OK during races lasting a couple of hours, but anything longer was where things could go awry.

I would often find I’d either bonk and/or cramp or even end up vomiting precious calories up at a time when I needed them most.

In retrospect and armed with more knowledge following 15 years of racing, I realise I was guilty of one of two errors. Either under-nourishing at a time when I would have been burning through more sugar than a Creme Brûlée factory, or over-saturating my gut with sugar (gels and regular sports drink can create a hypertonic formula that becomes difficult to digest in the absence of enough water). Very rarely was I getting it right to the point of consistent energy and a happy gut.

Over time I accumulated enough bad experiences and researched sports nutrition articles to understand what carbohydrate targets needed to look like during endurance events going beyond two hours.

I arrived at 60-90 grams per hour which is a target that still holds validity today.

When it comes to the type of fuel, I'm more inclined to utilise a mixture of glucose and fructose gels to allow for greater carbohydrate absorption. I've also managed to train my stomach to tolerate more solid foods, such as bars and chews, which seem to promote better satiety alongside a more sustained energy release.

Image Credit: Dave Blow ©

Recognise the mechanical stresses of different activities

Experience has also taught me that the mechanical stresses on the gut during different modes of activity (e.g. running vs. cycling vs. kayaking vs. swimming) offer varying levels of fueling opportunities.

Running is certainly the hardest physical stress on my digestive system, whereas kayaking seems to allow an hourly carbohydrate target approximately double that of running.

These days my considerations are two-fold:

  1. How MUCH can I eat?
  2. And WHEN can I eat during various stages of a multisport or triathlon event?

When it comes to training this aspect of race day execution, it's again comparable to training our muscles. We don’t want to suddenly expose our muscles to a weight that's twice what they’re accustomed to, nor do we want to suddenly aim to put twice as many calories into our stomach. We need to build up (i.e. the progressive overload principal of athletic training).

So if 90 grams of carbohydrate per hour is your goal for race day, you might start with 40-50g per hour and add 10 grams per hour each week during your key long ride and/or run. Do this for the 4-5 weeks it takes to get to the top end of your target.

The key message here is, take a gradual approach to allow your gut to adapt and improve its ability to metabolise the fuel.

Image Credit: Finlay Woods ©

The relationship between exercise intensity, climate and carb intake

The next point I would make from experience is to respect the relationship between INTENSITY and glycogen usage.

If we're cycling at 150 watts and expecting to race at 250 watts, we can't expect our body to be able to process the same amount of fuel for each output. Does your car use the same amount of fuel to travel 50kph as it does 100kph? Of course not, so keep in mind the intensity you're pushing in training and how this matches up with the amount of fuel your 'tank' needs.

The same goes for climate. If you're training in a significantly cooler or warmer environment than you expect to race in, aim to simulate race day conditions during some sessions where testing your gut is a priority (for example, riding your wind trainer or running in extra clothing if preparing for a hotter race). The climate will affect how our body is able to respond to the fuel we're ingesting.

Generally, carbohydrates are of higher importance in cooler conditions while the hotter races tip the scales more in favour of respecting hydration (specifically fluid and electrolyte) requirements.

When should you train your gut?

The best time to trial race day levels of nutritional intake are during the key training sessions that simulate race day intensity.

A classic example might be in the 8 weeks leading into an IRONMAN when you might do a key endurance ride on the weekend which includes some long efforts riding to race target power or heart rates. This presents a perfect opportunity to test and train your gut with potential fuel options, while applying race day stresses on your body and gut.

Another example would be a key weekly run where you might include some race pace work as part of an endurance run or bricked run off the bike. Try those gels and fluid options here and look to gauge how your body responds. Even if you respond well, keep doing it in the weeks leading up to race day to allow the gut to continually optimise its ability to digest and absorb the fuel.

Image Credit: davidmillerphotography_ ©

The mistake I see athletes make is when they try their planned 2-3 gels per hour during an easy aerobic run and complain of bloating or nausea. Again, remember your car analogy. If you drove for a long period at 50kph but put the quantity of fuel into the tank that would facilitate 100kph, it's likely the tank is going to overflow in the same way our tummy will if the intensity doesn’t match the fuel intake.

So to summarise, remember your gut is an adaptable organ just like your muscles. Offer it a stress regularly enough and it will adapt its ability to cope. Make sure the stress is applied gradually to allow adaptation to keep pace with the demands and look to include race day fuels around key race simulation sessions in the weeks leading into the event.

Take advice from people in the sports nutrition industry to give yourself a head start, then go about trialling your options with an open mind. By the time race day rolls around, you want to know your gut is as prepared for the fuel as your quadriceps are for the watts!

Further reading