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Nutrition has the power to make or break your triathlon event, especially when it comes to the ‘Iron’ or ‘full’ distance. Often, when you talk to an individual whose Ironman hasn’t gone as planned, they will likely mention nutrition as one of the main reasons.  


In long distance triathlon, there are 3 main nutritional issues we come across: 

  • Running out of fuel! Hitting the wall, bonking, whatever you want to call it! Essentially, you’ve ran out of gas too soon and you’re not able to maintain your desired speed/pace which sometimes even results in a ‘DNF’.
  • Stomach issues! Each of us have different gastrointestinal (GI) systems with preferences or intolerances that will influence our nutrition. Stomach cramps, bloating or diarrhoea can all have a negative impact on your performance. 
  • Dehydration! An area often not thought about enough, where becoming progressively dehydrated throughout your event severely limits your performance. 


To start with, let’s think about your nutrition before and during your event.  

Before the race: Carbohydrate loading  

Essentially this is when we are trying to optimise our energy stores before an event. Carbohydrate loading protocols recommend consuming 10 to 12g of carbohydrate per kg of body weight for 2 days prior to the race. Although these recommendations are based on studies using male participants, there is evidence stating women can store muscle glycogen (the storage form of carbohydrates) as effectively as men. 

So, for example, if you are a 60kg female, this would work out as 600-720g of carbohydrate per day for 2 days. If you are an 80kg male, this would work out as 800-960g of carbohydrate per day for 2 days. Glycogen storage also requires water, therefore hydration in the days leading up to the race is just as important as hydration on race day.  

Practically this can be achieved by choosing to increase your carbohydrate portions in meals and should be spread throughout the day. Use foods you are familiar with and that you know agree with your stomach. Popular examples are oats, rice and pasta but you can also increase your carbohydrate intake with supplements such as our GO Energy Bakes (30g of carbohydrate) or our GO Energy Bars (26g of carbohydrate) as a snack throughout the day or through our GO Energy (47g of carbohydrate) or GO Electrolyte Powder (36g of carbohydrate) to sip on throughout the day if you’d prefer to consume more through fluids (bonus, this will also help ensure you are adequately hydrated before the event!). 

Top tip: If travelling abroad for your full distance triathlon, take some foods from home you are familiar with to avoid upsetting your stomach ahead of race day. 


Before the race: The morning of the race 

Your main aim is to top up your liver glycogen stores after being in a fasted state overnight, maintaining your hydration and keeping your hunger at bay.  

Your final meal before the race start should be 2-4 hours before to allow food to digest, which yes when you have a 7am race start time means you might not be very hungry, but it is vitally important to ensure you are fuelled beforehand. This also means you can focus on checking transition and warming up at the event start without focusing too much on food or going into the water on a very full stomach. 

In the last 30-60 minutes before your race starts, this is when we can add in another light carbohydrate focused pre-race snack for a final top up. Good examples here are a banana, dried fruit or an SiS GO Isotonic Energy Gel or SiS GO Energy Bar.  

Tim Don taking energy gel in water

Fuelling during the race 

Despite adequately carbohydrate loading before the event, you will still need to plan to fuel during the race as our internal stores are limited. How much to fuel with is very individual. In general, for any exercise over 2 and a half hours we tend to recommend aiming for between 80-120g of carbohydrate per hour. It has been shown that 120g of carbohydrate per hour can be consumed using our Beta Fuel range without presenting GI symptoms in this research study! 

However, if you’re of a lighter bodyweight, common amongst female athletes, you may want to reduce this further than 80g. For example, speaking from experience, as a 50kg female for my full distance Ironman I aimed for on average 60-80g of carbohydrate per hour. Exactly how much you fuel with will vary not just on body weight, but also your gastrointestinal system (i.e. digestion, absorption and gastric emptying) so build up gradually to find out what works best and is most tolerable for you. 

You will also need to consider your carbohydrate source, if you are trying to consume over 60g of carbohydrate per hour, you will need multiple carbohydrate types as the oxidation of single source carbohydrates such as glucose is limited to 60g per hour. You can increase this by introducing another carbohydrate source such as fructose. This is why we recommend our scientifically superior, high carbohydrate Beta Fuel range which is a mixture of maltodextrin and fructose. You can read more about the Science in Sport Beta Fuel range for triathlon here. 

During the swim, it’s not easy to take on fuel and most athletes will instead start their fuelling on the bike. It’s important to do this straight away, even if you don’t feel hungry.  

With it being the longest segment, most of your nutritional intake should take place on the bike. By default, it allows more space for nutrition to be carried or accessed whilst also providing ample opportunity to drink and chew properly compared to when on the run.  

You should plan to use your own nutrition where possible and make the most of ‘special needs bags’ if they are available at your event. Details on these and where/when you will be able to access them will usually be available in your event guide. These can contain your own nutrition on the course meaning you don’t need to experiment with what’s available at the aid stations on the day! Alternatively, look in the event guide before your race and find out what will be available at the aid stations so you can practice with the correct fuel beforehand. 

We spoke to professional triathlete, Tim Don, on how he fuels with SiS both in training and during a full distance triathlon:  

“The key for me and most Ironman athletes is to train to race, on your longer rides and runs really try to take in your race day fuel, if it’s Beta Fuel Gel’s train on them or Beta Fuel Chews, get use to them. No point training on bars and bananas if you are racing on something else, your gut won’t know what hit it. 

I try to take 80g of carbs per hour when I race using Beta Fuel and Beta Fuel gels, but I front load it on the bike as towards the end of 180km I find it hard to get the fuel in so in the first 2-3hours I try to take more like 100g per hour, also hydration is so key even if it’s not hot, as 8 hours is so long and I find the more fluid I take on the better the carbs absorb for me. 

I think it’s also so important to try a few different fuelling strategies, as not everyone is the same and what works for one might not work for another. There is no golden rule other than you need to fuel and hydrate to get the most out your body!” 

Energy Gel being put into back pocket

Avoiding stomach issues 

Research has shown more than a third of full distance Ironman competitors (both male and female) report GI complaints. This is a large percentage that you don’t want to be part of if you can avoid it! Usually, these GI issues become uncomfortable and have a knock-on effect on how much fuel you then want to continue taking on, leading to impaired performance.  

To minimise stomach issues, it is important to practice with your fuelling strategy regularly in training. Stick to what works for you. SiS GO Isotonic Energy Gels or our Beta Fuel Gels are very popular in triathlon as for most they are easy to consume on the go without the need for dilution, but if they are not for you, have a look at our alternatives to energy gels here. 

Factors that are likely to contribute to GI issues are temperature, hydration and anxiety.  

  • Exercising in the heat can worsen gut symptoms as your blood flow is redirected from your gut to your skin to help cool you down. Generally, reduced blood flow to your digestive organs is a common reason for gut symptoms to occur. 
  • Dehydration is also linked to increased GI distress as it leads to reduced blood supply to the gut. Read more about the importance of hydration and electrolytes here 
  • Before a race, it’s common to experience feelings of anxiety. This anxiety and stress can lead to a fight or flight response which in turn, can lead to increases in stress hormones, your heart rate and blood pressure. These changes can also disrupt blood flow to the gut. 

Some people are also just more prone to suffering with gut issues than others, this is a term called visceral hypersensitivity. It will take some trial and error to find the most effective fuel strategy for you and your gut. 


Common mistakes on race day 

Not having a plan and hoping for the best. Sometimes this can work, but for long distance triathlon, nutrition will be key to how you perform and feel. 

Thinking more is better. More fuel does not equal better performance and consuming too much will also lead to GI symptoms. This is why it’s important to have a plan and practice what works for you thoroughly in training before your event. 

Having a plan but not being adaptable. Sometimes things we don’t expect creep up on us, what if you lose a bottle on the bike for example? Although we cannot plan in detail what to do in such situations, we can have knowledge of nutrition available on the course or having spare nutrition available in transition or your ‘special needs bags’ if your event supplies them. You also need to be adaptable if you start with GI symptoms, you may have to take your foot of the gas per se and slow it down slightly. Finishing slower than you expected is better than not finishing at all! 

Experimenting on race day. Those shiny new products at the race expo can be tempting! Stick to what you know, unless for some reason you can’t access your own fuel, its best to bring your own, sticking to what you know and what you have practiced with. 


Take home messages  

 Your nutrition needs for a full distance triathlon should be thought out carefully and planned beforehand. It is also highly individual and trial and error during training will be needed to find out what works best for you. 

 At Science in Sport, we have a wide range of energy products available in the formats of gels, powders, chews, bars and bakes which can be used in both your carbohydrate loading strategy and during the race. 

 Nothing new on race day! Obviously in some cases unforeseen circumstances may mean this is not always an option but as a priority we should always try and prepare to use fuel you are familiar with. 

After training or racing, the body will be in a state of depletion. To reduce fatigue, the risk of injury and to promote physiological adaptations, it is important to recover well by refuelling. Read more about muscle recovery post exercise here. 

Written By

Dr Emily Jevons - PhD in Exercise Physiology & Nutrition, Clean Sport Advisor

Emily has worked with Science in Sport since 2021. With a PhD in Exercise Physiology & Nutrition, she currently provides nutritional advice for endurance athletes. Emily not only understands the science behind performance nutrition solutions, but also the physiological and psychological demands of sport after competing competitively in swimming and triathlon for a number of years.