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Our Comprehensive Ironman Fuel & Hydration Guide

Our comprehensive guide to help you prepare and execute a winning fuelling plan for your next Ironman.

There is a common saying among Ironman triathletes - "nutrition is the fourth discipline." When you’re competing in endurance events that can last anywhere from 8 to 18 hours, you need an eating and drinking strategy that will fuel you adequately all the way to the finish line. When you speak to fellow triathletes about what went wrong during their race, you often hear them talk about running out of energy (i.e. "bonking"), stomach problems and/or cramping. All of these problems have one thing is common - they are all caused by nutrition/hydration. This guide is designed to prepare you for your next Ironman from a nutrition and performance nutrition perspective - from planning in the weeks before, during the final days and from start-to-finish. It won't guarantee a good day, but it will certainly improve your chances of having one.

Carbohydrates for energy

The human body has two major fuel sources: fat and carbohydrate. Both are stored in the body and act as two separate "fuel tanks" that can provide energy for exercise. Carbohydrate can be used quickly by the body and can provide energy much faster than fat. The use of fat will also require more oxygen and when oxygen supply is limited, carbohydrate can still be used while fat cannot. Carbohydrate is therefore used in much larger amounts than fat when the intensity of exercise is very high, and the delivery of energy needs to be fast. In contrast, during low to low-to-moderate intensity exercise where the energy demands are lower, fat is an ideal energy source. One of the key differences between the two fuel sources is their storage.

How Carbohydrates are Stored

Carbohydrate is stored as glycogen, which is found in the liver and in muscles. The glycogen stored in muscles is used by the muscle it is in when energy demands are high, so running uses glycogen stored in the muscles of the thighs much faster than the glycogen in the arms. All of the muscle in the body combined may store 400 to 800g of glycogen, which depends on body size and how much carbohydrate is eaten. Liver glycogen is stored in much smaller amounts, around 80g, which is less variable between people. The glycogen stored in liver is broken down into glucose and transported in the blood to wherever it is needed, which includes organs like the brain and kidneys, but also muscles during exercise.

How Fats are Stored

Fat is mostly stored in fat tissue (adipose tissue) which is found all over the body. Even in most lean people who have relatively low body fat levels, the amount of energy stored in fat tissue is massive. Fat is also found in much smaller amounts inside muscles, but the majority of fat in the body is found in fat tissue, which are measured in kilograms. Even in very lean people, fat stores comprise tens of thousands (if not hundreds of thousands) of calories, while carbohydrate stores (as glycogen) only make up a few thousand even when glycogen stores are high. Fat stores are effectively unlimited with regard to fuel for exercise, while carbohydrate stores can limit endurance exercise.

"Hitting the wall" ("bonking")

Many triathletes will be familiar with the experience of "hitting the wall" or "bonking," which is effectively when glycogen stores start to run out and the available fuel cannot generate energy fast enough to sustain the high intensity exercise. Glycogen stores are usually enough to fuel 2 to 3 hours of exercise, depending on the exercise intensity and the amount of glycogen stores. Consuming carbohydrates before and during exercise can help to prolong performance and delay glycogen depletion. Unlike taking in extra carbohydrate around exercise, eating fat does not improve exercise performance because the supply of fat is not limiting in the way carbohydrate is. Therefore, ensuring a high availability of carbohydrate is essential for endurance exercise performance and many of the nutritional strategies used to enhance are related to ensuring carbohydrates are stored and available during competition.

Carb Loading in the days before an Ironman

Carb loading is a strategy that has been used by endurance athletes since the 1970s to maximise glycogen stores in the days leading up to an event. Although various different methods have been suggested and there are many different protocols, carb loading is relatively straightforward.

The most straightforward way to carb load is to eat 10 to 12g of carbohydrate per kg of body weight (g/kg) per day, for the 36 to 48 hours before your marathon. For a 70kg (154 lb.) person, this equates to 700 to 840g of carbohydrate per day, which is a very high intake. This should be complemented by a reduction in training volume in the few days before a competition, though complex super-compensation protocols are not necessary. For smaller triathletes and many women, this will mean “overeating” a little in the days before an Ironman (eating more calories than expended). In reality many triathletes opt for slightly lower intakes and 8 to 10g/kg/day will still give very high muscle glycogen stores.

Carb loading usually leads to a relatively large increase in body weight. This is partly due to the increase in glycogen stores, which will be a few hundred grams. However, when glycogen is stored in the muscle, water is stored with it, in a ratio of 3:1. Therefore, a carb-loading protocol that increases glycogen stores by 400g will lead to an increase in body weight of 1.6 to 2.0kg. An increase in body weight could impact performance, due to having to carry additional weight during the marathon. However, the possible negative of the additional weight is balanced out by the far greater glycogen availability plus some additional fluid that will become available.

Foods eaten while carb loading should be carbohydrate dense, palatable and familiar - high-carbohydrate foods include things like rice, pasta, cereals, white bread, honey, syrups, jams and pancakes. The fat and protein content of the diet should be relatively low, as these will contribute to fullness without delivering carbohydrate. The very high carbohydrate intake required to carb load involves eating large amounts of food, so feeling too full to continue eating can be a problem. Eating palatable, tasty foods can help to ensure you can eat the carbohydrate required. However, carb loading is not simply an excuse for overeating, which is how some athletes see it. The foods chosen should be high in carbohydrate (and thus lower in fat) and palatable but should not extend to large amounts of high fat/high carbohydrate foods such as cookies, ice cream, cakes or pizza. The total amount of fat and protein should be reduced to keep calorie intake relatively similar. Consuming carbohydrate containing juices or fizzy drinks (soda) can be an easy way to add carbohydrate to the diet with little impact on fullness.

Many triathletes also choose low-fibre foods when carb-loading. Fibre is indigestible carbohydrate, that remains in the digestive system for many hours, and eventually passes in stool. Many carbohydrate sources are high in fibre, and so a high carbohydrate diet can lead to large amounts of fibre being carried in the digestive tract. This adds additional weight, and so eating low-fibre foods may offer a benefit to those looking to keep a lower race weight.

Suggested meals for carb loading

Breakfast (170g Carbohydrate)
100g Breakfast Cereal
250ml Milk
2 Slices White Toast with Jam
1 Ripe Banana


Lunch (175g Carbohydrate)
150g (Dry Weight) Pasta with Tomato Sauce
500ml Orange Juice


Snack (120g Carbohydrate)
2 bagels with Honey
1 Low-Fat Cereal Bar


Dinner (155g Carbohydrate)
150g (Dry Weight) White Rice
Grilled Chicken Breast
330ml Soda


Bedtime snack (80g Carbohydrate)
80g Breakfast Cereal
150ml Milk


Race-Day Breakfast

Breakfast is often described as the most important meal of the day - something that may very well be true on the morning of a race. Depending on the start time of your Ironman, there may be a balance between waking up early to have breakfast, and ensuring you get enough sleep. When you wake up, liver glycogen stores will be relatively low having been used to supply blood glucose to the brain and other organs while sleeping. During the race, liver glycogen is needed to maintain blood glucose levels which will supply your working muscles, so replenishing these is the top priority to prevent hypoglycaemia. "Topping off" the liver with carbohydrates will require 100 to 200g of carbohydrate in the 3 to 4 hours before a race. This does not have to be in one large meal but can be spread out over multiple snacks or drinks. Using drinks or gels may help people who struggle to eat before a race due to nerves or anxiety. The foods chosen should generally not contain large amounts of fat, fibre and protein which all slow down digestion. This is particularly important for individuals who suffer from gastrointestinal problems before and during the race. Similarly, these people may want to avoid large amounts of dairy foods to minimise lactose intake if this causes them problems, though this is not a problem for most people. Ideal food sources are similar to those needed for carb loading, but for breakfast you may include foods like cereals, white bread, jam, honey, pancakes and juices. If you are staying in a hotel, many of these foods are freely available at a breakfast buffet. However, there is no guarantee of what will be available - so many athletes prefer to bring their own food for breakfast when staying in a hotel. Alternatively, it may be possible to contact the hotel in advance to find out more about their breakfast and bring a kitchen scale to keep track of food intake.

Pre-Race Fuel & Hydration

Even in the 30 minutes before the swim start, it is possible to "top off" liver glycogen and make even more carbohydrate available. 20 to 30 grams of carbohydrate will work well, which can be from a gel, sports drink, chew or solid food. We recommend taking this as close 10 minutes before the race start. However, it is important to practice this in training to ensure it does not trigger gastrointestinal symptoms. This carbohydrate will stay in the stomach for a short while, meaning some of this carbohydrate will become available during the race. Many people will opt for chews or gels here, but personal preference is important as well. It is important to start the race hydrated, and so drinking around 500ml of water or sports drink in the +/- 2 hours before start time will help to achieve this. Ideally, your urine colour should be pale before starting the race. The small amounts of carbohydrate taken before the race should be taken with some fluid, around 90 to 180ml (3 to 6 fl. oz.) of water. Too much carbohydrate with too little water will slow the release from the stomach, which could lead to discomfort.

Pre-Race Caffeine

Caffeine is an effective supplement for improving exercise performance. Relatively low doses of caffeine can improve performance - as low as 2 to 3mg per kg of body mass (140 to 210mg for a 70kg or 154 lb. person) 30 to 60 minutes before the start of the race. Caffeine can also be consumed during a race and still have an effect, though it takes +/- 60 minutes to reach its peak in the blood - so timing is important. Taking additional caffeine in doses of +/- 1.0 to 1.5 mg/kg in the later stages of the race can provide a good "boost" to performance. 400mg is the generally accepted daily limit for caffeine intake, as larger amounts of caffeine can start to cause unpleasant side effects without improving performance further. 400mg should not be exceeded over the course of the race day unless this has been practiced in advance, knowing that it will not provide a benefit over and above smaller amounts.


Sources of Caffeine

Caffeine can be obtained from a number of sources, and any of these sources can be effective at improving performance provided the effective doses are achieved. Some sources of caffeine include:

  • Caffeinated Gels - 50 to 100mg
  • Cola Drinks - 30 to 50mg per 330/355ml can
  • Energy Drinks - 75 to 80mg per 250ml can
  • Espresso - 80 to 100mg per shot

Although there are many ways to consume caffeine, some sources have downsides to them. Energy drinks and colas are carbonated, and the fizz can cause stomach problems for many people when consumed during exercise. Therefore, these drinks should be taken flatted/de-fizzed if used. Coffee is the main source of caffeine in the diet but increases the need to defecate (have a bowel movement) in some people. This is individual, so if this does not affect you, then coffee can be a great way to obtain caffeine before a race. Different coffees contain different amounts of caffeine, depending on how they were made, as well as the brand. Therefore, coffee can be an unreliable source of caffeine that may provide too much or too little caffeine, with no way of knowing until it is too late.


During The Race: How Much Carbohydrate?

Taking in extra carbohydrate during your Ironman has a number of positive effects, which combine to result in better performance and a faster race time. There is no need for any other macronutrients (protein, fat) during the race, as only carbohydrate is limited, and is the best fuel source. The recommendations for how much carbohydrate to take in vary from 60g to 120g per hour. For most people, a moderate carbohydrate intake of 60g per hour is enough to fuel performance, and although this is a moderate intake, these amounts should be practiced before the race. You should avoid delaying your carbohydrate intake until you start feeling tired and should start early on in the race with regular feedings every 20 to 30 minutes as preferred. For anyone competing at a very fast pace and at a high intensity, an intake of up 90g or even 120g per hour may be useful, because the higher speed will require more carbohydrate for energy. It is best to use a multiple transportable carbohydrate such as 2:1 glucose-to-fructose as your carbohydrate source. High carbohydrate intake is are not for everyone and should be reserved for relatively experienced triathletes competing at a relatively fast pace.

What is the best source of carbohydrate?

There are many choices of food or drink to obtain carbohydrate during an Ironman - including drinks, gels, chews and bars. All sources are similarly effective at delivering carbohydrate, though each has specific considerations. Drinks are very popular and can be ready-to-drink or available as powder to mix yourself. The concentration of carbohydrate is worth considering, as very high concentration carbohydrate drinks can slow down the emptying of the stomach, which causes discomfort and also limits how much carbohydrate can be absorbed. A concentration that is too low means that very large amounts of fluid have to be consumed to get enough carbohydrate, increasing the need to urinate. An ideal sports drink should have 6-7% carbohydrate concentration. Energy gels have very high carbohydrate concentrations (+/- 30%), meaning that if they are consumed without water, they can lead to stomach discomfort and fullness. Because they are so compact, is essential to drink fluid alongside gels. Gels have the benefit of being quite easy to consume compared to large amounts of carbohydrate drinks or foods that need to be chewed. Bars are very variable, and may contain very different amounts of carbohydrate, fat and fibre. Therefore, it is essential to check the nutritional information before choosing bars. Choose low-fat, low-fibre and low-protein energy bars, which will be quick and easy to digest. Bars have the benefit of preventing feelings of hunger or having an empty stomach which is common near the end of a race. However, they can be difficult to chew and swallow while competing. Some bars have very high carbohydrate content (e.g. 60g) which may be your carbohydrate requirements for a whole hour, so breaking up the bar into regular feedings every 15 to 30 minutes may be best to ensure a constant delivery of carbohydrate. This high carbohydrate content makes them an efficient way to carry lots of carbohydrate. Chews provide some of the benefits of gels, as they are very high carbohydrate with minimal/no protein, fat or fibre and because of their high carbohydrate concentration, they need to be consumed with fluids. Like bars, they need to be chewed which can be difficult while racing. The bottom line of choosing carbohydrate sources for during the race is to choose options that are comfortable for you and that you have practiced. All choices are similar for providing carbohydrate - but provided you are getting the right amount of carbohydrate, then the way in which you do it is down to personal preference.

Fluids and hydration

Dehydration is a potential cause of fatigue in an Ironman - as water is lost through sweat to maintain a normal body temperature, and through breathing. More intense exertion causes greater heat production - and therefore more sweating. In hot conditions, this is exacerbated as the body tries to lose more heat. Losing too much water can lead to dehydration, which makes it harder to maintain a normal body temperature. Some level of dehydration is not a problem, but losses of 3% of body weight or higher may start to impact performance. Starting the race well-hydrated is important, as is staying hydrated once it has started. Aiming to consume a little less fluid than you lose from sweat is good practice, with longer races being closer to your sweat rate. Drinking to your level of thirst is a good strategy for slower triathletes, though faster athletes can benefit from a specific plan. Most carbohydrate drinks contain sodium and other electrolytes which are lost in sweat and consuming these electrolytes can help with absorption of fluid. Because later in the race your gastrointestinal tract is worse at absorbing carbohydrate and fluid, it is best to start with fluids and carbs early on in the race. You should expect to lose a little weight over the course of the race (perhaps 1 to 2kg), which is a mixture of glycogen and water. It is key not to drink too much fluid during the race, as this can lead to a potentially fatal condition called hyponatraemia. Use common sense in deciding how much fluid to drink and ensure that you don’t gain weight over the course of a race, which would suggest the potential for hyponatraemia. If you find yourself with gastrointestinal symptoms like stomach fullness or bloating, this can be remedied by stopping fluid and carbohydrate intake and reducing running speed temporarily to allow the stomach to empty and the fluid and carbohydrate to be absorbed.

“Training your Gut"

Much like you can train your body for the event you are planning to do, you can (and should) train your body for the foods, drinks and nutritional practices you plan to use. Good nutrition has the potential to improve marathon performance in a number of ways but used improperly even ideal nutrition can cause problems if you are not used to them.

Taking in food or drinks during exercise triggers gastrointestinal problems for some athletes, including symptoms like reflux, heartburn, stomach fullness, vomiting, bloating, stitches and diarrhoea. If you suffer from symptoms like these during training when taking carbohydrates, then it is likely these same symptoms will appear during your Ironman. Practicing with the carbohydrate amounts, carbohydrate type and format (gel, drink, bar) that you plan to use in the race during your training can help to reduce symptoms and maximise the amount of carbohydrate you can take in.

Race Day Plan

Eating and drinking in an Ironman, like all endurance events, is a matter of striking a balance between giving your body sufficient fuel and what your body can tolerate from an intake perspective. If you consume too little, you risk bonking or underperforming. If you consume too much or the wrong products, you could experience stomach problems as your gut is unable to process the nutrition, especially at race effort. As a general rule, Ironman athletes should aim to eat 60 to 120 grams of carbohydrate per hour during Ironman events, with athletes typically able to consume more carbohydrate and fluids during the bike leg and fewer during the run portion of the event. You also need to consider fluid needs as part of your overall nutrition strategy. During an Ironman, you lose fluids through sweat, respiration (breathing), oxidation of what you’re eating as well as the output of bodily waste. Even a small percentage loss of bodily fluid can be harmful to both performance and your overall health. When competing in an Ironman, you should start the event well-hydrated and aim to replace most of the fluid and some of the electrolytes (mainly sodium) as you go. The rate at which you sweat is quite variable person-to-person and based on your level of exertion - but temperature and humidity also play a big role. The hotter and more humid it is - the more fluids you will need to replace. Your goal in an Ironman should be to drink when you are thirsty, but you should also be mindful of your sweat rate. You should aim to replace most of what you sweat out.



Consume 1 x C30 Energy Gel with 120ml of water 15 minutes before swim start. The purpose is to top-off your glycogen and fluids, and to pre-load your swim with fuel and fluid. It is important to practice this in training and to be aware of the potential for rebound hypoglycaemia.


    Transition 1 [T1]

    Consume 1 x C30 Energy Gel - with or without caffeine. Caffeine sensitivity varies quite a bit athlete-to-athlete, so this should be tested in training. Our caffeine gels contain 75mg of caffeine per gel.



      Consume 70 to 120 grams of carbohydrate per hour through a combination of drink mixes, energy gels and energy bars (as all solid foods, like our C30 Fuel Bar, are associated with a higher risk of GI problems than gels and drink mixes, it is important to test them in training). It is important to also consider your on-bike fluid needs which are largely dependent on individual sweat rate and weather conditions. First, calculate your total fuel need by the number of hours you will be on-bike. As example, an elite triathlete may need 360 grams total carbs on bike, where a strong age-grouper may need 450 grams. A third consideration is practicality. Water is typically available on course, but fuel is not. Many Ironman veterans add 2-3 x C90 High Carb Mix sachets into each of 2 x 750ml bottles and use a third bottle solely for water (they refill water on course). This way they have 360g to 540g of carbs already pre-mixed as a fuel "IV." This strategy reduces the number of gels you need to carry and simplifies your overall on-bike intake. You can substitute C90 High Carb Mix with multiple scoops of C30 Sports Drink Mix as well. The compositions of C30 Sports Drink and C90 High Carb Drink Mix are identical. Using multiple scoops of C30 Sports Drink will deliver more sodium than C90 alone based on a 90g carbohydrate serving (C30 Sports Drink will deliver 600mg per 90 grams. C90 will deliver 200mg per 90 grams). This may be a better choice for athletes who are heavier sodium sweaters.

      Neversecond Products
      C90 High Carb Mix or C30 Sports Drink Mix
      C30 Energy Gel
      C30 Fuel Bar



      Transition 2 [T2]

      Consume 1 x C30 Energy Gel - with or without caffeine. Caffeine sensitivity varies quite a bit athlete-to-athlete, so this should be tested in training. Our caffeine gels contain 75mg of caffeine per gel.


      Consume 45 to 90 grams of carbohydrate per hour by energy gels and on-course water. To keep gel intake as efficient as easy as possible, we recommend using a soft flask that can hold multiple energy gels. Our 500ml soft flask may be a good option for you.

      Neversecond Products
      C30 Energy Gel



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