Ironman Nutrition

The following article explains how you should prepare for, execute and recover from an IronMan race nutritionally. Any text marked with an asterisk such as " *3 " has a definition supplied in the glossary the bottom of the page.

The preparation:

In the days before the race a diet containing mostly carbohydrates of low glycaemic index*1 along with adequate amounts of essential fats and proteins can ensure that your body will use a greater proportion of fatty acids when exercising, which should lead to better endurance. Ensure that you eat enough fats as reducing fat intake to below 20% of calories can lead to increased use of carbs during exercise and less fat burning. A few hours before the race starts it is a good idea to eat some carbohydrate containing foods, preferably a mix of complex carbohydrates with a low glycaemic load*3 and simple carbs with a higher glycaemic load. A good example of a pre race breakfast would be: Porridge with berries and seeds or wholegrain toast with jam.

The race:


During the swim you will be unable to feed and therefore will use up the energy reserves and water that you consumed prior to the race start. A 1-2 hour swim will use up quite a few calories and after the swim leg you will have mostly used up the fuel that you supplied earlier in your pre-event breakfast. This is not a major concern as you will be starting to supply fuel to your body soon after starting the bike leg. Bear in mind however that many athletes avoid eating immediately after their swim as the previously experienced turbulence can cause nausea in the first few minutes after the completion of the swim. Take your time and start feeding on the bike after about 20-30 minutes. Drinking should take place between 5-20 minutes after completion of the swim.

Bear in mind that the swim leg is far shorter than the other two and that time lost during it can be made up easily on the bike and run legs. As such you may find your best strategy to be taking it easy on the swim. This will burn less calories and ensure you are in a good nutritional state when you start your bike leg. However, if you are a very good swimmer it may be appropriate to put in a hard effort to escape the inevitable melee at the beginning of an Ironman swim in which case you will finish sooner and be able to start feeding on the bike more quickly anyway.


On the bike your needs will roughly be 300-400kcals per hour. This is dependent on your size and your work rate. One 750ml sports drink containing 75ml (75g) of powder roughly contains 300kcals. Carbohydrates contain 4kcal per gram. So dependent on your fluid needs you may need one bottle per hour providing 300kcals and you could then top up with a gel if you needed more calories but no more water. Alternatively make your drink up at 12% concentration giving 360kcals per hour from a 750ml bottle. Your water requirement is unlikely to exceed 1litre per hour. Drinking more than this can put you in danger of hyponatraemia*4, a state in which your body fluids become too dilute. The condition can be fatal and has caused a number of deaths in marathons and other endurance events over the years.

Many athletes like to consume some solid food on the bike as an additional way of gaining nutrients. It also provides a focus and something to look forward to. A good time for some solids may be after about 4 hours on the bike, leaving you some time to digest it before the run. Solid foods should be reasonably easy to digest, such as a white roll with ham and cheese. I personally find many bars difficult to consume while exercising so experiment with different types of food in training first to see what you get on with. Prior to finishing the bike leg it may be worth forcing a little more drink down to enable you to settle into your run before drinking. This is especially important if you find it difficult drinking on the run.


On the run your needs will roughly be 300-400kcals per hour. It is very difficult to consume solid foods on the run and so you have a choice of combining gels and water or of drinking sports drinks. A typical gel such as powergel contains 41g of which 27g are carbohydrate. This provides 108kcals. As such, one sachet every 20mins provides the 300+kcals that are required to balance energy needs during an Ironman event. With each sachet aim to drink about 200ml of water. Note that some gels are isotonic and that these gels are designed to be consumed without water. The "Go" gels by Science In Sport - SIS come into this category. Modify your water intake appropriately if you are using these. An example intake during an Ironman run would be:- One gel every 20 minutes. This will often correspond to picking up a gel at every other feed station on many Ironman courses. Try to maintain your drinking rate on the run. This may necessitate stopping periodically. Drink 200ml of fluid per gel. If the salty soup, bouillon is provided then take a small cup of this twice during the run in order to maintain your sodium intake. On the last 10k try coca cola or red bull as sugar boost to push you towards the finish.

The recovery:

After an Ironman race you will be severely depleted of muscle glycogen and you will also be depleted generally. To recover fully you will require not just carbohydrates to replace your muscle glycogen stores, but also fats, proteins, vitamins and minerals to rebuild all those parts of the body that have been affected by your extreme effort. If your stomach allows it you should eat and drink as soon as possible. This will allow your body to start the recovery process sooner and lead to a faster recovery. Recovery drinks are a reasonable solution if you do not have immediate access to a good square meal straight away, however the best way to replenish yourself is with a rounded meal that supplies all the nutrients you need to recover. A source of protein as well as the ever popular carbs and fats should be on the plate. Make sure you drink plenty before and after the meal. Ideally you will combine these nutrition strategies with a very easy stretch when you are recovered sufficiently. Make sure you are protected from the elements whether sun, cold winds or rain and afterwards put your legs up for a good long rest.


*1 The glycaemic index - GI is a measure of the speed with which you can absorb the sugars from a given food into your bloodstream. Some foods will lead you to absorb sugars fast. If you eat enough of these types of food in one sitting this will give you a surge of insulin, which may destabilise your blood sugar levels, especially if you are not active at that time. For an explanation of insulin see

*2 below. *2 Insulin is a hormone that allows sugar to enter our muscles from our bloodstream. If too much is produced it may lead to low bllod sugar known as hypoglycaemia. This can cause feelings of tiredness and low energy.

*3 The glycaemic load - GL is the overall effect a carbohydrate meal or snack has on your blood sugar levels. It is basically the amount of a food multiplied by its glycaemic index. So for instance a piece of baguette is high GI. However a crumb of it isn't going to affect you, but a whole stick will in all likelihood raise your blood sugar levels, lead to an insulin surge that will probably an hour or so later give rise to low blood sugar.

*4 Hyponatraemia is a condition where the blood contains very low levels of salt. It can occur if an athlete drinks too much water and in so doing dilutes his or her body fluids too much. If the salt levels get too low then coma and death can result. Before this stage an athlete with hyponatraemia can display symptoms not unlike those of dehydration. Needless to say it is vital for medical staff to differentiate between the two states as they need very different kinds of treatment.

Don't miss.... more from Robin.

  • Sports gels are sticky sugar solutions used to fuel endurance efforts. They are used to provide energy in a palatible form that is easy to digest. Some people like them, others avoid them. When should we use them and which ones should you choose?
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