Ironman Nutrition - Avoiding the BONK!


Ironman Cairns 2018

Avoiding the BONK!!

Lining up on the pontoon; nerves running high, a million thoughts running through your head – should I have gone to the toilet again? Has my training paid off? Did I eat too mu… before you know it you’re in the water.

Endurance sport, whether it be an Ironman event or an ultra-endurance event is a constant fight against your mental and physical strength. DO YOU HAVE WHAT IT TAKES?


The marathon leg of an Ironman can make…. or break.. your race. Many athletes tend to race a step above their training pace particularly if they are quite new to Ironman. It is not surprising when some athletes slow to a jog or even walk during the running leg, and in some cases HIT THE WALL or experience THE BONK.


To prevent disaster from striking it is critical to be conservative during the first two legs of an Ironman. The onset of BONKING is often related to an energy deficit. Conserving energy is critical to allow your body to have sufficient energy availability during the marathon, as your energy stores will become exhausted the longer the duration of the event.


The common energy deficit occurs due to the body’s ability to burn up to 180g of carbohydrate per hour yet only absorb approximately 60-90g of carbohydrate per hour. This will put an athlete in a calorie deficit during an Ironman when analysing energy derived from carbohydrate. Those athletes that have trained their body to metabolize a percentage of fat for fuel will still be in an energy deficit, as carbohydrate is required to initiate the break down of fat. When competing at a high intensity majority of our energy is derived from carbohydrate, which is why we need to top up these stores regularly and adequately. It is essential that athletes (Elite and weekend warriors) have a good nutrition plan in place which starts carbohydrate feeding early and continues throughout the race.


To ensure athletes are continually topping up their carbohydrate stores, they could try some of the following options, keeping in mind as the pace generally picks up it may become harder to eat and swallow food:

· Gels (GU, SIS)

· Energy Bars (Cliff Bar)

· Vegemite sandwich, jam rolls or rice cakes

· Fluids

· Jelly babies

· Sports drink

· Flat coke


To prevent hitting the wall athletes are encouraged to wear a device to monitor their heart rate and race pace, which will athletes ensure they are competing in an aerobic state which allows athletes to use a higher percentage of fat as fuel.


For further support regarding training and race day nutrition, a Sports Dietitian can help you develop an individualised nutrition plan to help you smash your goals and address your own personal characteristics including energy expenditure, sweat rates, food intolerances, gut response and body composition. The advice given to a triathlete may contradict the advice given to the general public however endurance athletes energy requirements are significantly higher than an average Australian as they are completing high volumes of training.

For more information contact one of your local Sport Dietitians at Health Management Dietitians via www.healthmanagement.com.au or phone (07) 40519093 .


Author: Matt Hart (Sports Dietitian - Health Management)

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