This is a great question and one which will be in the minds of many triathletes now and during the next few months. Let’s start with a few basic facts.
- All triathlon events, but especially Ironman, are primarily fuelled by fat stores which most athletes, even the leanest ones, have in large quantities.
- Assuming that you start your event with full glycogen stores then there will also be a store of 400-500g of glycogen which is enough for approximately 2 hours of moderate to fast paced activity. Please be aware that during a race you may burn up to 10,000 calories, but that it is impossible to replace all of these. Your goal is just to keep your blood sugar levels high enough to maintain your pace.
- Most sports nutritionists suggest that 60-90g/hr of carbohydrate is the maximum that can be digested, but you do need to practice for many weeks to train your body to deal with this volume of carbs. For a 15 hours race that would mean roughly 15 x 60 = 900g of carbohydrates (3600kcal) from start to finish.
- You can deliver this to the muscles in many forms – bars, gels, carb drink, a combination of all 3, or you could even try ‘real’ food. The danger with this last choice is that digestion will take longer. Later on during the bike and on the run it might cause some gastro-intestinal (GI) problems.
The harder one races and the higher the intensity, the more carbohydrates required. Athletes aiming for the 9, 10 or 11 hour mark will likely push harder and dip into their glycogen stores more. Arguably if you are out there for 15 hours your intensity may be lower and therefore you could actually get through the race consuming less carbs and relying on your fat stores, especially if you are racing on a flat course where your pace is likely to be fairly constant. This compared to a hilly race where even with the best intentions some of the hills might push you into a zone where you have to burn carbohydrates.
I would aim for an intake of around 40g/hour, mostly on the bike. It seems that many athletes tolerate solid foods (i.e. energy bars) early on in the ride, which allows them to digest before the run. As they near the last 25-30 miles of the bike they switch to liquids or gels. On the run aim to consume 1 gel every 60 minutes or so (if you can still face doing this by then). If you can’t, then an alternative is to sip coke
Before your race, prepare a robust nutrition plan and make sure that you have a contingency if you lose your food (many people lose theirs when it falls out of their bike pocket), or if you feel nauseous. Look at the bike map of the course and understand where the feed stations are. You may be able to use nutrition provided. Remember your hydration needs as well (possibly including salt). Some races have a ‘special needs’ table at roughly 90k which might relieve you of having to carry all of your nutrition for the entire race.
Feel free to email me back if you need any further clarification on these suggestions.