While you can get away with a slap dash approach in sprint and Olympic distance racing. Half Ironman distance and above requires planning. You can be as fit as a fiddle but if you are not taking on the right amounts of food & drink this can have a serious impact on your race performance. I see so many age group athletes with their hands on their knees mid run - typically through too much or too little food and drink!
Whilst I appreciate nutrition is highly individual, I have put a few guidelines below...
1. You need to take a minimum of 1 gram of carbohydrate per kg of body weight per hour. This requires training in itself. During the bike, carbohydrate powder with water can work well (again all very individual, SIS / OTE are safe brands to go with here). At first this amount of carbohydrate may seem large so start practicing at a lower level and build up. Little and often is the best approach here. IF you are competing in Ironman any solids are best taken in the first half of the bike course only.
2. You need to drink min of 750ml per hour but everybody is different so it can be that you need to drink up to 1250ml per hour (depending on weather/temperature) drinking to thirst is my best advice here.
3. You need salt when it is hot, up to 1 gram per hour but some individuals need 2 grams (without test difficult to tell exactly, some sweat more salt than others).
4. Gels for the run but with every gel 100-200ml water. Appreciate its more difficult to take on the calories during the run and this is why the breakfast, pre swim and bike fueling is so important. Up to 3 gels per hour here. Check the grams of carbohydrate on the gels, most are 20g per gel. I would avoid those of 30g per gel or higher because they can be too highly concentrated and give you gut problems.
5. Practice in training to see what products work for you & check with race organisers what products are available on the day (advice here is always stick with what you know!) The 3 days pre race is also a critical time and this is an area I will talk through with athletes 1-1.