If you're finding that in an Ironman you always have stomach issues you can't explain? Feeling frustrated that suddenly your nutrition plan fails you? Maybe the problem isn't your nutrition?
There is no nutritional plan on earth which will make up for poor pacing. If you push the bike, you run the risk of the stomach becoming too sensitive to handle what you put in it. Put more in your stomach, and the odds of GI distress is even greater.
If you race at a higher intensity than you train at, and then suffer stomach issues, the issue isn't the nutrition so much as it's the intensity and pacing during your race.
Many athletes think the goal is to shove as many calories in their gut as it can process. The goal should be to simply take in as much as you need to accomplish your goal. Anything more than that, and the risk of GI distress greatly increases, ESPECIALLY if the athlete is racing HARD.
If your training doesn't match your race intensity, and you are having stomach issues, look at your training and race execution, first, and then match your caloric intake and concentrations to that proper intensity.