When should you take them?
Like almost every other facet of running, the timing of when you should take your gels is very individual. Each runner absorbs and processes carbohydrates at a different rate — some can feel the effect within three minutes while others might take up to 15 minutes.
This variation in absorption rate has to do with how well your stomach reacts to the gel. When running hard, your body often diverts blood away from the digestive track to help your legs continue to move forward (your body naturally conserves resources). Sometimes, however, your body shuts the stomach down completely while other times it just slows down. This is why it isn’t uncommon to see runners throw up fluids or gels right after ingesting them late in the race.
Therefore, you want to begin taking gels relatively early in the race. By taking the gels early, your body shouldn’t be under great duress and you have a better chance of processing the sugars faster and without stomach issues. I suggest taking your first gel somewhere between 45-60 minutes, depending on how well you generally react to gels in training.
Some runners like to take a gel right before the race starts. While there is no problem with this from a physiological standpoint, I find it better to consume a more substantial breakfast, with less simple sugars. This helps you avoid eating nothing but simple sugar for 3-4 hours.