Taking nutrition on the run can be more complicated than it sounds, in large part, because the digestive process becomes compromised when we run. In order to support the energy needs of running, the body is designed to divert blood flow away from internal organs, like our gastrointestinal tract, and re-direct it to the large exercising muscles. The large muscles of the body need to be well supplied with oxygen-enriched blood to sustain the activity.
The more intense the run, the greater percentage of blood flow is diverted away from the GI tract, which means that digestion can become difficult. Some lucky runners seem to have "iron" stomachs and can tolerate any and everything regardless of pace or distance. For the rest of us, the key is finding the right product(s) that work best, and then figuring out the right amount and timing. Essentially, it comes down to trial and error.
If you're having a difficult time getting any product down, try a minimal approach. Begin experimenting by consuming as little as possible. Rather than eating an entire packet of a product, try consuming only a portion of it. Try just one mouthful and be sure to wash it down with water. If you experience no negative side effects, you can gradually increase the amounts ingested.
Also, take the product early in the run and more frequently, before your digestive system shuts down completely. Try taking some nutrition around mile 3 or 4 rather than waiting until mile 6 or 8 or an hour or more into the run. By fueling early, you may be able to keep your digestive system working better during the entire run. In addition, practice taking nutrition even on short runs to help train your body to accept nutrition while running. (For help choosing the right energy boost to pack, check out our article, Caffeine Pills Vs. Energy Gels.)