Here’s a Q & A Marc did a few years ago discussing how to fuel while you’re running.
For your upset stomach, you should immediately avoid whatever variables you can. In other words, see if you can figure out what works for you: accounting for taste, consistency, time before your run, etc will help eliminate the cause of your stomach issues. If it’s the flavor of the gu, then try a new flavor (or brand) as some brands have different consistencies and ingredients. If you find that it’s gu-typed products that you can’t stand; don’t use them. Sometimes, just changing the flavor can make all the difference though.
If the flavor isn’t the problem and you’re still having issues, you may want to try different products. Professional runners, like Lauren Fleshman & co, make products that are good (healthy and tasty) for runners. You may want to reach out to your friends and see what they use. There are also different companies making various products that can help aid runners in fueling. There’s always the prospect that you find something homemade that you like and can stand to eat mid-run. The possibilities are endless!
Another potential change you can make is in your pre-running ritual. Make sure you’re planning not only your run but also your nutrition leading up to and after your run. Try eating foods that are bland in the hours before your run. Avoid eating heavy or dense foods before you run because they will “sit” in your stomach and eventually give you plenty of issues during your run. Remember, the more variables you can account for, the quicker you’ll be able to find the problem and fix it.
Prior to long runs, give yourself enough time to eat, drink and let food begin to digest. All runners are different, so only you know what your stomach can handle. Again, keep to simple foods (half a bagel and some peanut butter, water…) so that your body has a better chance to digest the food.
With my own personal training, I like to sip on a sports drink beforehand and something with a good ratio of carbs and protein afterward. I eat half a bagel and either cream cheese or peanut butter on top. I consume both of these roughly 1 hour before I run and I will be both comfortable (stomach-wise) and full of energy for an entire long run. For really long runs, generally over 90 minutes, I also have a gu (strawberry-banana or vanilla) roughly 50 minutes into the run.
Again, make sure you consume foods that you feel comfortable eating on the run. Plan your hydration, meals and mid-run snacks well in advance. Remember to experiment with any foods that you plan on using on race day in practice and during long runs or workouts well before race day. Know exactly how your body is going to react to the specific foods that you are going to use on race day.