There have been quite a few posts on different trail and ultrarunning blogs about people getting upset stomachs from gels (see e.g. http://blog.irunfar.com/2008/12/energy-gel-comparison.html). I truly believe some runners CANNOT tolerate prolonged gel consumption. However, before you conclude that you're one of those people, let's make certain something else isn't the problem. [Some of what follows can also be found in the special topics article "Biochemical Strategies for Ultramarathon Running" as part of a broader discussion of carbohydrate and fat calorie needs for running.]

Long distance running is inherently stressful to the stomach. Very small tears occur in the stomach and intestinal lining. This is not dangerous, but it does significantly upset the stomach. Moreover heavy exercise diverts blood from the stomach to the muscles, thereby decreasing the digestive and nutrient uptake activity that the stomach can support. Race stress further affects the stomach. Your stomach may be unhappy regardless what you eat! If you are running a sub-20 hour pace for a 100 mile race, you are pushing the limits of human aerobic energy metabolism. I really applaud you, but it is fairly certain you will encounter stomach problems.

There is an established upper limit to how much carbohydrate a runner can consume per hour before their stomach shuts down. For a 150 lb runner, that limit is 100-130 grams (about 350 calories or a little over 3 gels). If you weigh less or more than 150 lb, scale that number down or up accordingly. A few elite ultrarunners may be able to get away with consuming 3 gels per hour, but most of us mere mortals are much safer with 2 gels per hour. OK, when I say 2 gels per hour I mean 2 gels per hour—I do NOT mean zero gels one hour and 4 gels the next. Even at 2 gels per hour, you can still find yourself in trouble if you consume a sports drink or eat large amounts of other food. Sports drinks contain upwards of 50 grams carbohydrate per liter (quart).

Ultrarunners often get asked the question "How can you run that far?" For several years now there has been a provocative scientific hypothesis which says distance running was a significant part of human evolution. According to that hypothesis [D. M. Bramble and D. E. Lieberman, "Endurance running and the evolution of Homo." Nature, vol. 432, pp. 345-352, 18 Nov. 2004], such well-known human traits as lack of body hair, high perspiration rate, upright stature, high arches, broad shoulders, and large gluteus maximus were selected through evolution to make it easier for us to run long distances.