There always seems to be some confusion regarding the types of sugars in energy gels. Here is an overview of the sugars most commonly used in the most popular gels. Maltodextrin is the most easily digested form of carbohydrate, 36% faster than glucose, making it ideal in a gel. More importantly, Maltodextrin requires far less water to be isotonic than glucose or fructose. The flavor is mild even at high concentrations.
Fructose is useful as it is absorbed via different pathways increasing the total carbohydrate absorption above what is possible with Maltodextrin alone. Fructose must be converted into glucose by the liver before it can be used as a fuel by the body. As a result, when fructose is consumed it is a “slower release” carbohydrate. Consuming a gel with both Maltodextrin and Fructose is what I have found works best for me giving your body 2 different pathways of sugar. Some athletes do have an issue digesting Fructose. Fructose malabsorption is an increasingly recognized condition where the person has an inability to absorb fructose from the small intestine into the blood. The fructose passes through to the large intestine, where naturally occurring bacteria digest the fructose and produce gas. People with fructose malabsorption experience abdominal discomfort, bloating, gas if they eat large quantities of fructose. If you have an issue or think you might have an issue with Fructose, Hammer Gel is noteworthy as one of the easiest to digest gels. It is nearly all Maltodextrin dissolved in a greater volume of fluid than other gels. The low level of sugars and electrolytes makes this far easier on the digestive system, and is recommended for athletes who have issues with other types of gel. I do prefer the taste of Hammer gels myself.
The gel I do prefer most of the time is GU gels. It contains 5g of Fructose, with the rest of the calories coming from the easily digested Maltodextrin. I just seems to give me the best "kick", though the taste is sometimes a little strong or excessively sweet. Fructose is 1.7x as sweet as sugar.
Some gels do contain caffeine which is great for improving performance and speeding the absorption of Carbohydrates. With my love affair with caffeine I normally choose gels that contain as least a little.