There are so many different types of nutritional products available for endurance exercise. If you are new to endurance exercise and new to what is available or if you've been around for awhile and have nutritional issues that are plaguing your ability to perform at longer distances, hopefully I can help you.
Much of what you will tolerate is via trial and error. Each person is different and has a different digestion rate during exercise. Some have the ability to absorb liquids and semi-solids quickly, for others it takes an extended amount of time. This is where the ingredients become important. You have to read the labels and you need a basic understanding of the types of carbohydrates available to you and a little bit of knowledge as to how they are digested. Endurance athletes must rely on additional sources of carbohydrates to delay the onset of fatigue and to help maintain performance levels. You have some stored muscle sugar (glycogen) but it's best to preserve these reserves as long as you can. If you have nothing available to you to ingest, and you run out of sugar available in your blood, your body will metabolize (breakdown) the glycogen stored in your muscle tissue. There isn't that much there, marathoners that drink nothing but water and take in no gel call this "hitting the wall", cyclists and triathletes refer to this phenomenon as "the bonk". Bonking or hitting the wall boils down to you having used up all your sugar storage leaving only fat for fuel. Fat does not burn as quickly even though you have plenty of stored calories to use so you slow down, and often feel horrible with little energy left to complete the task. It's easily avoidable with the right amount of calories. The average person can take in 250-300 cals per hour with exercise and those calories mostly come in the form of sugars. It's the type of sugar that makes all the difference for some people.
Carbs break down into sugars. The type of carbohydrate you ingest dictates the sugar(s) it breaks down into.
Monosaccharides: the simplest sugars. They include glucose (a.k.a. dextrose), fructose and galactose. These sugars differ based on their structure which we don't need to get into. Note that they have a high glycemic index meaning that they typically tend to break down quickly and enter the bloodstream faster than other sugars.
Disaccharides: two sugar molecules combined. These include: Sucrose (table sugar) = glucose + fructose; Lactose (milk sugar) = glucose + galactose; and Maltose = glucose + glucose. Some people cannot break the bond that holds the sugars together; like the lactose intolerant and bloating, cramping, gas, and diarrhea become common symptons since the sugar cannot be digested and absorbed into the bloodstream.
Polysaccharides: Long chains of glucose. These are your starches. A polysaccharide like maltodextrin is a typical component of many sports drinks, it is just a long chain of three or more glucose molecules held together.
So what does this mean for the endurance athlete?
Many companies use combinations of monosaccharides and disaccharides only. Others use predominantly maltodextrin. Your ability to digest certain sugars depends on your gastric (stomach) emptying rate, the concentration of the fluids or gels you are taking in and how hard you are working. Higher heart rates make it more difficult to digest and absorb nutrients as the blood is shunted away from the core and towards the muscles for powerful contractions, and increased oxygen and waste removal demands. If you have been suffering from GI distress during long workouts and races, check the label! If you are taking in sugars in the form of fructose- it may be the culprit. Fructose malabsorption is common, symptoms may include stomach pain, gas and diarrhea. If you notice that you have difficulty digesting anything with fructose in it while exercising, watch out for sucrose! Sucrose breaks down into fructose and glucose. While glucose is actively absorbed in the small intenstine, fructose is not and it may pose difficulty in absorption. Fructose must also be sent to the liver first since the enzyme fructokinase to metabolize it is only found in large quantities there. The other main thing to look out for is the concentration of the fluids and gels use take in. Take gels with water only. If you are taking your gels in conjunction with a sugary fluid replacement drink the concentration (osmolality) of the mixture increases and makes it more difficult to digest and absorb. You need water too! Don't forget it! Many people rely soley on the sugary drinks and gels and forget to take in water. Listen to your thirst, when you are craving water, make sure to get some! GI distress distress can also be caused by ingesting too much salt, so be wary. If you have any questions, feel free to ask. Exercise nutrition is very personal and it helps to have an objective opinion. Send me your issues in a comment or by email and I'll be happy to post a Q and A blog that may help many other people out there! Physical training is important for long endurance racing but you will never finish within your potential unless your calorie intake is nailed down. Let the questions begin!