The human body can last, on average, 60-90 minutes on its stored muscle glycogen. Muscle glycogen (carbohydrate) is the primary fuel source used by the body for most endurance events. Fat is also a fuel but typically we have enough fat stored to last several days. If our muscle glycogen stores run out then it is known as hitting the wall or bonking. Due to the length of a marathon or long triathlon event, fueling becomes essential for most people. Below are a few tips you can use to determine how much you should consume.
Consume 1-2 grams of carbohydrate per kilogram of body weight per hour. If the event you are doing will take you less than an hour you may not need to fuel at all.
The higher the intensity of the event (the higher your effort level) generally the harder it is to consume and absorb calories or carbohydrates. Thankfully the faster you are going typically the shorter the race, which may mean you require less fueling. For a marathon you will need to consume some kind of carbohydrate no matter who you are. If you are an elite runner staying close to your 85% threshold effort then you will likely need to be down around 1 gram of carbohydrate per kilogram of body weight per hour. However if you are in a 3-hour plus event and not at the elite level you may need to be consuming closer to 1.5 grams of carbohydrate per kilogram of body weight per hour. If you are in the 4 plus hour category and your effort level is down in the 60% category then you can probably consume 1.5-2 grams per hour. It is very important to know where you fit and practice any nutritional strategies.
Consuming small amounts of fuel frequently is the best way to avoid stomach problems. No matter what effort you are at, if you dump too much carbohydrate into your system at once it may come back to haunt you. The digestive system can only handle so much at one time and as your effort increases the amount you can tolerate will typically decrease. Aid stations usually occur every mile or every 5-12 minutes during the race so it is a good idea to practice this fueling using this time frame in training.
Everyone is different when it comes to nutrition and understanding what you can consume is absolutely essential to a successful process!