Your body uses two primary sources of fuel to feed the muscles when you’re running — fat and carbohydrate. Fat is a largely abundant resource, but is broken down into usable energy slowly, making it an ineffective fuel source when running anything faster than about 60-70% of your VO2max (roughly equivalent to your aerobic threshold or marathon pace).
Therefore, your body relies on carbohydrate as its primary fuel source when racing. Generally, the faster you run, the greater the percentage of your fuel will come from carbohydrates. The problem with carbohydrate is that we can only store a limited amount in our muscles — even when you load up. Typically, we can store about 90 minutes of muscle glycogen when running at half marathon pace and about 2 hours worth when running at marathon pace. So, if you’re not an elite athlete, you’ll be running out of muscle glycogen long before you cross the finish line.
Simply speaking, energy gels are designed to replenish carbohydrate stores that are depleted when running. Sounds like energy gels are a savior, right?
Unfortunately, energy gels don’t provide a simple one-to-one replacement (something you won’t read on the label of your favorite gel) because the glycogen we ingest from gels doesn’t always make its way to the working muscles. So why use them?
Different Types of Energy Gels
Isotonic Energy Gels Isotonic means that they have already been mixed to the correct water / electrolyte balance. Therefore, you don't need to take extra water. These are excellent if you are worried about getting the right electrolyte / water concentration. They will provide energy and also help provide optimal hydration. The drawback is that they are a little more bulky to carry around. But, it also saves carrying both gels and water. They are great for training runs for simultaneously topping up both water and energy levels.
Glucose / Fructose 2:1 Studies have suggested that a combination of glucose and fructose in a 2:1 ratio can lead to a higher uptake of carbohydrate than just relying on glucose. Glucose has a very high GI index (increases blood sugar immediately) Fructose has a lower GI index and raises your blood sugar levels more gradually. Therefore, this combination can be good for long distance running who are struggling to take on enough carbohydrate during a race.
Caffeine Some energy gels also include caffeine - a legal stimulant. Studies suggest that ingesting caffeine can boost your performance, though it can vary between individuals. In long distance races, some runners like to take a caffeine energy gel towards the end of the race when they are becoming tired. Some studies have suggested caffeine is less effective in hot weather because it can lead to dehydration. Caffeine also acts as a diuretic making you need a toilet break in a race - another reason to save for later in the race. Whether you want to use caffeine will come down to personal preference whether you want to benefit from this everyday stimulant.