How They Work
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?