In simple terms, our fuel tanks include our muscles, liver, and blood. The amount of fuel stored in our tanks depends on our training and diet in the days leading up to the race. A fully stocked human fuel tank allows us to travel at maximum effort for about 90 to 120 minutes-assuming no additional fuel is added to the system. The need to supplement fluids and fuel for events lasting over about 90 minutes is fairly common knowledge. The problem comes when people wait until the 90-minute mark to begin rehydrating and refueling.
Individual differences play a huge role when it comes to fluid losses due to sweat.
Some of us can really sweat up a storm. This does help cool the body, but the bad news is some of us sweat out more precious fluid than we are able to absorb in a given time period. In other words, the rate that sweat pours out of our bodies can be greater than our fluid absorption rate. Let's say your body sweats out 1500 ml per hour, but can only absorb 1000 ml per hour. Given such a situation, you will slowly dehydrate at a rate of 500 ml per hour.