Food is fuel.
As an athlete, you know that.
You consume three forms of potential fuel or macronutrients/food – carbohydrates, fat and protein. Depending on how hard you work (the intensity) and how long you go (the duration) during exercise determines what proportion and when your body uses them.
Glucose (sugar) is the principle energy source for your body. Blood glucose can be used immediately or stored in your liver and muscles as glycogen. During exercise, muscle glycogen is converted back into glucose to be used as fuel for your working muscles. As your muscles are converting their glycogen into usable glucose, your liver is converting its glycogen back into glucose as well. The liver’s glucose is released directly into the bloodstream to maintain your blood glucose level. Your working muscles “pick up” this blood glucose and use it in addition to the glucose from their own private stores. The primary source of energy for your brain is from that same blood glucose. So keeping your body sufficiently fueled during exercise is extremely important.
Your body is constantly using and replenishing its glycogen stores. Whether you eat a diet high in carbohydrates or not, influences the size of your glycogen stores. This is why you hear athletes say they need to “top off” their glycogen stores. The more you have to tap into or convert, the longer you can go…in theory anyway. There are other contributing factors as to how long you can exercise. The fact that your body can only store approximately 1800-2000 calories or enough fuel for 90-120 minutes of continuous vigorous exercise is one of them. It takes approximately 20 hours for your body to restore it’s depleted glycogen stores. So your rest days are doubly important. Your body needs rest to repair it’s muscles for performance gains and to restore
its glycogen stores. Imagine it this way: if you do not allow sufficient rest, it’s like going into a race without a full tank of gas. You would never set off on a road trip without a full tank. If you did, you’d have to stop sooner than the rest of the field to refuel, right? Seems silly when you can start off full!
Ok, so we’ve talked about how your body uses carbohydrates as fuel during exercise. Now let’s talk about another source of fuel-fats. Stored fats are broken down into fatty acids that are transported in your blood to working muscles. This process is considerably slower than the conversion of carbohydrates for fuel. In addition, fats require oxygen to be broken down. Fats cannot be converted and mobilized quick enough to meet the energy demands of sprinters or athletes performing hill repeats or interval sessions. In these shorter more intense bouts of exercise, carbohydrates are the only form of fuel that can be used to generate ATP anaerobically. Think of ATP molecules as chemical energy.
In comparison, during moderate to intense exercise lasting 4-6 hours, fat contributes up to 70% of your body’s energy needs. Highly trained endurance athletes use more fat for energy while sparing their muscle glycogen stores compared to less fit athletes at the same level of exercise. However, as you pick up the pace, your body shifts from using fats and carbohydrates as fuel to burning more muscle glycogen. Your muscles and brain need a steady supply of glucose to keep working. Remember this : Fats burn in a
carbohydrate flame. Sport drinks containing carbohydrates provide working muscles with a readily available source of glucose. Sport drink carbohydrates enter the blood stream for immediate use. If you do not have enough carbohydrates to be broken down as glucose for muscle and liver demands, your body will shut down. “Hitting the wall’ essentially means there is no more glucose available to your working muscles and “bonking” is used to describe the insufficient blood glucose (or low blood sugar) needed for proper brain function.
Lastly, protein may only contribute up to 15% of your energy needs when your glycogen stores run low. Protein is necessary to repair exercise induced muscle damage and to replace amino acids lost during prolonged exercise, but it is not an immediate source of fuel for exercise. It can be converted into glucose and used as fuel during prolonged bouts of exercise. It may even delay fatigue and enhance mental strength, but it is not your body’s first choice for fuel.
In the end, the most influential factor in your performance (injury and ability aside) is your body’s limited carbohydrate stores. Even though the leanest runner may have enough fat stored to supply up to 100,000 calories, enough for over 100 hours of marathon running, you will experience fatigue and be unable to maintain your current pace, if you deplete your muscle glycogen stores.
As an athlete, you need to educate yourself. If you physically train your body for peak performance, you need to fuel your body for peak performance. Your body is a perfectly engineered machine…a divine creation. Your bodys energy systems work in perfect harmony, switching between one source of fuel to the other, depending on what you ask it to do. Your job is to supply your body with adequate fuel to keep that harmony. Practice with different fuels to find the perfect cocktail for race day. Food is as important
a piece of your athletic puzzle as your training. Train smart. Fuel smart.