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Preventing the "Low-Fuel Light" in Endurance Exercise

Written by Andrea Hacker Thompson M.S., R.D.

Endurance athletes spend months training for a marathon or triathlon with a performance goal in mind. Often, nutrition is mistakenly left out of their plan. Nutrition should be a fundamental tool in every endurance athlete’s toolbox. During the training season, athletes should practice and sharpen their nutrition plan so they feel confident in it on race day.

ACSM defines an endurance athlete as one who trains and competes for 90 minutes or longer. A nutritional plan is especially important for endurance athletes because they are at high risk of bonking, or as I call it, having the “low-fuel light” come on.

The body of an endurance athlete is like a race car with two fuel tanks. The duration and intensity of the activity determines which tank is the primary fuel source. Tank A is the body’s fat stores, which contain about 70,000 calories of fat that are available during lower-intensity aerobic exercise. Tank B is the body’s carbohydrate stores, which are glycogen stored in the muscle and liver. As the intensity of a workout increases, the ability to use the fat in tank A as fuel decreases, and the body becomes more dependent on carbohydrates in tank B for fuel. The body can only store around 2,000 calories of glycogen at a time, which fuels both the working muscles and brain. When our glycogen stores get low, the low-fuel light comes on. Both the brain and muscles send signals of fatigue.

When we exercise for less than 90 minutes, tank B has sufficient stores to power us through the activity. However, when we exercise for more than 90 minutes, we need to have a nutritional plan to prevent the low fuel light from turning on.

There are four key areas to focus on if you want to prevent a low-fuel light—fueling before exercise, fueling during exercise, fueling after exercise and daily fueling.

Fueling Before Exercise A race car never starts a race without new tires and a full tank of gas, so an endurance athlete should not start a workout without fueling. Eating before a workout guarantees that the body starts with a full tank of glycogen.

If you have three or four hours, eat 300-600 calories, primarily of carbohydrate (2-3g/kg body weight), moderate in protein and low in fat. Minimize the amount of fiber in this meal to prevent stomach discomfort during exercise. Even if you are not hungry, you should have something to eat before a long workout. Think of it as fueling your body so it can perform optimally.

Pre-exercise meals can include:

  • Oatmeal with milk, fruit and nuts

  • Turkey sandwich with fruit

  • Cottage cheese with crackers and fruit

  • Toast and peanut butter

Three to four hours before you work out, drink 2-4 cups of fluids. One hour before you work out, drink 1-2 cups of fluids.

Fueling During Exercise

This fueling opportunity is the well-planned “pit stop.” The fuel should be simple, easily digestible carbohydrates that the body needs to maintain energy and prevent fatigue.

Fuel every 45-60 minutes during a long workout. ACSM guidelines recommend 30-60 grams of carbohydrate (120-240 calories) per hour. Remember that for optimal performance, we also need to provide the body with fluids and electrolytes. If the workout is less than 90 minutes, but at a high intensity, you may want to drink an energy drink instead of water or bring an energy gel with you.

Mid-exercise foods can include:

  • Gels

  • Energy beans

  • Energy beverages

  • Honey

  • Bananas

  • Oranges

During endurance exercise, drink 6-12 oz. of sports drink or water every hour.

Eating After Exercise

The goal for post-workout fueling is recovery. Fueling will help you replenish glycogen stores used during the workout, optimize protein synthesis to repair damaged muscle tissue and stimulate the development of new tissue, and replace fluids and electrolytes that were lost in sweat.

Within 30 minutes of exercise, an endurance athlete should have a snack of 300-400 calories containing carbohydrate (75-100 grams) and protein (6 grams). The carbohydrate-to-protein ratio should be 2:1 in short, low- to medium-intensity workouts or 3:1 in long, high-intensity workouts.

Post-exercise foods can include

  • Chocolate milk

  • A high-density nutrition bar (e.g., Clif bar)

  • Smoothie with yogurt and fruit

After exercise, drink two cups of fluid for every pound of body weight lost.

Daily Fueling

The fourth way to prevent your low-fuel light from turning on is to eat a diet consistently high in carbohydrates. A diet full of whole grains, fruits, vegetables and lean protein (not in cookies and chips) will ensure that your muscles have fuel when you hit the road.

A good pit crew at the Indianapolis 500 is a key part of a winning performance. A good fueling plan is equally as important for an endurance athlete, making the difference between running out of fuel and taking a victory lap.

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