DON’T HIT THE “WALL” : Calculating Your Calorie Needs Helps You Finish Strong


by John A. Ruibal, MS, RD,C SSD, LD

Nutrition plays an important role in how well and how long we can exercise. What you need to determine is how much fuel or calories you need before working out and how much you need to put back in your body after your workout.

How much fuel do I have?

A well-fueled athlete will have approximately 500 grams of carbohydrates stored prior to a workout or race. Of that, about 300-400 grams will be stored in the muscle glycogen, 90-110 grams stored in the liver and 25 grams circulating in our blood. Each gram of carbohydrate contains 4 calories.

Let’s do the math. I use the lower numbers to ensure I don’t overestimate how much fuel I have for my work out.

  • 300 grams from Muscle glycogen = 300 x 4 = 1,200 calories
  • 90 grams from liver glycogen = 90 x 4 = 360 calories
  •  25 grams of blood glucose = 25 x 4 = 100 calories

If you start your workout or race properly fueled, you should have between 1,500-1,660 calories in your tank before your “low fuel” light comes on.

How much fuel do I burn during a workout?

This will vary depending on how fast you are running and how much you weigh. There are plenty of energy expenditure calculators available online, but for runners a good rule of thumb is 100 calories per mile.

Next, it’s important to estimate how far we you could run before your low fuel light comes on. Again, I like to use the lower numbers of my estimated fuel reserve to build in a safety factor. Plug the numbers into your calculator: 1,500 divided by 100 equals 15 miles. As I said earlier, more precise expenditure calculators can be found online.

How much do I need to consume to finish strong?

Now that we know how much fuel you are burning, we can determine how much fuel you need to consume to avoid “hitting the wall.” The first thing to note about fueling for your workout is if you wait until your low fuel light comes on, it’s too late. It takes time for the carbohydrates you consume to be broken down into glucose for your working muscles to use.

Current research recommends that you consume 100 calories from carbohydrates every 45-60 minutes during training and racing if the total workout time is 90 minutes or longer. The good news is there are a large variety of carbohydrate gels, chews and jelly beans that are about 100 calories per serving. The important thing to remember is that fluids are required to digest these carbohydrate fuels. A good rule of thumb is to take your energy packets while you are hydrating and consume 4 to 6 ounces of fluid per packet.

If you do not tolerate the pre-packaged carbs well, be sure to hydrate with sports drinks instead of water to get your calories. Drinking 8 ounces of sports drink every 45 minutes is a good substitute for a carbohydrate packet. Also ensure the sports drink you are hydrating with (or the one served at your race) has more than just electrolytes. Many sports drinks on the market are low calorie or calorie free and are designed to address solely your electrolyte needs.

Remember to train smart with your nutrition during workouts so that you know you tolerate the product. By practicing good fueling techniques during your workouts you may never have to experience the feeling of hitting the wall.

iRunJohn Ruibal is a registered dietitian who is board-certified in sports nutrition from the academy of Nutrition and Dietetics. He coordinates the distance running classes available through both iRun Texas locations in San Antonio. He has been running for more than 40 years and coaching for 25. To contact John or learn more about his classes, visit

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