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By Shawn Dolan PhD, RD, CSSD

The primary reason to consume calories during endurance exercise is to provide carbohydrate to the working muscle and brain. This becomes increasingly important as the duration of training/competition lasts longer than 60-90 minutes due to the body’s limited capacity to store carbohydrate (glycogen). The ability to digest and utilize carbohydrate is dependent on the amount and types of carbohydrate consumed. Individual carbohydrate sources have different transport mechanisms in the gastrointestinal tract which allow for different rates of absorption. There is good physiological data to support an upper limit in our ability to breakdown carbohydrate during exercise. However, the upper limit is increased if there are multiple sources of carbohydrate consumed. By selecting a product with more than one source (e.g. sucrose and fructose), you are able to absorb more than when you consume a single source (e.g. glucose only).

The general recommendation is to consume 30 – 60 grams of carbohydrate per hour or 120 – 240 calories per hour during endurance exercise. Although at face value the recommendation appears straightforward, there are several influential factors each athlete should consider. Furthermore, the discussion should include both the science and the specific application in order for each athlete to develop an individual fueling plan and put it into practice.

Take a look at the following athlete scenarios and identify the differences.

Joe is an Ironman distance triathlete who typically finishes a race in 10 – 11 hours. He consumes 300 – 360 calories per hour (75 – 90 grams of carbohydrate) during the bike portion of the event, 200 – 250 calories per hour (50 – 60 grams of carbohydrate) during the run portion of the event. He consumes EFS Sports Drink and EFS Liquid Shot to obtain carbohydrate (calories). These products provide multiple sources of carbohydrate.

Then consider Brent – an ultrarunner who typically finishes a 100 mile race in 24 – 26 hours. He consumes 100 – 175 calories per hour (25 – 45 grams of carbohydrate). He relies on EFS Liquid Shot (with water) and snacks provided on the course. Again, multiple sources of carbohydrate are included in the shot.

Why does Joe consume more calories on the bike than the run? Why does Joe consume more calories than Brent when running, even though it is the same mode of exercise?

Consider the following factors that might alter the number of calories (carbohydrate) consumed.

Intensity of Exercise:  When the intensity of exercise is high (beyond lactate threshold), more carbohydrate is used as a fuel source, possibly requiring more to be consumed (depending on duration).

Duration of Exercise: When the duration of moderate to high intensity exercise lasts longer than 2 – 3 hours, more carbohydrate is recommended (up to 90 grams per hour) as long as multiple sources are consumed; however, if the duration is significantly longer (> 10 hours),  lower recommendations typically apply (especially when a single mode of exercise is performed).

Mode of Exercise: Athletes are typically able to consume larger amounts of carbohydrate during cycling than running due to the different nature of the exercise.

Fitness Level and Training Method: Athletes who have a higher level of fitness and train their aerobic metabolism have a greater ability to use fat as a fuel source during exercise and therefore rely less on carbohydrate.

Number of Carbohydrate Sources in Product:  If a training session lasts longer than 2 – 3 hours, select a product with multiple carbohydrate sources. If the training session is less than 2 – 3 hours, one source (e.g. glucose) may be adequate.

Source of Carbohydrate:  Some carbohydrate sources are more readily absorbed than others; glucose, sucrose, and maltodextrin are absorbed more quickly than fructose, galactose, and amylose.

Heat and Humidity:  In a hot and/or humid environment the body requires a greater amount of carbohydrate; however, the GI system is often less able to digest and absorb carbohydrate, requiring a fine balance between supplying enough but not too much.

Altitude:  At higher altitudes there is an increase in utilization of carbohydrate as a fuel source, which may require larger amounts to be consumed.

As you plan for your next race, consider the factors listed above to determine the best number of calories for YOU to consume per hour. Take a look at your sport nutrition product label to identify the carbohydrate sources and amount per serving to help you individualize your nutrition plan.  Taking the extra time to analyze your carbohydrate intake and modifying it to optimize your performance just may pay off.