How Many Carbs?

Carbohydrate is the most important fuel during exercise because it can be broken down quickly to give your muscles a boost of energy.

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Carbohydrate is the most important fuel during exercise because, unlike proteins and fat, it can be broken down quickly to give your muscles a boost of energy.

Photo: Ryan Bethke

How many carbs you can ingest is limited by the rate of absorption from the intestines into the bloodstream. Peak rates from a single carbohydrate (i.e. glucose) appear to be around 1.2 grams per minute.

However, when different types of carbohydrates are combined—most commonly glucose and fructose—the rate can increase to about 1.75 grams per minute, almost a 50 percent increase. This is because fructose and glucose use different transporters that absorb faster into the bloodstream, delivering energy to your muscles quicker.

Consuming a mixed carbohydrate sports drink or gel rather than a glucose-only drink allows you to use carbohydrate at a faster rate, and the sugar blend reduces feelings of stomach discomfort. Most sports products will use a mix of carbohydrate sources.

How many carbs you should take in per hour is influenced by exercise intensity and duration, carbohydrate availability, your fitness level and environmental conditions. Here are my carb intake recommendations for specific workouts or races.

Sprint races and short workouts

You need: A swig of a sports drink
Your muscles and liver have enough stored glycogen for short efforts, so additional carbohydrate is usually unnecessary. However, research has shown that a sports drink can still be beneficial. Studies of athletes who exercise for 60 minutes have found improved performance when they take in carbohydrate, despite having adequate body stores. Even just rinsing your mouth with a carbohydrate-containing fluid (such as a sports drink) can enhance performance. Although the explanation for this improved performance is unclear, it seems that a small amount of carbohydrate may enhance exercise performance simply by stimulating the brain!

Workouts > 90 minutes

You need: 30-60 grams per hour, single or mixed carbs
This is the point when muscle glycogen stores are stressed and you need to take in additional carbohydrate to maintain blood-glucose levels and provide fuel for working muscles. How much carbohydrate you need depends on many factors (pre-carbo-loading, fitness, body size, etc.) and can range from 30-60 grams per hour. Because you are not trying to absorb as much energy as you can, a mixed carbohydrate sugar source is not as vital.

Half- or full-iron-distance races

You need: 80-100 grams per hour (males), 60-90 grams (females), mixed carbs
In Ironman races, where carbohydrate is an important fuel for the working muscles, think about consuming as much as you can (sensibly) or as tolerated without causing GI problems. The bike during an iron-distance race could be considered a “racing smorgasbord”—tolerance, opportunity to eat and availability of food and fluids are high compared to the swim and run. Males should aim to consume 80-100 grams of carbohydrate per hour from mixed carbohydrate sources (more than one type of sugar) throughout the ride. For females, 60-90 grams is a reasonable hourly target.

Gregory Cox is a registered dietitian and has a master’s degree in sports nutrition.

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