How eating carbs or fat before exercise affects energy use and performance

Increased dietary carbohydrate intake prior to competition can enhance exercise performance in endurance events lasting 90 minutes or more.

• Higher muscle glycogen levels from carbohydrate intake may improve endurance performance.
• Carbohydrate consumption 3-4 hours before exercise can boost liver and muscle glycogen stores.
• Effects of carbohydrate ingestion on blood glucose and fat levels during exercise can last for at least 6 hours.
• Increased plasma insulin from pre-exercise carbohydrate may inhibit fat breakdown and liver glucose output, potentially causing hypoglycemia.
• There is no strong evidence that hypoglycemia from carbohydrate ingestion consistently impairs exercise performance.
• Increasing fat availability before exercise may reduce carbohydrate use but does not seem to provide performance benefits.

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