For athletes involved in preparation for prolonged heavy exercise, it is recommended that percent of their calories should be obtained from carbohydrates.

60-70%

Explanation:

Carbohydrate is the most important source of energy needed in preparation for prolonged exercise that consumes energy and requires endurance and strength to combat fatigue.  

Carbohydrate is a more efficient fuel than fat as carbohydrates are easily broken down  to supply an athlete with glucose as readily available energy, and also glycogen that is stored in the muscles, which are used as a fuel source during exercise. Hence, for prolonged heavy exercise, it is recommended that athletes need high carbohydrate consumption of 60-70 percent of their calories to maintain optimal performance.

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