[METHODS] After 2 days on a fat-rich ( 24.2 % 0.8 % CHO , 60.4 % 0.3 % fat , and 15.5 % 1.0 % protein ) , CHO-rich ( 69.3 % 1.2 % CHO , 15.9 % 2.1 % fat , and 15.1 % 1.1 % protein ) , or habitual ( 56.1 % 7.0 % CHO , 27.5 % 4.9 % fat , and 16.5 % 4.0 % protein ) diet , 19 trained adolescent boys ( 15.21.5 years ) performed a 10-minute constant run at 65 % VO2peak to determine the respiratory exchange ratio ( RER ) during exercise and 10,000-m running on an outdoor track .