HAGERMAN, F. C. Energy metabolism and fuel utilization. Med. Sci. Sports Exerc., Vol. 24, No. 9 Supplement, pp. S309-S314, 1992. Key Points: 1) Total energy expenditure during marathon training and competition is among the highest reported for any endurance activity. 2) Although training intensity for the marathon may vary between 60–125% of maximal physical working capacity, elite runners must run consistently at about 80–90% of VO2max in order to achieve successful competitive performances. 3) Earlier studies demonstrated that carbohydrate was the preferential fuel for endurance exercise, but more recent studies have suggested a greater role for lipids than previously thought. Also, proteins may contribute as much as 5–10% of the total energy to a marathon performance. 4) The preponderance of Type I and IIA skeletal muscle fibers among marathon runners and their significantly improved storage capacity for intramuscular lipids and increased mitochondrial density may account for increased reliance on oxidation of fats.