CARBOHYDRATE DEPENDENCE DURING MARATHON RUNNING

To test the hypothesis that marathon running is dependent on lipid oxi dation, 12 post-absorptive males (31.9 +/- 2.1 yr) ran a treadmill mar athon and substrate utilization was assessed. Subjects were placed int o a fast (F less-than-or-equal-to 2 hr, 45 min; 73.3% VO2max), or a sl ow (S less-than-or-equal-to 3 hr, 45 min; 64.5% VO2max) Marathon group . The day before testing subjects rested, but ate their normal diet. S ubjects were tested in the morning after an overnight fast, and only t ap water, at a rate of 1 l.h, was ingested during exercise. Blood gluc ose concentration rose at exercise onset, peaked at approximately an h our, but then decreased over time remaining at or above resting levels . Free fatty acids and glycerol rose continuously. No significant diff erences in plasma FFA, glycerol, or blood glucose concentrations were observed between F or S groups during the marathon. Mean blood lactate concentration was significantly higher (P < 0.05) in the F (2.1 +/- 0 .3 mM) group than the S (1.2 +/- 0.2 mM) during exercise. Mean plasma epinephrine was significantly higher in the F (0.9 +/- 0.2 ng.ml-1) th an the S (0.6 +/- 0.2 ng.ml-1) group; norepinephrine was also higher i n F (3.9 +/- 1.4 ng. ml 1) than the S (2.5 +/- 0.9 ng.ml-1, P less-tha n-or-equal-to 0.05). Blood lactate and epinephrine concentrations corr elated significantly (r = 0.76 and 0.78 in F and S groups, respectivel y). The average respiratory gas exchange ratio (R = VCO2/VO2) was high er in F (0.99 +/- 0.01) than S (0.90 +/- 0.01, P less-than-or-equal-to 0.05). A direct relationship between carbohydrate oxidation and runni ng speed during marathon running is indicated. Estimated carbohydrate combustion [(F: 2,414.3 +/- 43.0 kcal (575 +/- 10 g); S: 2,890.0 +/- 1 59.0 kcal (688 +/- 38 g)] exceeded estimated glycogen stores in active muscle and liver (475 g = 375 g (muscle) + 100 g (liver)]. Therefore, total body glycogen stores were made available for combustion. All cl asses of energy substrates participate, but carbohydrate, not lipid, i s the primary fuel for marathon running.