Effect of Two Different Intramyocellular Lipid Contents on Substrate Utilization During Endurance Exercise and Performance

Abstract

1494 In the field of sport, a deal of effort is placed on devising ways to increase the contribution of lipid energy during exercise (EX), with the aim to spare muscle glycogen for the later part of a race. It is a matter a debate to what extent intramyocellular lipids (IMCL) are used during EX and whether they contribute to performance when glycogen status is controlled. PURPOSE: To investigate the effect of a higher content of IMCL before EX on the availability of local fuels (IMCL and glycogen) and its effect on subsequent performance where glycogen is theoretically the limiting factor. METHODS: Eleven endurance-trained male athletes (age: 31 ± 6 years, mean ± SD; body mass (BM): 74 ± 8 kg; VO2 peak: 63 ± 7 ml/min/kg BM; maximal workload (Wmax): 365 ± 29 Watts) were investigated twice in a randomized crossover design following two types of diet: low fat diet (LF: fat <0.5g/kg BM during 2.5 days) and high fat diet (HF: fat <0.5g/kg BM during 1 day followed by fat = 3.5g/kg BM during 1.5 days). Carbohydrate (CHO) intake was identical on both occasions (7g/kg BM/d for 3 days). Before and after a 3-h EX on a bicycle-ergometer (50% of Wmax) IMCL- and glycogen were measured in the thigh muscle by 1H (IMCL) – and 13C (glycogen) – magnetic resonance spectroscopy (MRS) on a 1.5 Tesla MR-system using a flexible, 1H/13C double-tuned coil. VO2 uptake, VCO2 production, glucose, insulin, FFA and TG were determined at baseline and at regular intervals throughout the EX. The MRS-measurement after EX was followed by a 20-km time trial (TT) that subjects performed on a racing bicycle with the back wheel fixed to a magnetic brake. RESULTS: IMCL levels at rest were significantly higher (P<0.001) after HF compared to LF whereas glycogen levels were the same. Subjects burnt the same amount of glycogen during EX, independent of the preceding diet. IMCL utilization was significantly larger after HF (P<0.001), leading to the same post EX concentrations of IMCL after either diet. During EX, average total fat- and CHO-oxidation as well as energy expenditure were the same after HF and LF diet. During EX blood TG were significantly lower after HF (P<0.01) than after LF. There was a significant correlation between IMCL and insulin concentrations at rest after HF (R = 0.619) but not after LF (R = 0.045). Subjects were 25 sec faster in the TT after HF compared to LF but this was not significant. A correlation (HF: R = 0.539; LF: R = 0.782) between IMCL depletion during the 3-h EX and time used for TT was found. CONCLUSION: The higher the initial IMCL levels are, the larger is their contribution during submaximal exercise. Already a short (1.5 days) high fat diet modulates IMCL and influences fat metabolism. Supp. by Swiss National Fund (3100–065315.01).