Carbohydrate dependence during prolonged simulated cycling time trials

We determined the effect of suppressing lipolysis via administration of Nicotinic acid (NA) and pre-exercise feeding on rates of whole-body substrate utilisation and cycling time trial (TT) performance. In a randomised, single-blind, crossover design, eight trained male cyclists/triathletes completed two series of TTs in which they performed a predetermined amount of work calculated to last similar to 60, 90 and 120 min. TTs were undertaken after a standardised breakfast (2 g kg(-1) BM of carbohydrate (CHO)) and ingestion of capsules containing either NA or placebo (PL). Plasma [free fatty acids] were suppressed with NA, but increased in the later stages of TT90 and TT120 with PL (p < 0.05). There was no treatment effect on time to complete TT60 (60.4 +/- A 4.1 vs. 59.3 +/- A 3.4 min) or TT90 (90.4 +/- A 9.1 vs. 89.5 +/- A 6.6 min) for NA and PL, respectively. However, TT120 was slower with NA (123.1 +/- A 5.7 vs. 120.1 +/- A 8.7 min, p < 0.001), which coincided with a decline in plasma [glucose] during the later stages of this ride (p < 0.05). For TTs of the same duration, the rates of whole-body CHO oxidation were unaffected by NA, but decreased with increasing TT time (p < 0.05). CHO was the predominant substrate for all TTs contributing between 83 and 94 % to total energy expenditure, although there was a small use of lipid-based fuels for all rides. (1) NA impaired cycling TT performance lasting 120 min, (2) cycling TTs lasting from 60 to 120 min are CHO dependent, and (3) there is an obligatory use of lipid-based fuels in TTs lasting 1-2 h.