An interrelationship between lactate and carbohydrate utilisation explains the cross-over point inter-individual variability

The cross-over point (COP) is defined as the power output where energy from CHO derived fuels predominates over energy from lipids, and shown to be sensitive to fitness status, reflecting the biochemical adaptations that affect CHO and fat oxidation at COP (Brooks & Mercier, 1994:Journal of Applied Physiology, 76, 2253–2261). High inter-individual differences have been reported at several intensities where CHO and fat interact, including those at COP (Venables et al., 2005:
Journal of Applied Physiology, 98, 160–167). Various BLC levels have been suggested to explain these inter-individual differences including thresholds at 2
and 4 mmol.l-1 (Bircher et al., 2005: European Journal of Clinical Investigation, 35, 491–498; Billat et al., 2004: European Journal of Applied Physiology, 447, 875–883). Based on the idea that lactate may be used to indicate pyruvate availability, an interrelationship between BLC and relative rates of pyruvate combustion (RPY) has been proposed (Beneke et al., 2005: ECSS 10th Annual Congress, 93). The latter suggests that RPY changes may be reflected by individual BLC levels, as indicated by a constant (kel) describing RPY as a function of BLC. The present study investigates whether kel serves as an independent factor to explain COP variance.
Twenty one healthy males (Age mean 26.3, s ¼ 6.0 years, Height 179.4, s ¼ 8.1 cm, Body mass (BM) 74.9, s ¼ 12.5 kg) completed an incremental cycling test. The workload was initiated with 1 W kg71 BM and increased by 0.5 W.kg-1 BM every 2 min stage. Capillary blood samples were drawn at rest and at the end of each stage. Respiratory VO2 and VCO2 were measured and analysed for estimating CHO and fat oxidation using indirect calorimetry. RPY was calculated as a percentage of full pyruvate combustion (when VO2 equals VCO2). kel was approximated based on the following interrelationship between RPY and BLC: RPY ¼ 100/(1+kel/BLC2). BLC thresholds at 2 (BLC2) and 4 (BLC4) mmol.l-1 were determined based on corresponding intensities relative to peak power. COP was calculated as the intensity where <50% of energy is derived from CHO. COP was 161.35, s ¼ 48.02 W corresponding to 42.14, s ¼ 10.07% of peak power and 49.46, s ¼ 10.53% VO2peak and BLC levels of 1.5, s ¼ 0.4 mmol-1. Estimates of kel were 1.75, s ¼ 1.10 (mmol.-1)2 . kel explained 30.6% of the variance in COP (P< 0.01). This was improved to 76.2% when adding BLC2 as a predictor (P< 0.001), whereas BLC4, peak power, and VO2peak were not predictors. This suggests that kel may serve as a useful indicator for COP, and may explain inter-individual variability at that point.