Can an interrelationship between lactate and carbohydrate utilisation explain the inter-individual variations in Fatmax?

INTRODUCTION: High inter-individual variations in carbohydrate (CHO) and fat utilisation have frequently been reported during incremental exercise testing. Maximal fat oxidation (Fatmax) and the intensity at Fatmax often corresponded to different levels of blood lactate concentration (BLC) of 1-2.2 mmol.l-1, corresponding to exercise intensities of approx. 30-75 % (Achten and Jeukendrup 2004; Bircher et al. 2005). An interrelationship between BLC and the relative rates of pyruvate combustion (RPY) has previously been proposed based on the idea that lactate may be used to indicate pyruvate availability, and shown to be independent of cycling cadence (Alkhatib and Beneke 2006). The latter interrelationship suggests that RPY changes may be reflected by BLC levels, and that the variations in CHO and fat utilisation may be indicated by the magnitude of this BLC-RPY interrelationship. This study investigates whether the magnitude of the BLC-RPY interrelationship is related to the variations in Fatmax or the intensity at Fatmax.
METHODS: 21 healthy males (Age 26.3±6.0 years, Height 179.4±8.1 cm, Body mass 74.9±12.5 kg) completed an incremental load cycling test at 50 rpm. The workload was initiated with 1 W.kg-1 body mass, and increased by 0.5 W.kg-1 body mass every 2 min stage. Capillary blood samples were drawn from the hyperaemic earlobe at rest and at the end of each stage. Oxygen uptake ( ) and carbon dioxide production ( ) were measured, and analysed for estimating CHO and fat oxidation using indirect calorimetry. The relative rate of pyruvate combustion (RPY) was calculated as a percentage of the full pyruvate combustion (when equals ). RPY was further approximated as a sigmoid function of lactate (RPY = 100 / (1+kel / BLC2) where kel is the constant of half maximal pyruvate combustion. Absolute pyruvate and fat combustion rates were calculated based on the O2 required to oxidise glycogen and fatty acid, and further used to determine Fatmax and the intensities at Fatmax.
RESULTS: Fatmax was (Mean±SD (Min/Max)) 0.46±0.17 (0.10/0.77) g.min-1 corresponding to an intensity of 40.0 ± 9.7 % (25.0 / 55.0) % of peak power equivalent to 47.2±9.7 (30.1/63.9) % , and to BLC levels of 1.4 ± 0.4 (0.6 / 2.3) mmol.l-1.Estimates of kel were 1.82±0.95 (0.54/4.4) (mmol.l-1)2. Levels of kel correlated positively with Fatmax (r = 0.511, p<0.05), and the intensity at Fatmax (r = 0.485, p<0.05).
CONCLUSION: The high kel variability may reflect the high variance in Fatmax, corresponding BLC, and exercise intensity. At a given BLC and , higher levels of kel seem to reflect higher intensity at Fatmax, which implies higher individual’s ability to reserve CHO known to delay glycogen depletion and related fatigue mechanisms.