Fuel utilisation during prolonged low-to-moderate intensity exercise when ingesting water or carbohydrate

Abstract

Previously, we examined the effects of carbohydrate (CHO) ingestion on glucose kinetics during exercise at 70% of maximum O2 uptake (VO2, max). Here we repeat those studies in heavier cyclists (n = 6 per group) cycling for 3 h at a similar absolute O2 uptake but at a lower (55% of VO2, max) relative exercise intensity. During exercise, the cyclists were infused with a 2-3H-glucose tracer and ingested U-14C glucose-labelled solutions of either flavoured water (H2O) or 10 g/100 ml glucose polymer, at a rate of 600 ml/h. Two subjects in the H2O trial fatigued after 2.5 h of exercise. Their rates of glucose appearance (Ra) declined from 2.9 +/- 0.6 to 2.0 +/- 0.1 mmol/min (mean +/- SEM) and, as their plasma glucose concentration [Glu] declined from 4.7 +/- 0.2 to below 3.5 +/- 0.2 mM, their rates of glucose oxidation (Rox) and fat oxidation plateaued at 2.7 +/- 0.4 and 1.7 +/- 0.1 mmol/min respectively. In contrast, all subjects completed the CHO trial. Although CHO ingestion during exercise reduced the final endogenous Ra from 3.4 +/- 0.6 to 0.9 +/- 0.3 mmol/min at the end of exercise, it increased total Ra to 5.5 +/- 0.5 mmol/min (P < 0.05). A higher total Ra with CHO ingestion raised [Glu] from 4.3 +/- 0.3 to 5.3 +/- 0.1 mM and accelerated Rox from 3.5 +/- 0.2 to 5.9 +/- 0.2 mmol/min after 180 min of exercise (P < 0.05). The increased contribution to total energy production from glucose oxidation (34 +/- 1 vs. 20 +/- 1%) decreased energy production from fat oxidation from 51 +/- 2 to 40 +/- 5% (P = 0.08) and produced patterns of glucose, muscle glycogen (plus lactate) and fat utilisation similar to those during exercise at 70% of (V&dot;O2, max). Thus, CHO ingestion is necessary to sustain even prolonged, low to moderate intensity exercise and when ingested, it suppresses the higher relative rates of fat oxidation usually observed at exercise intensities less than 60% of VO2, max.

Fuel kinetics during intense running and cycling when fed carbohydrate

“…The rates of total blood glucose oxidation in the present study (-1 g.min-1) are highly comparable with other investigations with trained subjects cycling at either 55% (Rauch et al 1995) or 70% of V0 2 max (Bosch et al 1993a;Broberg and Sahlin 1989;Hawley et al 1994). However, the peak rates of ingested glucose oxidation observed in the current study (-0.5 g.min-1) are much lower than those typically measured during prolonged, moderateintensity exercise (Bosch et al 1993a;Hawley et al 1992Hawley et al , 1994Rauch et al 1995). In those studies ingested glucose oxidation typically peaks at -1.0 g.min-1 (for review see Hawley, Dennis and Noakes 1992) and contributes approximately 20% of total CHO oxidation.…”

“…The effect of carbohydrate (CHO) ingestion on energy metabolism and endurance capacity during prolonged (>90 min), low-to-moderate intensity (55-70% of maximal oxygen uptake [V0 2 maxn exercise has largely been studied in cyclists protocols (Coggan & Coyle 1988, 1989Coyle et al 1983Coyle et al , 1986Flynn et al 1987;Ivy et al 1983;Neufer et al 1987;Hawley et al 1992a & b;Rauch et al 1995). Only a few groups have examined fuel kinetics during running (Wilber & Moffat 1992;Madsen et al 1990;Tsintzas et al 1993a,b).…”

Oxidation of [ 13 C]glucose ingested before and/or during prolonged exercise

“…However, endogenous glucose oxidation was not significantly modified. These data are in line with several previous studies in which glucose was ingested immediately before and/or during exercise period (Adopo et al 1994;Bosch et al 1994;Jeukendrup et al 1999;Rauch et al 1995;Wagenmakers et al 1993). However, endogenous glucose oxidation was significantly reduced by 21.4% when glucose was ingested both before and during exercise.…”

“…In these studies the metabolic fate of exogenous glucose was not measured, but the rate of plasma glucose appearance or disappearance (R d ) reported by Febbraio et al (2000b) was similar when $2 g min )1 of glucose were ingested during, and both before and during exercise (0.3-0.4 g min )1 contributing to 8)11% of the energy yield, assuming that 100% plasma glucose R d was oxidized; Jeukendrup et al 1999). These values for plasma glucose labelled glucose have, indeed, shown that when large amounts of exogenous carbohydrates are ingested (1.0-3.0 g min )1 ) during exercise performed at 50-70% maximal O 2 uptake ( _ V O 2max ) the rate of plasma glucose oxidation (Angus et al 2002) or of exogenous glucose oxidation (which is at most equal to plasma glucose oxidation; Jeukendrup et al 1999) could reach 0.9-1.7 g min )1 (Couture et al 2002;Jeukendrup et al 1999;McConell et al 1999;Pe´ronnet et al 1998;Rauch et al 1995;Wagenmakers et al 1993; see also Hawley et al 1992 andPe´ronnet et al 1992 for a review).…”

Sélection des substrats lors de l’exercice prolongé avec ingestion de glucides : études par traçage au 13C

“…(2001), les études conduites utilisant les techniques de traçage montre que l'ingestion de glucides, à des taux variant de 0.6 à 3.0 g/min, ne modifie pas l'oxydation du glycogène musculaire (Febbraio et al, 2000a,b;Horowitz et al, 1999;Jentjens et al, 2006;Jeukendrup et al, 1999a;Rauch et al, 1995;. À titre d'exemple, lorsqu'une dose importante (3 g/min) de glucose fut administrée à des sujets effectuant 120 minutes d'exercice à 50 %VO 2 max, Jeukendrup et al (1999a) n'ont pas observé de ré-109 oads taken independently concentration correlated oxidized (Fig.…”