Glucose-fructose beverages do not alter the effects of training on lactate metabolism
Rosset R., Egli L., Cros J., Schneiter P. and Tappy L. and Lecoultre V.
Department of Physiology, University of Lausanne, Lausanne, Switzerland.
Introduction
It is generally accepted that lactate is produced by skeletal muscle during exercise, and is
either used in adjacent muscle fibers (lactate shuttle) or recycled to glucose in the liver. We
have shown that ingestion of fructose-containing drinks stimulates lactate production and
release from the liver during exercise, and that fructose-derived lactate is subsequently used
as an energy substrate by muscle. The regulation of this liver to muscle fructose-lactate shuttle
remains unknown. In this study, we assessed whether consumption of fructose-containing
beverages alters the effects of training on fructose and lactate metabolism.
Methods
Two groups of eight sedentary male subjects were endurance-trained for three weeks while
ingesting 489 mL/h of either a 9.8%-glucose 6.2%-fructose beverage (GLUFRU) or water (C)
during exercise training sessions. An incremental test to exhaustion and a metabolic test were
performed before and after the interventions to assess training adaptations and substrate use
during endurance-type exercise. Indirect calorimetry, [1-13C]lactate and [6,6-2H2]glucose were
used to calculate plasma lactate appearance, clearance and oxidation and glucose kinetics.
Results
Anthropometrics and performance parameters were similar in both groups at baseline. Plasma
glucose concentrations (+1±3 vs. +3±3 % vs. baseline values), glucose rate of appearance
(+3±7 vs. +2±3 %) and metabolic clearance (+6±8 vs. +1±5 %) remained stable after both
GLUFRU and C training (all p=n.s.). Overall, lactate concentrations were decreased after
intervention in both GLUFRU and C, but not differently between groups (-10±5 vs. -20±4 %;
p<0.01 vs. baseline, p=n.s. between GLUFRU and C), as a result of an increased lactate
metabolic clearance (+26.5±11.4 vs. +17.5±10.2 mL·min-1; p=0.01 vs. baseline, p=0.56
between GLUFRU and C). Lactate appearance (+10±6 vs. -4±9 %) and oxidation (+9±6 vs. -
6±9 %) remained unchanged across time and conditions (all p=n.s.). Maximal oxygen
consumption (+287±53 vs. +249±104 mL·min-1) and power eliciting lactate threshold (+25±5
vs. +25±8 W) were similarly increased in GLUFRU and C (both p<0.01 vs. baseline, p=n.s.
between GLUFRU and C).
Discussion
These data corroborate our earlier observation that fructose is converted into lactate by the
liver and subsequently oxidized during exercise. Endurance training did not alter liver lactate
release, but increased lactate metabolic clearance. The effects of endurance training were not
differently altered by the consumption of fructose during training sessions, however.