Oxidation and Metabolie Effects of Fructose or Glucose Ingested Before Exercise*

Abstract

The aim of this study was to compare the effects of fructose (F) and glucose (G) intake before exercise on oxidation of the ingested substrate, glycogen utilization, work output, and metabolic changes.

Ten trained subjects ingested F or G (1 g/kg), both of which were naturally enriched in ¹³C. After 1 h of rest, they exercised on an ergometer at 61% of their maximal oxygen uptake (V̇O₂ max) for 45 min, which was immediately followed by 15 min at their maximal voluntary output.

During the resting hour, blood insulin and glucose were lower (P < 0.05) and respiratory quotient and blood lactate higher (P < 0.01) after F. During exercise, the differences disappeared, apart from a transient but moderate (4.3 mmol/l) hypoglycemia after G compared to F. No difference between F and G was observed for uric acid, glycerol, FFA, and glucagon. Glycogen decrements in the vastus lateralis muscle were 67 ± 9 (F) and 97 ± 15 (G) mmol/kg, values not significantly different from each other (P > 0.05). The maximal voluntary work produced during the last 15 min did not differ between treatments. During the 2 h after sugar ingestion, 30 + 3g of F and 26 ± 3g of G were oxidized to ¹³CO₂.

These findings indicate that fructose ingested before exercise was utilized at least as well as glucose, allowed a more stable glycemia, and did not modify performance.

Key words

Glycogen - ¹³C naturally occurring labels - work output - blood lactate - uric acid