Carbohydrate dose influences liver and muscle glycogen oxidation and performance during prolonged exercise

Abstract

This study investigated the effect of carbohydrate (CHO) dose and composition on fuel selection during exercise, specifically exogenous and endogenous (liver and muscle) CHO oxidation. Ten trained males cycled in a double-blind randomized order on 5 occasions at 77% V˙O2max for 2 h, followed by a 30-min time-trial (TT) while ingesting either 60 g·h^-1 (LG) or 75 g·h^-113 C-glucose (HG), 90 g·h^-1 (LGF) or 112.5 g·h^-113 C-glucose-¹³ C-fructose ([2:1] HGF) or placebo. CHO doses met or exceed reported intestinal transporter saturation for glucose and fructose. Indirect calorimetry and stable mass isotope [¹³ C] tracer techniques were utilized to determine fuel use. TT performance was 93% "likely/probable" to be improved with LGF compared with the other CHO doses. Exogenous CHO oxidation was higher for LGF and HGF compared with LG and HG (ES > 1.34, P < 0.01), with the relative contribution of LGF (24.5 ± 5.3%) moderately higher than HGF (20.6 ± 6.2%, ES = 0.68). Increasing CHO dose beyond intestinal saturation increased absolute (29.2 ± 28.6 g·h^-1 , ES = 1.28, P = 0.06) and relative muscle glycogen utilization (9.2 ± 6.9%, ES = 1.68, P = 0.014) for glucose-fructose ingestion. Absolute muscle glycogen oxidation between LG and HG was not significantly different, but was moderately higher for HG (ES = 0.60). Liver glycogen oxidation was not significantly different between conditions, but absolute and relative contributions were moderately attenuated for LGF (19.3 ± 9.4 g·h^-1 , 6.8 ± 3.1%) compared with HGF (30.5 ± 17.7 g·h^-1 , 10.1 ± 4.0%, ES = 0.79 & 0.98). Total fat oxidation was suppressed in HGF compared with all other CHO conditions (ES > 0.90, P = 0.024-0.17). In conclusion, there was no linear dose response for CHO ingestion, with 90 g·h^-1 of glucose-fructose being optimal in terms of TT performance and fuel selection.

Keywords: Carbohydrate ingestion; exercise; metabolism; muscle glycogen; stable isotope.

Figure 1

(A) ¹³ CO ₂:¹² CO ₂ (δ¹³C) in expired air over the 2 h ride and (B) ¹³C:¹²C in plasma glucose during the second hour of the 2 h ride. (a) denotes CHO significantly greater than PLA (P = 0.00–0.047), (b) denotes HGF significantly greater than LG & HG (P = 0.01 & 0.02), (c) denotes LGF significantly greater than LG & HG (P = 0.0–0.024), (d) denotes LGF significantly greater than LG and HG (P = 0.026–0.045), (e) denotes 90 min significantly greater than 60 min.

Figure 2

Percentage energy contributions from various substrates during the second hour of the 2 h ride. (a) denotes LGF significantly different to LG (P = 0.00), (b) denotes LGF significantly different to HG (P = 0.00–0.019), (c) denotes LGF significantly different to HGF (P = 0.014)

Figure 3

Sources of oxidised glucose and muscle glycogen during the second hour of the ride. A: Carbohydrate from exogenous sources (g.min⁻¹) B: Plasma glucose oxidation (g.min⁻¹) C & D: Liver and Muscle glycogen oxidation respectively (g.min⁻¹) Data are means ± sd. a denotes LGF significantly lower than HG

Figure 4

Circulatory metabolites, plasma glucose and lactate, serum‐free fatty acids and insulin concentrations during the 2 h ride. Data are means ± SD. See text for statistical and ES comparisons.