Carbohydrate dose influences liver and muscle glycogenoxidation and performance during prolonged exercise

This study investigated the effect of carbohydrate (CHO) dose and composi-tion on fuel selection during exercise, specifically exogenous and endogenous(liver and muscle) CHO oxidation. Ten trained males cycled in a double-blindrandomized order on 5 occasions at 77%_VO2maxfor 2 h, followed by a30-min time-trial (TT) while ingesting either 60 g�h�1(LG) or 75 g�h�113C-glucose (HG), 90 g�h�1(LGF) or 112.5 g�h�113C-glucose-13C-fructose ([2:1]HGF) or placebo. CHO doses met or exceed reported intestinal transportersaturation for glucose and fructose. Indirect calorimetry and stable mass iso-tope [13C] tracer techniques were utilized to determine fuel use. TT perfor-mance was 93% “likely/probable” to be improved with LGF compared withthe other CHO doses. Exogenous CHO oxidation was higher for LGF andHGF compared with LG and HG (ES>1.34,P<0.01), with the relative con-tribution of LGF (24.5�5.3%)moderatelyhigher than HGF (20.6�6.2%,ES=0.68). Increasing CHO dose beyond intestinal saturation increased abso-lute (29.2�28.6 g�h�1,ES=1.28,P=0.06) and relative muscle glycogenutilization (9.2�6.9%, ES=1.68,P=0.014) for glucose-fructose ingestion.Absolute muscle glycogen oxidation between LG and HG was not significantlydifferent, but wasmoderatelyhigher for HG (ES=0.60). Liver glycogen oxida-tion was not significantly different between conditions, but absolute and rela-tive contributions weremoderatelyattenuated 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 otherCHO conditions (ES>0.90,P=0.024–0.17). In conclusion, there was no lin-ear dose response for CHO ingestion, with 90 g�h�1of glucose-fructose beingoptimal in terms of TT performance and fuel selectio

Fuel Use during Exercise at Altitude in Women with Glucose–Fructose Ingestion

Purpose: This study compared the co-ingestion of glucose and fructose on exogenous and endogenous substrate oxidation during prolonged exercise at terrestrial high altitude (HA) versus sea level, in women. Method: Five women completed two bouts of cycling at the same relative workload (55% Wmax) for 120 minutes on acute exposure to HA (3375m) and at sea level (~113m). In each trial, participants ingested 1.2 g.min-1 of glucose (enriched with 13C glucose) and 0.6 g.min-1 of fructose (enriched with 13C fructose) before and every 15 minutes during exercise. Indirect calorimetry and isotope ratio mass spectrometry were used to calculate fat oxidation, total and exogenous carbohydrate oxidation, plasma glucose oxidation and endogenous glucose oxidation derived from liver and muscle glycogen. Results: The rates and absolute contribution of exogenous carbohydrate oxidation was significantly lower at HA compared with sea level (ES>0.99, P<0.024), with the relative exogenous carbohydrate contribution approaching significance (32.6±6.1 vs. 36.0±6.1%, ES=0.56, P=0.059) during the second hour of exercise. In comparison, no significant differences were observed between HA and sea level for the relative and absolute contributions of liver glucose (3.2±1.2 vs. 3.1±0.8%, ES=0.09, P=0.635 and 5.1±1.8 vs. 5.4±1.7 grams, ES=0.19, P=0.217), and muscle glycogen (14.4±12.2% vs. 15.8±9.3%, ES=0.11, P=0.934 and 23.1±19.0 vs. 28.7±17.8 grams, ES=0.30, P=0.367). Furthermore, there was no significant difference in total fat oxidation between HA and sea level (66.3±21.4 vs. 59.6±7.7 grams, ES=0.32, P=0.557). Conclusion: In women, acute exposure to HA reduces the reliance on exogenous carbohydrate oxidation during cycling at the same relative exercise intensity

The effect of pre-exercise galactose and glucose ingestion on high-intensity endurance cycling.

This study evaluated the effects of the pre-exercise (30 minutes) ingestion of galactose (Gal) or glucose (Glu) on endurance capacity as well as glycemic and insulinemic responses. Ten trained male cyclists completed 3 randomized high-intensity cycling endurance tests. Thirty minutes before each trial, cyclists ingested 1 L of either 40 g of glucose, 40 g of galactose, or a placebo in a double-blind manner. The protocol comprised 20 minutes of progressive incremental exercise (70-85% maximal power output [Wmax]); ten 90-second bouts at 90% Wmax, separated by 180 seconds at 55% Wmax; and 90% Wmax until exhaustion. Blood samples were drawn throughout the protocol. Times to exhaustion were longer with Gal (68.7 ± 10.2 minutes, p = 0.005) compared with Glu (58.5 ± 24.9 minutes), with neither being different to placebo (63.9 ± 16.2 minutes). Twenty-eight minutes after Glu consumption, plasma glucose and serum insulin concentrations were higher than with Gal and placebo (p < 0.001). After the initial 20 minutes of exercise, plasma glucose concentrations increased to a relative hyperglycemia during the Gal and placebo, compared with Glu condition. Higher plasma glucose concentrations during exercise, and the attenuated serum insulin response at rest, may explain the significantly longer times to exhaustion produced by Gal compared with Glu. However, neither carbohydrate treatment produced significantly longer times to exhaustion than placebo, suggesting that the pre-exercise ingestion of galactose and glucose alone is not sufficient to support this type of endurance performance

The effect of pre-exercise galactose and glucose ingestion on high-intensity endurance cycling.

This study evaluated the effects of the pre-exercise (30 minutes) ingestion of galactose (Gal) or glucose (Glu) on endurance capacity as well as glycemic and insulinemic responses. Ten trained male cyclists completed 3 randomized high-intensity cycling endurance tests. Thirty minutes before each trial, cyclists ingested 1 L of either 40 g of glucose, 40 g of galactose, or a placebo in a double-blind manner. The protocol comprised 20 minutes of progressive incremental exercise (70-85% maximal power output [Wmax]); ten 90-second bouts at 90% Wmax, separated by 180 seconds at 55% Wmax; and 90% Wmax until exhaustion. Blood samples were drawn throughout the protocol. Times to exhaustion were longer with Gal (68.7 ± 10.2 minutes, p = 0.005) compared with Glu (58.5 ± 24.9 minutes), with neither being different to placebo (63.9 ± 16.2 minutes). Twenty-eight minutes after Glu consumption, plasma glucose and serum insulin concentrations were higher than with Gal and placebo (p < 0.001). After the initial 20 minutes of exercise, plasma glucose concentrations increased to a relative hyperglycemia during the Gal and placebo, compared with Glu condition. Higher plasma glucose concentrations during exercise, and the attenuated serum insulin response at rest, may explain the significantly longer times to exhaustion produced by Gal compared with Glu. However, neither carbohydrate treatment produced significantly longer times to exhaustion than placebo, suggesting that the pre-exercise ingestion of galactose and glucose alone is not sufficient to support this type of endurance performance

Glucose and fructose hydrogel enhances running performance, exogenous carbohydrate oxidation, and gastrointestinal tolerance

Purpose: Beneficial effects of carbohydrate (CHO) ingestion on exogenous CHO oxidation and endurance performance require a well-functioning gastrointestinal (GI) tract. However, GI complaints are common during endurance running. This study investigated the effect of a CHO solution-containing sodium alginate and pectin (hydrogel) on endurance running performance, exogenous and endogenous CHO oxidation and GI symptoms. Methods: Eleven trained male runners, using a randomised, double-blind design, completed three 120-minute steady state runs at 68% V[Combining Dot Above]O2max, followed by a 5-km time-trial. Participants ingested 90 g·h-1 of 2:1 glucose:fructose (13C enriched) either as a CHO hydrogel, a standard CHO solution (non-hydrogel), or a CHO-free placebo during the 120 minutes. Fat oxidation, total and exogenous CHO oxidation, plasma glucose oxidation and endogenous glucose oxidation from liver and muscle glycogen were calculated using indirect calorimetry and isotope ratio mass spectrometry. GI symptoms were recorded throughout the trial.RESULTS: Time-trial performance was 7.6% and 5.6% faster after hydrogel ([minutes:seconds]19:29 ± 2:24; p &lt; 0.001) and non-hydrogel (19:54 ± 2:23, p = 0.002), respectively, versus placebo (21:05 ± 2:34). Time-trial performance after hydrogel was 2.1% faster (p = 0.033) than non-hydrogel. Absolute and relative exogenous CHO oxidation was greater with hydrogel (68.6 ± 10.8 g, 31.9 ± 2.7%; p = 0.01) versus non-hydrogel (63.4 ± 8.1 g, 29.3 ± 2.0%; p = 0.003). Absolute and relative endogenous CHO oxidation were lower in both CHO conditions compared with placebo (p &lt; 0.001), with no difference between CHO conditions. Absolute and relative liver glucose and muscle glycogen oxidation were not different between CHO conditions. Total GI symptoms were not different between hydrogel and placebo, but GI symptoms was higher in non-hydrogel compared with placebo and hydrogel (p &lt; 0.001). Conclusion: Ingestion of glucose and fructose in hydrogel form during running benefited endurance performance, exogenous CHO oxidation and GI symptoms, compared with a standard CHO solution