Fate of14C Glucose Ingested During Prolonged Exercise*

A systematic review and meta-analysis of carbohydrate benefits associated with randomized controlled competition-based performance trials

Background

Carbohydrate supplements are widely used by athletes as an ergogenic aid before and during sports events. The present systematic review and meta-analysis aimed at synthesizing all available data from randomized controlled trials performed under real-life conditions.

Methods

MEDLINE, EMBASE, and the Cochrane Central Register of Controlled Trials were searched systematically up to February 2015. Study groups were categorized according to test mode and type of performance measurement. Subgroup analyses were done with reference to exercise duration and range of carbohydrate concentration. Random effects and fixed effect meta-analyses were performed using the Software package by the Cochrane Collaboration Review Manager 5.3.

Results

Twenty-four randomized controlled trials met the objectives and were included in the present systematic review, 16 of which provided data for meta-analyses. Carbohydrate supplementations were associated with a significantly shorter exercise time in groups performing submaximal exercise followed by a time trial [mean difference −0.9 min (95 % confidence interval −1.7, −0.2), p = 0.02] as compared to controls. Subgroup analysis showed that improvements were specific for studies administering a concentration of carbohydrates between 6 and 8 % [mean difference −1.0 min (95 % confidence interval −1.9, −0.0), p = 0.04]. Concerning groups with submaximal exercise followed by a time trial measuring power accomplished within a fixed time or distance, mean power output was significantly higher following carbohydrate load (mean difference 20.2 W (95 % confidence interval 9.0, 31.5), p = 0.0004]. Likewise, mean power output was significantly increased following carbohydrate intervention in groups with time trial measuring power within a fixed time or distance (mean difference 8.1 W (95 % confidence interval 0.5, 15.7) p = 0.04].

Conclusion

Due to the limitations of this systematic review, results can only be applied to a subset of athletes (trained male cyclists). For those, we could observe a potential ergogenic benefit of carbohydrate supplementation especially in a concentration range between 6 and 8 % when exercising longer than 90 min.

Electronic supplementary material

The online version of this article (doi:10.1186/s12970-016-0139-6) contains supplementary material, which is available to authorized users.

The effects of ingesting a carbohydrate-electrolyte beverage 15 minutes prior to high-intensity exercise performance

The aim of this study was to examine the effect of ingesting a commercially available carbohydrate-electrolyte (CHO-E) solution on strenuous exercise performance. Ten apparently healthy male volunteers (Mean +/- SD; age 20 +/- 2 yrs; height 178 +/- 7 cm; body mass 77 +/- 10 kg; estimated VO(2 max) 56 +/- 3 ml x kg(-1) x min(-1) completed three experimental trials in random order separated by a minimum of 7 days. For each trial, subjects consumed (8 ml x kg(-1) body mass) either a CHO-E solution (6% carbohydrate, 50 mg Na/500 ml), a non-CHO-E placebo, or no fluid, 15 minutes prior to exercise. The exercise involved intermittent shuttle (20 m apart) running for 1 hr followed by an incremental shuttle running test to exhaustion. Subjects displayed longer exercise times when the CHO-E solution was ingested compared with placebo or no fluid groups (exercise time to exhaustion - CHO-E 649 +/- 95 s, vs. placebo 601 +/- 83 s, vs. no fluid 593 +/- 107 s, P < 0.05). There was a main effect for time for specific gravity of urine (P < 0.05 vs. postexercise, pooled data) and body mass (P < 0.05 vs. postexercise, pooled data). The main finding from this investigation indicates that drinking a CHO-E solution 15 minutes prior to exercise improves performance. This study has practical implications for those sports where drinking during activity is restricted.

Exogenous carbohydrate utilisation: effects on metabolism and exercise performance

It is generally recognized that a decrease in carbohydrate availability can lead to the development of fatigue during prolonged exercise in humans. Administration of glucose or other carbohydrates before or during exercise has been shown to postpone fatigue, conserve muscle glycogen and improve performance. Carbohydrates can be categorised according to their ability to increase blood glucose concentration (known as glycaemic index) and by the extent they stimulate the release of insulin. The glycaemic index is reflected in the rate at which consumed carbohydrate is made available in the blood. Glucose is the only type of carbohydrate that can readily be oxidised by skeletal muscle for energy production. Gastric emptying is the primary factor limiting the rate of carbohydrate delivery to the blood and therefore influences the utilisation of exogenous carbohydrate ingested before or during exercise. Various methods have been used to assess the oxidation of exogenous carbohydrates during exercise. Peak rates of CHO oxidation during exercise have been reported between 0.4 and 1.0 g/min, and the rates of oxidation do not appear to be influenced to a major extent by the use of multiple drinking schedule in comparison with a single bolus schedule. Previous studies also suggest that the ingestion of fructose during exercise does not offer any additional benefits over ingestion of glucose or glucose polymer solutions of similar concentration. The hormones insulin, glucagon and adrenaline together with cortisol and growth hormone play key roles in the regulation of carbohydrate metabolism during exercise. Ingestion of moderately concentrated carbohydrate solutions (4-8%) enhances prolonged exercise performance and is appropriate for optimising energy and fluid delivery without causing adverse effects. The ergogenic effects of carbohydrate ingestion on performance during intermittent exercise such as competitive sports are less well established, although the evidence to date suggests diminished performance when carbohydrate are limiting.

Effects of carbohydrate feeding before and during prolonged exercise on subsequent maximal exercise performance capacity

The study examined the effect of carbohydrate ingestion on exercise performance capacity. Nine male cyclists performed two separate trials at 70% VO2max for 60 min followed by a maximal ride for 10 min. During trials subjects were fed either an 8% glucose solution (CHO) or a placebo solution (PL), which were administered at rest and during and immediately after submaximal exercise. Statistical analyses indicated that glucose levels at rest increased significantly 15 min after the ingestion of CHO compared to PL. At 30 and 60 min during submaximal exercise, plasma glucose levels decreased significantly in the CHO but not in the PL trial. Following the performance ride, glucose levels increased significantly only during the CHO test trial. Free fatty acids did not change significantly during testing trials. The maximal performance ride results showed that in the CHO trial, a significantly greater external work load was accomplished compared to the PL trial. It is concluded that CHO ingestion improves maximal exercise performance after prolonged exercise.