Benefits of Carbohydrate Feedings During Exercise
The ability to improve exercise performance during different sporting activities (especially during continuous, prolonged events – e.g. marathons) is dependent on promoting a variety of physiological and metabolic adjustments in order to allow an individual to increase their power output and delaying the onset of fatigue. This may be achieved by mediating an athlete’s feeding strategy and manipulating the volumes of nutrients consumed to elicit the most significant improvement. Research has found that of the macronutrients, carbohydrate (CHO) feedings during exercise are most likely to improve an athlete’s performance in terms of exercise capacity and power output through mechanisms such as maintenance of glycaemia (blood-glucose) levels and by sparing endogenous glycogen by increasing carbohydrate oxidation. An adequate level of glucose availability is directly associated in a positive-dose relationship with physical activity and improvement of endurance performances, where increased exercise duration observed in subjects is around 45 minutes or longer. In addition, CHO feedings during exercise bouts have been shown to spare liver glycogen levels – although there is still some controversy as to whether this sparing phenomenon may be seen in the muscle glycogen levels, with conflicting studies arising.
Carbohydrates, or saccharides, are important macronutrients consisting of carbon, hydrogen, and oxygen atoms that are responsible for providing the body with energy through a process called carbohydrate oxidation that results in production of ATP from glucose molecules. Carbohydrates can be stored in large quantities in the body as chains of glucose molecules when an excess quantity is consumed in the diet (e.g. glycogen storage in skeletal muscles) and can later be used for the production the energy source ATP when immediate stores are depleted. This mechanism of storage is essential for the maintenance of euglycaemia (normal blood-glucose concentrations). The metabolism of CHOs in the human body begins with the initial breakdown of complex carbohydrates into simple monomers (e.g. glucose, fructose, galactose) by enzymatic activity. Glucose is the main constituent to be derived from CHOs, making up approximately 80% of the resulting products from digestion and can be either broken down further to produce ATP, or stored as glycogen. Metabolism then continues with CHO oxidation – which is the breakdown of one molecule of glucose by aerobic respiration and involved both glycolysis and the citric-acid cycle, yielding between 30 and 32 molecules of ATP. Approximately, the oxidation of one gram of carbohydrate will yield 4 kcal of energy.
Energy production in skeletal muscle is supplied by a reciprocal relationship between carbohydrate oxidation and fat (lipid) oxidation, whereby they will be simultaneously activated during exercise in order to keep up with the energy demands for various metabolic processes. However, when a steadier state is found at a given aerobic exercise intensity, and the metabolic demands are established, there can be fluctuations in the proportion of CHO and fat that are oxidised. The interaction between these two forms of oxidation is dependent on the extracellular and intracellular metabolic environments – wherein an availability of the substrates (e.g. levels of CHO or fat available) the intensity of the exercise and its duration, can all be influential.
Whilst it is known that CHO has the biggest impact on performance in athletes, research has shown that different carbohydrates constituents do not oxidise at the exact same rate and therefore may not be as equally effective in feedings. It is thought that the following forms of carbohydrates are oxidised at high rates; glucose, sucrose, maltose, maltodextrins, and amylopectin, whereas the following are found to oxide at lower rates; fructose, galactose, and amylose. Given that an athlete’s training status does not impact the level of exogenous carbohydrate oxidation, determining which is the most carbohydrate constituent is the most effective can be important for establishing the best feeding strategy. These rates, however, can be manipulated through different combinations of multiple transportable CHO in order to increase the absorption level and total exogenous oxidation rate.
In individuals who were given adequate level of glucose during exercise, the maximal exogenous carbohydrate oxidation rate possible was observed to be ~1 g. This restriction is thought to be the result of limited capacity of glucose uptake in the intestine, as well as possible retention of glucose in the liver. Intestinal limitation usually occurs when the sodium-dependent glucose transporter 1 (SGLT1) becomes saturated with glucose molecules. However, there are other intestinal CHO transporters that have been associated with the uptake of different carbohydrate constituents (e.g GLUT2 for glucose, or GLUT2, GLUT5, GLUT8, and GLUT12 for fructose) which allow for simultaneous absorption of different CHOs to occur (thus optimising exogenous carbohydrate oxidation rates). Fundamentally, this means that an inclusion of 2 or 3 different CHOs in the feedings may increase absorption and oxidation levels. A study confirmed this co-ingestion theory during a cycling endurance test, whereby oxidation was shown to significantly improve by 35-55% following consumption of a 2:1 glucose:fructose solution ingested at a rate of 1.75 g.min-1 and 14.4% concentration, in comparison to an glucose-only solution (~1 g·min−1). Fructose in isolation, however, has a lower rate of oxidation which may be a result of the requisite conversation into glucose in the liver before metabolism can occur (usually a slow process). Although, the oxidation rates were found to be similar in glucose- and fructose-only solutions when the individuals were in a fasted state (and the gluconeogenic pathways were activated). Sucrose is a naturally-occuring disaccharide that combines both glucose and fructose monomers and is thought to provide an effective dietary source for the glucose-fructose co-ingestion solution. Moreover, studies have suggested that because sucrose is able to be directly transported as an intact disaccharide across the intestinal membrane – it is consequently not limited by the CHO transporters as seen with glucose-only feedings. Studies have confirmed its ability to increase exogenous CHO oxidation levels during exercise in comparison to these glucose-only solutions (with a rate of 1.2–1.3 g·min−1), but does not seem to improve on the rate seen in fructose-glucose co-ingestion studies (1.3–1.8 g·min−1).
Furthermore, the volume of CHO in the solutions provided will also be a determining factor in performance gain. Recent studies have shown that a capacity for peak exogenous carbohydrate oxidation rate to reach 1.26 – 1.75 g·min-1 when ingesting high doses (i.e. 1.8 – 2.4 g·min-1) of mixed carbohydrates (e.g. a glucose-fructose solution).
Carbohydrates, and their various constituents, are important for muscle contraction during prolonged exercise bouts and can aid in the prevention of fatigue (often associated with muscle glycogen depletion and hypoglycaemia) by optimising the availability of muscle and liver glycogen and euglcaemia levels. A high carb diet during recovery from the prolonged periods of variable speed running restores muscle glycogen and subsequent performance.
In conclusion, carbohydrate consumption during prolonged, endurance-type exercise can increase an individual’s power output, exercise capacity, and overall performance. The improvements seen during carbohydrate feedings are thought to be the result of maintaining eugycaemia levels and increasing the level of carbohydrate oxidation during late stages of exercise. Whilst glucose-only solutions have been seen to increase the oxidation levels during exercise, limitations can occur and is likely due to an oversaturated SGLT1 from glucose. Therefore, a co-ingestion of a glucose-fructose solution may be a better feeding strategy (despite fructose’s low oxidation rate) as there is an overall better increase to the intestinal absorption levels (and thus the exogenous carbohydrate oxidation rates) – this is due to the different transporter routes they use. Although, sucrose may be an even more effective at increasing CHO oxidation levels over glucose-only feedings, due to certain limitation, fructose co-ingestion seems to be the best carbohydrate feeding strategy.
