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A look under the hood at carbohydrate intake during exercise

Higher carbohydrate intake during submaximal exercise can help boost performance. This study explores why.

Study under review: Metabolic Responses to Carbohydrate Ingestion during Exercise: Associations between Carbohydrate Dose and Endurance Performance.

Introduction

During exercise, our working muscles use a variety of fuel sources. Which fuel source is tapped depends on factors like exercise intensity and nutrient availability. Fatty acids contribute more energy during lower intensity exercise, while stored glycogen in our muscles and liver, along with glucose circulating in our blood, make up a greater proportion of fuel as exercise intensity increases. This concept becomes particularly relevant when considering some of the known causes of fatigue during endurance exercise, which include depletion of glycogen, a drop in circulating plasma glucose[1] concentration, and dehydration.

As shown in Figure 1, the ingestion of carbohydrates during exercise has been well established[2] to improve performance. These improvements come about through several mechanisms: carb ingestion sustains a high rate of carbohydrate oxidation[3] (more carbs per minute can be burned), maintains blood glucose[4] levels (it prevents an exercise-induced blood-sugar drop), spares liver glycogen[5], and improves the central nervous system’s[6] response to fatigue.

Previous studies[7][8] have examined the dose-dependent effects of ingesting carbohydrates during exercise on endurance cycling performance, and it has been suggested[9] that performance can be improved by maximizing exogenous carbohydrate oxidation rate. However, the limited number of studies looking at performance when consuming carbohydrate beyond the peak exogenous carbohydrate oxidation rates (that is, ingesting more carbs than your body’s capable of burning) have shown mixed results.

One possible benefit of ingesting excess carbs is that more dietary carbs could spare glycogen in the liver, which may have a relevant impact on performance, though research looking at direct effects are lacking. During moderate intensity exercise, consumption of 100 grams[10] of carbohydrate per hour suppressed hepatic (liver) glucose output by 51%, compared to a placebo trial. Another study[11] found that neither 30 or 180 grams of carbohydrate per hour affected muscle glycogen breakdown, but breakdown of liver glycogen was reduced by 30 grams and completely inhibited by 180 grams of carbohydrate per hour. At the same time, a recent study[12] showed that the carbohydrate ingestion rate should reach, but not exceed, the saturation limits for intestinal transporters for either glucose or glucose-fructose. Consumption of 90 grams of carbohydrate per hour (2:1 glucose:fructose) led to better performance than 60 and 75 grams per hour of glucose, and was also better than 112 grams per hour of a glucose/fructose combination.

Study Deep Dives #10 covered a study[13] that examined how performance was affected by varying carbohydrate intake during exercise. Twenty trained male cyclists completed a 35-minute time trial after cycling for two hours at about 60% of their VO2max while consuming either zero, 20, 39, or 64 grams of carbohydrate per hour (glucose only). In that study, intakes between 39 and 64 grams per hour improved performance in a similar manner, compared with drinking water only. That study did not present the metabolic data to look further at the performance changes that were observed. However, the same group of authors has recently published additional findings from the same project in order to further explore the metabolic responses to submaximal endurance exercise with a varying range of carbohydrate intakes. In this study, the authors report what was happening that made 39 and 64 grams per hour better than zero or 20 grams per hour.

Carbohydrate ingestion during endurance exercise is known to improve performance, but the optimal intake amounts are still not fully understood. The purpose of the study under review was to examine the metabolic responses to a range of carbohydrate intakes and determine their associations with performance outcomes.

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