Energex - Glucose, Insulin, and Glycogen
QUESTION: Can a low glycemic carbohydrate restore muscle glycogen as well as simple sugar?
- Glycogen is a carbohydrate stored in muscles for use as an energy source during exercise.
- Foods with a high glycemic index (GI), such as sucrose, can be used to replace muscle glycogen after exercise.
- High GI foods also induce higher insulin responses than lower GI foods.
- High insulin levels are associated with long-term development of insulin resistance and short-term suppression of fat utilization.
- Feeds that can restore glycogen without inducing high insulin levels are of interest.
Aim of Study
To investigate if a low GI carbohydrate solution (slower energy release) can restore muscle glycogen post-exercise as well as a high GI solution (sucrose).
- Blinded = researchers did not know which carbohydrate solution horses received.
- Randomized = which horses were given placebo was pre-determined by a random system rather than a person deciding at the time of seeing a horse. This removes bias in the results caused by selecting only certain horses (e.g. less lame horses) to have a particular treatment.
Muscle glycogen replacement was studied in 11 horses which underwent treadmill exercise to deplete muscle glycogen followed by a carbohydrate replacement treatment. Treadmill exercise consisted of 15 min at a heart rate of 130-140 bpm with a treadmill incline of 6-10°, 15 min at a slope of 6-10° and heart rate between 150-180 bpm, and then 5 min at a slope of 0-10° with a heart rate between 180-200 bpm. Horses were rested for 30 min and then performed six 1 min sprints at a slope of 4-10° with a target heart rate of 200-220 bpm, with 5 min walking at the same slope between sprints. Thirty minutes after exercise horses were randomly allocated to receive either a low glycemic index or high glycemic index carbohydrate solution by stomach tube. The low glycemic index carbohydrate supplement consisted of 1.3 g/kg EnerGex (Science Supplements). The high glycemic index carbohydrate supplement was given as an equivalent calorie, same volume solution of sucrose. Muscle biopsies were taken from the middle gluteal muscle, at a depth of 8 cm, in each horse before exercise, immediately post-exercise, and at 24 h following treatment. Blood samples for glucose and insulin analysis were taken before, and at 0, 15, 30, 45, 60, 90, 120, 150, 180, 210, 240, and 1440 min after supplementation.
- There was no significant difference in pre-exercising blood glucose concentrations in horses administered sucrose or EnerGex (Fig. 1) indicating that any changes during the trial were due to the carbohydrates given.
Figure 1: Blood glucose concentration post-exercise following nasogastric administration of EnerGex™ or sucrose.
- Sucrose caused a significantly higher insulin response than EnerGex (Fig. 2). Blood glucose and insulin were both elevated for significantly longer in horses treated with sucrose versus EnerGex.
Figure 2: Blood insulin concentration post-exercise following nasogastric administration of EnerGex™ or sucrose. Timepoints with different letters denotes significantly different mean blood insulin concentration (P<0.05) between sucrose and EnerGex supplemented horses.
- Muscle glycogen replacement was similar for both carbohydrate supplements (Fig. 3). At 24h following exercise, muscle glycogen recovery was 63% with EnerGex compared to 58% with sucrose.
Figure 3: Effect of EnerGex™ and sucrose supplementation on mean (±SEM) muscle glycogen concentration before (pre-exercise), 0 h (post-exercise but prior to supplementation) and 24 h (24 h post-exercise). dw= dry weight.
Take Home Messages
- The low GI carbohydrate (EnerGex) replaced the same amount of muscle glycogen as the high GI solution (sucrose) within 24 h of intense treadmill exercise.
- Administration of sucrose to horses resulted in higher blood glucose levels, which stimulated an increased insulin response. In contrast, EnerGex supplementation elicited a significantly lower insulin response, possibly due to a slower, steadier glucose digestion and absorption in the small intestine.
- EnerGex given before and during aerobic exercise may preserve glycogen stores for a longer duration of time, thus delaying time to fatigue. Additionally, due to the lower blood insulin response to EnerGex, there is a greater potential for the body to utilize fatty acid oxidation during exercise, thus sparing muscle glycogen.