Pre-exercise Carbohydrate Intake and the Effect on Performance and Blood Glucose Levels.

Pre-exercise Carbohydrate Intake and the Effect on Performance and Blood Glucose Levels.

There has been a lot of research concerning performance enhancement with early carbohydrate (CHO) supplementation and the effects that pre-exercise supplementation has on blood glucose levels during exercise. Several studies have linked the glycemic index (GI) of CHO to improved performance and time to exhaustion (TTE), but also they have tried to look for a link to hypoglycemia that can occur in some athletes within the first 15 minutes of exercise. Is there a time associated with CHO consumption? Is there a link with the GI of the CHO? The results might be just as vague as the questions. Hypoglycemia in the beginning of exercise is not directly linked to consumption of CHO prior to exercise, but pre-exercise intake of CHO will increase TTE and improve performance capabilities.  

Researchers in the past ten plus years have looked at the time of CHO intake, the amount of CHO consumed, and the GI of the CHO consumed to determine the best consumption rate for performance enhancement and lowest risk for exercise induced hypoglycemia. The experimented time frames for pre-exercise consumption are broad and this can produce inconclusive evidence when looking through the research. Time frames range from 2 hours prior to 15 minutes prior to an exercise bout. The dose of 1- 1.5 g/kg of CHO was fairly consistent throughout all of the studies. However, the time of CHO intake will influence the amount of CHO consumed more than time of intake will play a part in determining how successful your exercise bout will be,  because even small doses of CHO consumed prior to exercise will positively improve one’s ability to perform endurance and resistance exercises (Karelis, Smith, Passe,  & Péronnet., 2010). The evidence for improved  performance, or TTE, when endurance training takes place is now common place knowledge. Most studies use endurance athletes when arguing their thesis on the use of CHO prior to exercise for improved performance. In one study examining runners there was an improved TTE of 12.8% in subjects that ingested 1 g/kg 15 min prior to exercise (Tokmakidis, & Karamanolis., 2008). Another study looking at run time found that subjects ran 2.8% faster during a time trail where 1.5 g/kg of either low GI or high GI was consumed 2 hr prior to the run (Wong, Siu, LOK, Chen, Morris, & Lam., 2008).

Though CHO consumption has been shown to improve endurance activity capabilities, there is also evidence that pre-exercise CHO consumption can improve weight training performance (Karelis et al., 2010). One study using weight training to determine the effectiveness of CHO on resistance exercise had the experimental group consume 125 g of CHO prior to their exercise bout, and the number of sets and reps increased while performing leg extensions at 80% of max. Another study showed that a similar group when given 240 g of CHO prior to exercise were able to increase their work intensity from 38.1 kj to 41.1 kj (Karelis et al., 2010). Improvements in performance with CHO supplementation or pre-exercise consumption might be due to the following: decrease in fatigue, improved muscle contraction excitation, muscle glycogen sparing, improved glucose oxidation and metabolic pathways, and a reduction in muscular strain caused from exercise (Karelis et al., 2010). One downside to CHO consumption 30-60 min prior to exercise is the sparing of free fatty acids (FFA) in the blood, also several studies showed that consumption of low GI CHO (glucose) increased the chances of exercise induce hypoglycemia in exercisers at the start of their exercise bout (Savvas,  Tokmakidis, & Karamanolis., 2008). However, all of the studies showed that blood glucose levels returned to normal within 60 minutes for the athletes that experienced exercise induced hypoglycemia (Savvas,  Tokmakidis, & Karamanolis., 2008), this is one reason that experimentation during the training period is important. Experimenting with a CHO solution on the day of competition could decrease your performance if your body happens to experience a drop in blood sugars, or you experience gastric upset from for your pre-exercise CHO intake. . This drop in blood sugars will increase fatigue, decrease concentration, decrease muscle contraction excitation, and a drop in performance. Though this is a rare occurrence, it is possible.

There appears to be some similarities in the studies where clients that ingested CHO 30 min or earlier to their exercise had blood glucose levels drop to near hypoglycemic levels, however, none of the studies’ subjects demonstrated symptoms associated with hypoglycemia. There were no hypoglycemic symptoms even though blood glucose levels dropped to near hypoglycemic levels (Wong, et al. 2008). Many studies have also shown that blood glucose and insulin levels return to normal within the first 60 minutes of activity when levels reach near hypoglycemic levels within the first 30 min of activity (Tokmakidis & Karamanolis., 2008). There also appears to be no major concern over exercise induced hypoglycemia within the first 30 min of exercise when CHO is consumed prior to exercise. Consumption of CHO prior to exercise may actually keep blood glucose levels above hypoglycemic levels by maintaining blood glucose levels at the beginning of an exercise (Karelis et al, 2010; Wong et al, 2008), especially when low GI CHO are consumed. This could be a good thing for those with diabetes or other issues regarding maintaining blood sugar levels.

When looking at whether low GI or high GI were the preferred nutrient of choice for exercise there was not a clear and simple answer. Research on the effect of GI on exercise was first started in the 1980’s to look at the effects of different types of CHO on diabetic clients (Donaldson, Perry, & Rose., 2010), and has now expanded that research to the athletic population. It is not easy to determine how well an individual will respond to different GI groups. Several factors that play a part in the athletes ability to utilize low or high GI are the athlete’s sex, age, body mass index, and even the athlete’s ethnicity; however, gender and training status seems to play a larger role  (Donaldson, Perry, & Rose., 2010). None of the studies were clear as to how these statuses would increase or decrease an athletes ability.

Wong et al, 2008 showed an increase run time that was 2.8% faster in athletes that consumed a low GI meal or snack prior to running 15 km. Also, fat oxidation rates were higher in athletes that consumed low GI; rates were up 19% higher is subjects that consumed low GI over a high GI pre-exercise meal (Wong et al, 2008). This would mean that the subjects burned more fat during exercise when they consume low GI over the subjects that consumed high GI. This may be due to that the breakdown of low GI would release slower into the system and would be around as a fuel source allowing the body’s natural desire to select fat as fuel source during a longer bout of exercise. However, low GI pre-exercise intake might increase the chances of gastric upset in some individuals due to its slower absorption rate (Donaldson, Perry, & Rose., 2010), for this reason it might be advantageous for an athlete to consume low GI 30 minutes or longer before an exercise session and to experiment with different CHO sources prior to competition.  On the bright side; consumption of low GI prior to exercise has been shown to increase endurance, FFA levels, and fat oxidation in both endurance and strength athletes (Karelis et al., 2010). One benefit for strength athletes is that CHO consumption can result creatine sparing (Karelis et al., 2010). This would reduce the need to supplement with creatine to increase power, TTE, force exertion, and muscle size.

Overall the research for low GI and high GI CHO shows that the low is better, not only for diabetics, but for all athletes. Low GI CHO metabolizes slower meaning that there will be more fuel to power the athlete. It should be noted that research has shown that supplementation of any CHO can benefit performance, but low GI will maintain blood glucose levels longer, promote muscle glycogen sparing, increase FFA oxidation rates, and give you energy for the long haul. As mentioned earlier take the time to experiment with CHO as a pre-exercise performance enhancer before competion. The last thing a competitive athlete needs is an upset stomach during the competition.    

Works Cited

Donaldson, C. M., Perry, T. L., & Rose M. C. (2010). Glycemic index and endurance performance. International Journal of Sport Nutrient and Exercise Metabolism, 20. 154 – 165.

Karelis, A. D., Smith, JE. E., Passe, D. H., & Peronnet, F. (2010). Carbohydrate administration and exercise performance: What are the potential mechanisms involved. Sports Medicine, 40(9). 747 – 763.

Tokmakidis, S. P., & Karamanolis, L. A. (2008). Effects of carbohydrate ingestion 15 min before exercise on endurance running capacity. Applied Physiology of Nutrition and Metabolism, 33. 441 – 449.

Wong, S. H. S., Siu, P. M., Lok, A., Chen Y. J., & Morris, J. (2008). Effect of the glycaemic index of pre-exercise carbohydrate meals on running performance. European Journal of Sport Science, 8(1). 23 – 33.

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