In
an effort to increase multisport athlete awareness surrounding nutrition,
recovery, racing, training, and bioenergetic adaptation, summaries, briefs, and
peer reviewed literature will be discussed in order to stimulate thinking and
overall knowledge about the sports we love. I hope you enjoy, and please feel
free to ask questions (lovelace.ben@gmail.com) if you ever don’t understand
something or would like more information regarding the specific topic being
discussed.
The
article bellow will be discussed: Mark
A. Febbraio, Alison Chiu, Damien J. Angus, Melissa j. Arkinstall, and john a
Hawley. Effects of carbohydrate ingestion before and during exercise on glucose
kinetics and performance. J Appl Physiol. 89: 2220-2226, 2000.
Many athletes, specifically endurance athletes, take in some sort of carbohydrate during exercise to prolong their inevitable end to their exercise bout. It is this practice, that allows folks to compete in ironman triathlons, long distance running or cycling events! Often compared to a cars fuel utilization, the human body is thought to burn everything it takes in, but, what if it was not so simple. . . . . .
Carbohydrate (CHO) ingestion has been shown to improve performance and increase time to fatigue. Because of this, Febbraio and colleagues observed seven trained men cycling for 120 min at 63% peak power output, followed by a 7kj/kg body weight (wt) time trial (TT). On four separate occasions subjects received either a placebo beverage before and during (PP); Placebo 30 min before and 2 g/kg body/wt of CHO in 6.4% CHO beverage during (PC); 2g/kg body wt of CHO in a 25.7% CHO beverage 30 min before and a placebo throughout (CP); or 2g/kg body wt of CHO in a 25.7% CHO beverage 30 min before and 2 g/kg of CHO in a 6.4% CHO solution throughout (CC).
When carbohydrate intake was maintained throughout, plasma glucose concentrations were significantly greater after 90 minutes when compared to placebo. There were no reported differences in plasma glucose between all trials before 90 minutes. This suggests that the individuals exercising with placebo were able to utilized gluconeogenic processes in the liver (i.e the liver changing glycogen into glucose) to maintain blood sugar at levels necessary to continue exercising. Pre-exercise carbohydrate ingestion resulted in higher insulin levels at the beginning of exercise, however, after exercise started there were no observed difference in insulin concentrations between trials suggesting that insulin plays a small role in glucose uptake at the muscle during exercise.
Between all trials there was no difference between plasma free fatty acids and glycerol (markers found in the blood suggesting fat metabolism) for the first 60 minutes. However, during the last 60 minutes, both plasma free fatty acids and glycerol concentrations were elevated in only the placebo (PP) group when compared to the CC, CP, and PC group. The reason for the no difference for the first 60 minutes is possibly due to the heavy reliance on muscle glycogen for the first 60 minutes in all trials. As muscle glycogen begins to decrease, the PP group experiences a slight shift to adipose tissue as a substrate and in doing so an elevation of free fatty acids and glycerol are present in the blood.
Individuals that took in CHO prior to exercise but not throughout exercise oxidized significantly more glucose, in the last 20 minutes, then the individuals that took CHO in during exercise. The observed findings suggest that the individuals that took in glucose prior to exercise experienced a reactive hypoglycemic response that drove down lyposis (fat break down) and increased reliance on muscle glycogen as a fuel source, thus, creating a small, but significant difference in glucose oxidation between these two groups in the later portion of exercise. Different oxidation rates were also seen in individuals that took in no exogenous glucose prior to, and during, their exercise bout. Individuals in the PP group experienced a shift in fuel utilization, were they were oxidizing more free fatty acids in the last 60 minutes of exercise when compared to the CP, PC and CC group. While there are differences in fuel utilization between treatments, they are observed in the later portion of exercise and the shift in fuel utilization did not result in performance benefits. The findings outlined suggest that athletes exercising at ~70% of VO2peak for ~2hrs, regardless of the athletes pre-exercise meal or during exercise nutritional practice oxidize glucose and free fatty acids similarly.
While oxidation of exogenous glucose is minimal (11% or .4g/min), and oxidation of free fatty acids and glucose are similar between all groups, it is clear through the exercise time trials that exogenous glucose intake during exercise increases performance. The observed results, however, do not support performance benefits for individuals that supplement with CHO prior to exercise. The current findings suggest that maintenance of plasma glucose during exercise has a positive effect on mechanisms other than those associated with substrate oxidation (what the body uses as fuel) and muscle contraction. Therefore, performance gains are possibly due central nervous system preservation and homeostasis rather then direct fuel utilization at the muscle.
The above findings by Febbraio et al. suggest that athletes must supplement with exogenous glucose (with sugars taken in by mouth) in order to perform at a higher level. The finding flys in the face of common thought that suggest that exogenous glucose is fully oxidized at the muscle during exercise (i.e like our car uses gas). Rather, Febbraio et al. findings suggest that the exogenous glucose, often taken in the form of gels, drinks and bars are essentially meant to keep your brain happy and this leads to increases in performance.
Many athletes, specifically endurance athletes, take in some sort of carbohydrate during exercise to prolong their inevitable end to their exercise bout. It is this practice, that allows folks to compete in ironman triathlons, long distance running or cycling events! Often compared to a cars fuel utilization, the human body is thought to burn everything it takes in, but, what if it was not so simple. . . . . .
Carbohydrate (CHO) ingestion has been shown to improve performance and increase time to fatigue. Because of this, Febbraio and colleagues observed seven trained men cycling for 120 min at 63% peak power output, followed by a 7kj/kg body weight (wt) time trial (TT). On four separate occasions subjects received either a placebo beverage before and during (PP); Placebo 30 min before and 2 g/kg body/wt of CHO in 6.4% CHO beverage during (PC); 2g/kg body wt of CHO in a 25.7% CHO beverage 30 min before and a placebo throughout (CP); or 2g/kg body wt of CHO in a 25.7% CHO beverage 30 min before and 2 g/kg of CHO in a 6.4% CHO solution throughout (CC).
When carbohydrate intake was maintained throughout, plasma glucose concentrations were significantly greater after 90 minutes when compared to placebo. There were no reported differences in plasma glucose between all trials before 90 minutes. This suggests that the individuals exercising with placebo were able to utilized gluconeogenic processes in the liver (i.e the liver changing glycogen into glucose) to maintain blood sugar at levels necessary to continue exercising. Pre-exercise carbohydrate ingestion resulted in higher insulin levels at the beginning of exercise, however, after exercise started there were no observed difference in insulin concentrations between trials suggesting that insulin plays a small role in glucose uptake at the muscle during exercise.
Between all trials there was no difference between plasma free fatty acids and glycerol (markers found in the blood suggesting fat metabolism) for the first 60 minutes. However, during the last 60 minutes, both plasma free fatty acids and glycerol concentrations were elevated in only the placebo (PP) group when compared to the CC, CP, and PC group. The reason for the no difference for the first 60 minutes is possibly due to the heavy reliance on muscle glycogen for the first 60 minutes in all trials. As muscle glycogen begins to decrease, the PP group experiences a slight shift to adipose tissue as a substrate and in doing so an elevation of free fatty acids and glycerol are present in the blood.
Individuals that took in CHO prior to exercise but not throughout exercise oxidized significantly more glucose, in the last 20 minutes, then the individuals that took CHO in during exercise. The observed findings suggest that the individuals that took in glucose prior to exercise experienced a reactive hypoglycemic response that drove down lyposis (fat break down) and increased reliance on muscle glycogen as a fuel source, thus, creating a small, but significant difference in glucose oxidation between these two groups in the later portion of exercise. Different oxidation rates were also seen in individuals that took in no exogenous glucose prior to, and during, their exercise bout. Individuals in the PP group experienced a shift in fuel utilization, were they were oxidizing more free fatty acids in the last 60 minutes of exercise when compared to the CP, PC and CC group. While there are differences in fuel utilization between treatments, they are observed in the later portion of exercise and the shift in fuel utilization did not result in performance benefits. The findings outlined suggest that athletes exercising at ~70% of VO2peak for ~2hrs, regardless of the athletes pre-exercise meal or during exercise nutritional practice oxidize glucose and free fatty acids similarly.
While oxidation of exogenous glucose is minimal (11% or .4g/min), and oxidation of free fatty acids and glucose are similar between all groups, it is clear through the exercise time trials that exogenous glucose intake during exercise increases performance. The observed results, however, do not support performance benefits for individuals that supplement with CHO prior to exercise. The current findings suggest that maintenance of plasma glucose during exercise has a positive effect on mechanisms other than those associated with substrate oxidation (what the body uses as fuel) and muscle contraction. Therefore, performance gains are possibly due central nervous system preservation and homeostasis rather then direct fuel utilization at the muscle.
The above findings by Febbraio et al. suggest that athletes must supplement with exogenous glucose (with sugars taken in by mouth) in order to perform at a higher level. The finding flys in the face of common thought that suggest that exogenous glucose is fully oxidized at the muscle during exercise (i.e like our car uses gas). Rather, Febbraio et al. findings suggest that the exogenous glucose, often taken in the form of gels, drinks and bars are essentially meant to keep your brain happy and this leads to increases in performance.
