Carbohydrate Consumption During Exercise

Also see:
Low carb + intensive training = fall in testosterone levels
Low Blood Sugar Basics
Carbohydrate Lowers Serotonin from Exercise
Serotonin, Fatigue, Training, and Performance
Ray Peat, PhD on Low Blood Sugar & Stress Reaction
PUFA Promote Stress Response; Saturated Fats Suppress Stress Response
Belly Fat, Cortisol, and Stress
Sugar (Sucrose) Restrains the Stress Response
Carbohydrate Lowers Exercise Induced Stress
Exercise Induced Stress
The Randle Cycle

J Nutr. 1992 Mar;122(3 Suppl):788-95.
Carbohydrate supplementation during exercise.
Coyle EF.
Muscle glycogen and plasma glucose are oxidized by skeletal muscle to supply the carbohydrate energy needed to exercise strenuously for several hours (i.e., 70% maximal O2 consumption). With increasing exercise duration there is a progressive shift from muscle glycogen to blood glucose. Blood glucose concentration declines to hypoglycemic levels (i.e., 2.5 mmol/L) in well-trained cyclists after approximately h of exercise and this appears to cause muscle fatigue by reducing the contribution of blood glucose to oxidative metabolism. Carbohydrate feeding throughout exercise delays fatigue by 30-60 min, apparently by maintaining blood glucose concentration and the rate of carbohydrate oxidation necessary to exercise strenuously. Carbohydrate feedings do not spare muscle glycogen utilization. Very little muscle glycogen is used for energy during the 3-4-h period of prolonged exercise when fed carbohydrate, suggesting that blood glucose is the predominant carbohydrate source. At this time, exogenous glucose disposal exceeds 1 g/min (i.e., 16 mg.kg-1.min-1) as evidenced by the observation that intravenous glucose infusion at this rate is required to maintain blood glucose at 5 mmol/L. However, at this time these cyclist cannot exercise more intensely than 74% of maximal O2 consumption, suggesting a limit to the rate at which blood glucose can be used for energy. It is important to realize that carbohydrate supplementation during exercise delays fatigue by 30-60 min, but does not prevent fatigue. In conclusion, fatigue during prolonged strenuous exercise is often due to inadequate carbohydrate oxidation. This is partly a result of hypoglycemia, which limits carbohydrate oxidation and causes muscle fatigue.(ABSTRACT TRUNCATED AT 250 WORDS)

Int J Sports Med. 1992 Oct;13 Suppl 1:S126-8.
Carbohydrate feeding during exercise.
Coyle EF.
During strenuous exercise (i.e. 70% maximal O2 consumption) there is a progressive shift from muscle glycogen to blood glucose oxidation with increasing duration of exercise. By maintaining blood glucose concentration and the rate of carbohydrate oxidation necessary to exercise strenuously, carbohydrate consumption throughout exercise delays fatigue by 30-60 min in endurance-trained subjects. This requires exogenous glucose supplementation at rates in excess of 1 gram/min (i.e., 16 mg/kg/min) as evidenced by the observation that intravenous glucose infusion at this rate is required to maintain blood glucose at 5 mM. Exogenous glucose must be infused at a rate of 2.6 gram/min (i.e., 37 mg/kg/min), which is similar to the total rate of carbohydrate oxidation, in order to maintain blood glucose at 10 mM after 2 h of exercise. However, carbohydrate supplementation during intense exercise does not spare muscle glycogen utilization in people. This suggests that over the course of 2-4 hours of exercise at 70% VO2max, muscle glycogen and blood glucose contribute equally to total carbohydrate oxidation. Furthermore, during the latter stages of prolonged exercise, exogenous blood glucose supplementation may be capable of supplying almost all of the carbohydrate requirements of exercise at intensities up to 70% VO2max.

J Appl Physiol. 1986 Jul;61(1):165-72.
Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrate.
Coyle EF, Coggan AR, Hemmert MK, Ivy JL.
The purpose of this study was to determine whether the postponement of fatigue in subjects fed carbohydrate during prolonged strenuous exercise is associated with a slowing of muscle glycogen depletion. Seven endurance-trained cyclists exercised at 71 +/- 1% of maximal O2 consumption (VO2max), to fatigue, while ingesting a flavored water solution (i.e., placebo) during one trial and while ingesting a glucose polymer solution (i.e., 2.0 g/kg at 20 min and 0.4 g/kg every 20 min thereafter) during another trial. Fatigue during the placebo trial occurred after 3.02 +/- 0.19 h of exercise and was preceded by a decline (P less than 0.01) in plasma glucose to 2.5 +/- 0.5 mM and by a decline in the respiratory exchange ratio (i.e., R; from 0.85 to 0.80; P less than 0.05). Glycogen within the vastus lateralis muscle declined at an average rate of 51.5 +/- 5.4 mmol glucosyl units (GU) X kg-1 X h-1 during the first 2 h of exercise and at a slower rate (P less than 0.01) of 23.0 +/- 14.3 mmol GU X kg-1 X h-1 during the third and final hour. When fed carbohydrate, which maintained plasma glucose concentration (4.2-5.2 mM), the subjects exercised for an additional hour before fatiguing (4.02 +/- 0.33 h; P less than 0.01) and maintained their initial R (i.e., 0.86) and rate of carbohydrate oxidation throughout exercise. The pattern of muscle glycogen utilization, however, was not different during the first 3 h of exercise with the placebo or the carbohydrate feedings. The additional hour of exercise performed when fed carbohydrate was accomplished with little reliance on muscle glycogen (i.e., 5 mmol GU X kg-1 X h-1; NS) and without compromising carbohydrate oxidation. We conclude that when they are fed carbohydrate, highly trained endurance athletes are capable of oxidizing carbohydrate at relatively high rates from sources other than muscle glycogen during the latter stages of prolonged strenuous exercise and that this postpones fatigue.

J Appl Physiol. 1993 Oct;75(4):1477-85.
Carbohydrate supplementation spares muscle glycogen during variable-intensity exercise.
Yaspelkis BB 3rd, Patterson JG, Anderla PA, Ding Z, Ivy JL.
Effects of carbohydrate (CHO) supplementation on muscle glycogen utilization and endurance were evaluated in seven well-trained male cyclists during continuous cycling exercise that varied between low [45% maximal O2 uptake (VO2 max)] and moderate intensity (75% VO2 max). During each exercise bout the subjects received either artificially flavored placebo (P), 10% liquid CHO supplement (L; 3 x 18 g CHO/h), or solid CHO supplement (S; 2 x 25 g CHO/h). Muscle biopsies were taken from vastus lateralis during P and L trials immediately before exercise and after first (124 min) and second set (190 min) of intervals. Subjects then rode to fatigue at 80% VO2 max. Plasma glucose and insulin responses during L treatment reached levels of 6.7 +/- 0.7 mM and 70.6 +/- 17.2 microU/ml, respectively, and were significantly greater than those of P treatment (4.4 +/- 0.1 mM and 17.7 +/- 1.6 microU/ml) throughout the exercise bout. Plasma glucose and insulin responses of S treatment were intermediate to those of L and P treatments. Times to fatigue for S (223.9 +/- 3.5 min) and L (233.4 +/- 7.5 min) treatments did not differ but were significantly greater than that of P treatment (202.4 +/- 9.8 min). After the first 190 min of exercise, muscle glycogen was significantly greater during L (79 +/- 3.5 mumol/g wet wt) than during P treatment (58.5 +/- 7.2 mumol/g wet wt). Furthermore, differences in muscle glycogen concentrations between L and P treatments after 190 min of exercise and in time to fatigue for these treatments were positively related (r = 0.76, P < 0.05). These results suggest that CHO supplementation can enhance prolonged continuous variable-intensity exercise by reducing dependency on muscle glycogen as a fuel source.

Med Sci Sports Exerc. 1992 Sep;24(9 Suppl):S331-5.
Nutritional manipulations before and during endurance exercise: effects on performance.
Coggan AR, Swanson SC.
1) Ingesting CHO during prolonged, moderate-intensity (60-85% VO2max) exercise can improve performance by maintaining plasma glucose availability and oxidation during the later stages of exercise. 2) Plasma glucose may be oxidized at rates in excess of 1 g.min-1 late in exercise. Athletes therefore need to ingest sufficient quantities of CHO in order to meet this demand. This can be accomplished by ingesting CHO at 40-75 g.h-1 throughout exercise or by ingesting approximately 200 g of CHO late in exercise. Ingesting CHO after fatigue has already occurred, however, is generally ineffective in restoring and maintaining plasma glucose availability, CHO oxidation, and/or exercise tolerance. 3) No apparent differences exist between glucose, sucrose, or maltodextrins in their ability to improve performance. Ingesting fructose during exercise, however, does not improve performance and may cause gastrointestinal distress. 4) The form of CHO (i.e., solid vs liquid) ingested during exercise is unlikely to be important provided that sufficient water is also consumed when ingesting CHO in solid form. 5) Ingesting 50-200 g of CHO 30-60 min before exercise results in transient hypoglycemia early in exercise, but this does not affect the rate of muscle glycogen utilization or, in most people, cause overt symptoms of neuroglucopenia. Whether performance is impaired, unaffected, or enhanced by such pre-exercise CHO feedings remains equivocal. 6) Ingesting 200-350 g of CHO 3-6 h before exercise appears to improve performance, possibly by maximizing muscle and/or liver glycogen stores or by supplying CHO from the small intestine during exercise itself.(ABSTRACT TRUNCATED AT 250 WORDS)

J Appl Physiol. 1983 Jul;55(1 Pt 1):230-5.
Carbohydrate feeding during prolonged strenuous exercise can delay fatigue.
Coyle EF, Hagberg JM, Hurley BF, Martin WH, Ehsani AA, Holloszy JO.
This study was undertaken to determine whether carbohydrate feeding during exercise can delay the development of fatigue. Ten trained cyclists performed two bicycle ergometer exercise tests 1 wk apart. The initial work rate required 74 +/- 2% of maximum O2 consumption (VO2 max) (range 70-79% of VO2 max). The point of fatigue was defined as the time at which the exercise intensity the subjects could maintain decreased below their initial work rate by 10% of VO2 max. During one exercise test the subjects were fed a glucose polymer solution beginning 20 min after the onset of exercise; during the other they were given a placebo. Blood glucose concentration was 20-40% higher during the exercise after carbohydrate ingestion than during the exercise without carbohydrate feeding. The exercise-induced decrease in plasma insulin was prevented by carbohydrate feeding. The respiratory exchange ratio was unchanged by the glucose feeding. Fatigue was postponed by carbohydrate feeding in 7 of the 10 subjects. This effect appeared to be mediated by prevention of hypoglycemia in only two subjects. The exercise time to fatigue for the 10 subjects averaged 134 +/- 6 min (mean +/- SE) without and 157 +/- 5 min with carbohydrate feeding (P less than 0.01).

“Many studies have found that sucrose is less fattening than starch or glucose, that is, that more calories can be consumed without gaining weight. During exercise, the addition of fructose to glucose increases the oxidation of carbohydrate by about 50% (Jentjens and Jeukendrup, 2005).” -Ray Peat, PhD

Br J Nutr. 2005 Apr;93(4):485-92.
High rates of exogenous carbohydrate oxidation from a mixture of glucose and fructose ingested during prolonged cycling exercise.
Jentjens RL, Jeukendrup AE.
A recent study from our laboratory has shown that a mixture of glucose and fructose ingested at a rate of 1.8 g/min leads to peak oxidation rates of approximately 1.3 g/min and results in approximately 55% higher exogenous carbohydrate (CHO) oxidation rates compared with the ingestion of an isocaloric amount of glucose. The aim of the present study was to investigate whether a mixture of glucose and fructose when ingested at a high rate (2.4 g/min) would lead to even higher exogenous CHO oxidation rates (>1.3 g/min). Eight trained male cyclists (VO2max: 68+/-1 ml/kg per min) cycled on three different occasions for 150 min at 50% of maximal power output (60+/-1% VO2max) and consumed either water (WAT) or a CHO solution providing 1.2 g/min glucose (GLU) or 1.2 g/min glucose+1.2 g/min fructose (GLU+FRUC). Peak exogenous CHO oxidation rates were higher (P<0.01) in the GLU+FRUC trial compared with the GLU trial (1.75 (SE 0.11) and 1.06 (SE 0.05) g/min, respectively). Furthermore, exogenous CHO oxidation rates during the last 90 min of exercise were approximately 50% higher (P<0.05) in GLU+FRUC compared with GLU (1.49 (SE 0.08) and 0.99 (SE 0.06) g/min, respectively). The results demonstrate that when a mixture of glucose and fructose is ingested at high rates (2.4 g/min) during 150 min of cycling exercise, exogenous CHO oxidation rates reach peak values of approximately 1.75 g/min.

Substrate interaction during exercise (randle cycle at work):
Can J Appl Physiol. 1998 Dec;23(6):558-69.
The role of glucose in the regulation of substrate interaction during exercise.
Sidossis LS.
Glucose and fatty acids are the main energy sources for oxidative metabolism in endurance exercise. Although a reciprocal relationship exists between glucose and fatty acid contribution to energy production for a given metabolic rate, the controlling mechanism remains debatable. Randle et al.’s (1963) glucose-fatty acid cycle hypothesis provides a potential mechanism for regulating substrate interaction during exercise. The cornerstone of this hypothesis is that the rate of lipolysis, and therefore fatty acid availability, controls how glucose and fatty acids contribute to energy production. Increasing fatty acid availability attenuates carbohydrate oxidation during exercise, mainly via sparing intramuscular glycogen. However, there is little evidence for a direct inhibitory effect of fatty acids on glucose oxidation. We found that glucose directly determines the rate of fat oxidation by controlling fatty acid transport into the mitochondria. We propose that the intracellular availability of glucose, rather than fatty acids, regulates substrate interaction during exercise.