Roy L. P. G. Jentjens

Determinants of Post-Exercise Glycogen Synthesis During Short-Term Recovery

AbstractThe pattern of muscle glycogen synthesis following glycogen-depleting exercise occurs in two phases. Initially, there is a period of rapid synthesis of muscle glycogen that does not require the presence of insulin and lasts about 30–60 minutes. This rapid phase of muscle glycogen synthesis is characterised by an exercise-induced translocation of glucose transporter carrier protein-4 to the cell surface, leading to an increased permeability of the muscle membrane to glucose. Following this rapid phase of glycogen synthesis, muscle glycogen synthesis occurs at a much slower rate and this phase can last for several hours. Both muscle contraction and insulin have been shown to increase the activity of glycogen synthase, the rate-limiting enzyme in glycogen synthesis. Furthermore, it has been shown that muscle glycogen concentration is a potent regulator of glycogen synthase. Low muscle glycogen concentrations following exercise are associated with an increased rate of glucose transport and an increased capacity to convert glucose into glycogen.The highest muscle glycogen synthesis rates have been reported when large amounts of carbohydrate (1.0–1.85 g/kg/h) are consumed immediately post-exercise and at 15.60 minute intervals thereafter, for up to 5 hours post-exercise. When carbohydrate ingestion is delayed by several hours, this may lead to ∼50% lower rates of muscle glycogen synthesis. The addition of certain amino acids and/ or proteins to a carbohydrate supplement can increase muscle glycogen synthesis rates, most probably because of an enhanced insulin response. However, when carbohydrate intake is high (≥1.2 g/kg/h) and provided at regular intervals, a further increase in insulin concentrations by additional supplementation of protein and/or amino acids does not further increase the rate of muscle glycogen synthesis. Thus, when carbohydrate intake is insufficient (<1.2 g/kg/h), the addition of certain amino acids and/or proteins may be beneficial for muscle glycogen synthesis. Furthermore, ingestion of insulinotropic protein and/or amino acid mixtures might stimulate post-exercise net muscle protein anabolism. Suggestions have been made that carbohydrate availability is the main limiting factor for glycogen synthesis. A large part of the ingested glucose that enters the bloodstream appears to be extracted by tissues other than the exercise muscle (i.e. liver, other muscle groups or fat tissue) and may therefore limit the amount of glucose available to maximise muscle glycogen synthesis rates. Furthermore, intestinal glucose absorption may also be a rate-limiting factor for muscle glycogen synthesis when large quantities (>1 g/min) of glucose are ingested following exercise.

Oxidation of carbohydrate feedings during prolonged exercise: current thoughts, guidelines and directions for future research.

AbstractAlthough it is known that carbohydrate (CHO) feedings during exercise improve endurance performance, the effects of different feeding strategies are less clear. Studies using (stable) isotope methodology have shown that not all carbohydrates are oxidised at similar rates and hence they may not be equally effective. Glucose, sucrose, maltose, maltodextrins and amylopectin are oxidised at high rates. Fructose, galactose and amylose have been shown to be oxidised at 25 to 50% lower rates. Combinations of multiple transportable CHO may increase the total CHO absorption and total exogenous CHO oxidation. Increasing the CHO intake up to 1.0 to 1.5 g/min will increase the oxidation up to about 1.0 to 1.1 g/min. However, a further increase of the intake will not further increase the oxidation rates. Training status does not affect exogenous CHO oxidation. The effects of fasting and muscle glycogen depletion are less clear.The most remarkable conclusion is probably that exogenous CHO oxidation rates do not exceed 1.0 to 1.1 g/min. There is convincing evidence that this limitation is not at the muscular level but most likely located in the intestine or the liver. Intestinal perfusion studies seem to suggest that the capacity to absorb glucose is only slightly in excess of the observed entrance of glucose into the blood and the rate of absorption may thus be a factor contributing to the limitation. However, the liver may play an additional important role, in that it provides glucose to the bloodstream at a rate of about 1 g/min by balancing the glucose from the gut and from glycogenolysis/gluconeogenesis. It is possible that when large amounts of glucose are ingested absorption is a limiting factor, and the liver will retain some glucose and thus act as a second limiting factor to exogenous CHO oxidation.

High oxidation rates from combined carbohydrates ingested during exercise.

UNLABELLED Studies that have investigated oxidation of a single carbohydrate (CHO) during exercise have reported oxidation rates of up to 1 g x min(-1). Recent studies from our laboratory have shown that a mixture of glucose and sucrose or glucose and fructose ingested at a high rate (1.8 g x min(-1)) leads to peak oxidation rates of approximately 1.3 g x min(-1) and results in approximately 20 to 55% higher exogenous CHO oxidation rates compared with the ingestion of an isocaloric amount of glucose. PURPOSE The purpose of the present study was to examine whether a mixture of glucose, sucrose and fructose ingested at a high rate would result in even higher exogenous CHO oxidation rates (>1.3 g x min(-1)). METHODS Eight trained male cyclists (VO2max: 64 +/- 1 mL x kg(-1) BM x min(-1)) cycled on three different occasions for 150 min at 62 +/- 1% VO2max and consumed either water (WAT) or a CHO solution providing 2.4 g x min(-1) of glucose (GLU) or 1.2 g x min(-1) of glucose + 0.6 g x min(-1) of fructose + 0.6 g x min(-1) of sucrose (MIX). RESULTS High peak exogenous CHO oxidation rates were found in the MIX trial (1.70 +/- 0.07 g x min(-1)), which were approximately 44% higher (P < 0.01) compared with the GLU trial (1.18 +/- 0.04 g x min(-1)). Endogenous CHO oxidation was lower (P < 0.05) in MIX compared with GLU (0.76 +/- 0.12 and 1.05 +/- 0.06 g x min(-1), respectively). CONCLUSION When glucose, fructose and sucrose are ingested simultaneously at high rates (2.4 g x min(-1)) during cycling exercise, exogenous CHO oxidation rates can reach peak values of approximately 1.7 g x min(-1) and estimated endogenous CHO oxidation is reduced compared with the ingestion of an isocaloric amount of glucose.

High rates of exogenous carbohydrate oxidation from a mixture of glucose and fructose ingested during prolonged cycling exercise

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.

Oxidation of combined ingestion of maltodextrins and fructose during exercise

PURPOSE To determine whether combined ingestion of maltodextrin and fructose during 150 min of cycling exercise would lead to exogenous carbohydrate oxidation rates higher than 1.1 g.min. METHODS Eight trained cyclists VO2max: 64.1 +/- 3.1 mL.kg.min) performed three exercise trials in a random order. Each trial consisted of 150 min cycling at 55% maximum power output (64.2+/-3.5% VO2max) while subjects received a solution providing either 1.8 g.min of maltodextrin (MD), 1.2 g.min of maltodextrin + 0.6 g.min of fructose (MD+F), or plain water. To quantify exogenous carbohydrate oxidation, corn-derived MD and F were used, which have a high natural abundance of C. RESULTS Peak exogenous carbohydrate oxidation (last 30 min of exercise) rates were approximately 40% higher with combined MD+F ingestion compared with MD only ingestion (1.50+/-0.07 and 1.06+/-0.08 g.min, respectively, P<0.05). Furthermore, the average exogenous carbohydrate oxidation rate during the last 90 min of exercise was higher with combined MD+F ingestion compared with MD alone (1.38+/-0.06 and 0.96+/-0.07 g.min, respectively, P<0.05). CONCLUSIONS The present study demonstrates that with ingestion of large amounts of maltodextrin and fructose during cycling exercise, exogenous carbohydrate oxidation can reach peak values of approximately 1.5 g.min, and this is markedly higher than oxidation rates from ingesting maltodextrin alone.

Caffeine Improves Physical and Cognitive Performance during Exhaustive Exercise

UNLABELLED Caffeine is thought to act as a central stimulant and to have effects on physical, cognitive, and psychomotor functioning. PURPOSE To examine the effects of ingesting a performance bar, containing caffeine, before and during cycling exercise on physical and cognitive performance. METHODS Twenty-four well-trained cyclists consumed the products [a performance bar containing 45 g of carbohydrate and 100 mg of caffeine (CAF), an isocaloric noncaffeine performance bar (CHO), or 300 mL of placebo beverage (BEV)] immediately before performing a 2.5-h exercise at 60% VO2max followed by a time to exhaustion trial (T2EX) at 75% VO2max. Additional products were taken after 55 and 115 min of exercise. Cognitive function measures (computerized Stroop and Rapid Visual Information Processing tests) were performed before exercise and while cycling after 70 and 140 min of exercise and again 5 min after completing the T2EX ride. RESULTS Participants were significantly faster after CAF when compared with CHO on both the computerized complex information processing tests, particularly after 140 min and after the T2EX ride (P < 0.001). On the BEV trial, performance was significantly slower than after both other treatments (P < 0.0001). There were no speed-accuracy tradeoffs (P > 0.10). T2EX was longer after CAF consumption compared with both CHO and BEV trials (P < 0.05), and T2EX was longer after CHO than after BEV (P < 0.05). No differences were found in the ratings of perceived exertion, mean heart rate, and relative exercise intensity (% VO2max; P > 0.05). CONCLUSION Caffeine in a performance bar can significantly improve endurance performance and complex cognitive ability during and after exercise. These effects may be salient for sports performance in which concentration plays a major role.

Nutritional considerations in triathlon

AbstractTriathlon combines three disciplines (swimming, cycling and running) and competitions last between 1 hour 50 minutes (Olympic distance) and 14 hours (Ironman distance). Independent of the distance, dehydration and carbohydrate (CHO) depletion are the most likely causes of fatigue in triathlon, whereas gastrointestinal (GI) problems, hyperthermia and hyponatraemia are potentially health threatening, especially in longer events. Although glycogen supercompensation may be beneficial for triathlon performance (even Olympic distance), this does not necessarily have to be achieved by the traditional supercompensation protocol. More recently, studies have revealed ways to increase muscle glycogen concentrations to very high levels with minimal modifications in diet and training.During competition, cycling provides the best opportunity to ingest fluids. The optimum CHO concentration seems to be in the range of 5–8% and triathletes should aim to achieve a CHO intake of 60–70 g/hour. Triathletes should attempt to limit body mass losses to 1% of body mass. In all cases, a drink should contain sodium (30–50 mmol/L) for optimal absorption and prevention of hyponatraemia.Post-exercise rehydration is best achieved by consuming beverages that have a high sodium content (>60 mmol/L) in a volume equivalent to 150% of body mass loss. GI problems occur frequently, especially in long-distance triathlon. Problems seem related to the intake of highly concentrated carbohydrate solutions, or hyperosmotic drinks, and the intake of fibre, fat and protein. Endotoxaemia has been suggested as an explanation for some of the GI problems, but this has not been confirmed by recent research. Although mild endotoxaemia may occur after an Ironman-distance triathlon, this does not seem to be related to the incidence of GI problems. Hyponatraemia has occasionally been reported, especially among slow competitors in triathlons and probably arises due to loss of sodium in sweat coupled with very high intakes (8–10L) of water or other low-sodium drinks.

Fructose and Galactose Enhance Post-Exercise Human Liver Glycogen Synthesis

PURPOSE Both liver and muscle glycogen stores play a fundamental role in exercise and fatigue, but the effect of different CHO sources on liver glycogen synthesis in humans is unclear. The aim was to compare the effect of maltodextrin (MD) drinks containing galactose, fructose, or glucose on postexercise liver glycogen synthesis. METHODS In this double-blind, triple crossover, randomized clinical trial, 10 well-trained male cyclists performed three experimental exercise sessions separated by at least 1 wk. After performing a standard exercise protocol to exhaustion, subjects ingested one of three 15% CHO solutions, namely, FRU (MD + fructose, 2:1), GAL (MD + galactose, 2:1), or GLU (MD + glucose, 2:1), each providing 69 g CHO·h(-1) during 6.5 h of recovery. Liver glycogen changes were followed using (13)C magnetic resonance spectroscopy. RESULTS Liver glycogen concentration increased at faster rates with FRU (24 ± 2 mmol·L(-1)·h(-1), P < 0.001) and with GAL (28 ± 3 mmol·L(-1)·h(-1), P < 0.001) than with GLU (13 ± 2 mmol·L(-1)·h(-1)). Liver volumes increased (P < 0.001) with FRU (9% ± 2%) and with GAL (10% ± 2%) but not with GLU (2% ± 1%, NS). Net glycogen synthesis appeared linear and was faster with FRU (8.1 ± 0.6 g·h(-1), P < 0.001) and with GAL (8.6 ± 0.9 g·h(-1), P < 0.001) than with GLU (3.7 ± 0.5 g·h(-1)). CONCLUSIONS When ingested at a rate designed to saturate intestinal CHO transport systems, MD drinks with added fructose or galactose were twice as effective as MD + glucose in restoring liver glycogen during short-term postexercise recovery.

OXIDATION OF COMBINED INGESTION OF GLUCOSE AND FRUCTOSE DURING EXERCISE

The purpose of the present study was to examine whether combined ingestion of a large amount of fructose and glucose during cycling exercise would lead to exogenous carbohydrate oxidation rates >1 g/min. Eight trained cyclists (maximal O(2) consumption: 62 +/- 3 ml x kg(-1) x min(-1)) performed four exercise trials in random order. Each trial consisted of 120 min of cycling at 50% maximum power output (63 +/- 2% maximal O(2) consumption), while subjects received a solution providing either 1.2 g/min of glucose (Med-Glu), 1.8 g/min of glucose (High-Glu), 0.6 g/min of fructose + 1.2 g/min of glucose (Fruc+Glu), or water. The ingested fructose was labeled with [U-(13)C]fructose, and the ingested glucose was labeled with [U-(14)C]glucose. Peak exogenous carbohydrate oxidation rates were approximately 55% higher (P < 0.001) in Fruc+Glu (1.26 +/- 0.07 g/min) compared with Med-Glu and High-Glu (0.80 +/- 0.04 and 0.83 +/- 0.05 g/min, respectively). Furthermore, the average exogenous carbohydrate oxidation rates over the 60- to 120-min exercise period were higher (P < 0.001) in Fruc+Glu compared with Med-Glu and High-Glu (1.16 +/- 0.06, 0.75 +/- 0.04, and 0.75 +/- 0.04 g/min, respectively). There was a trend toward a lower endogenous carbohydrate oxidation in Fruc+Glu compared with the other two carbohydrate trials, but this failed to reach statistical significance (P = 0.075). The present results demonstrate that, when fructose and glucose are ingested simultaneously at high rates during cycling exercise, exogenous carbohydrate oxidation rates can reach peak values of approximately 1.3 g/min.

HYPOGLYCEMIA FOLLOWING PRE-EXERCISE CARBOHYDRATE INGESTION IS NOT ACCOMPANIED BY HIGHER INSULIN SENSITIVITY.

Pre-exercise carbohydrate feeding may result in rebound hypoglycemia in some but not all athletes. The aim of the present study was to examine whether insulin sensitivity in athletes who develop rebound hypoglycemia is higher compared with those who do not show rebound hypoglycemia. Twenty trained athletes (VO(2max) of 61.8 +/- 1.4 ml.kg(-1).min(-1)) performed an exercise trial on a cycle ergometer. Forty-five minutes before the start of exercise, subjects consumed 500 ml of a beverage containing 75 g of glucose. The exercise trial consisted of 20 min of submaximal exercise at 74 +/- 1% VO(2max) immediately followed by a time trial. Based upon the plasma glucose nadir reached during submaximal exercise, subjects were assigned to a Hypo group (<3.5 mmol/L) and a Non-hypo group (> or =3.5 mmol/L). An oral glucose tolerance test was performed to obtain an index of insulin sensitivity (ISI). The plasma glucose nadir during submaximal exercise was significantly lower (p <.01) in the Hypo-group (n = 10) compared with the Non-hypo group (n = 10) (2.7 +/- 0.1 vs. 4.1 +/- 0.2 mmol/L, respectively). No difference was found in ISI between the Hypo and the Non-hypo group (3.7+/-0.4 vs. 3.8 +/- 0.5, respectively). The present results suggest that insulin sensitivity does not play an important role in the occurrence of rebound hypoglycemia.