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John A. Hawley, Steven C. Dennis and Timothy D. Noakes

Soccer requires field players to exercise repetitively at high intensities for the duration of a game, which can result in marked muscle glycogen depletion and hypoglycemia. A soccer match places heavy demands on endogenous muscle and liver glycogen stores and fluid reserves, which must be rapidly replenished when players complete several matches within a brief period of time. Low concentrations of muscle glycogen have been reported in soccer players before a game, and daily carbohydrate (CHO) intakes are often insufficient to replenish muscle glycogen stores, CHO supplementation during soccer matches has been found to result in muscle glycogen sparing (39%), greater second-half running distances, and more goals being scored with less conceded, when compared to consumption of water. Thus, CHO supplementation has been recommended prior to, during, and after matches. In contrast, there is currently insufficient evidence to recommend without reservation the addition of electrolytes to a beverage for ingestion by players during a game resulting in sweat losses of < 4% of body weight.

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Sonja Terblanche, Timothy D. Noakes, Steven C. Dennis, De Wet Marais and Michael Eckert

This study examined the effect of magnesium supplementation on muscle magnesium content, on running performance during a 42-kni marathon footrace, and on muscle damage and the rate of recovery of muscle function following the race. Twenty athletes were divided equally into two matched groups and were studied for 4 weeks before and 6 weeks after a marathon in a double-blind trial; the experimental group received magnesium supplement (365 mg per day) and the control group, placebo. Magnesium supplementation did not increase either muscle or serum magnesium concentrations and had no measurable effect on 42-km marathon running performance. Extra magnesium ingestion also had no influence on the extent of muscle damage or the rate of recovery of muscle function. The latter was significantly reduced immediately after the marathon but returned to normal within 1 week. Thus, magnesium supplementation in magnesium-replete subjects did not enhance performance or increase resistance to muscle damage during the race, or the rate of recovery of muscle function following the race.

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Clayton Zeederberg, Lloyd Leach, Estelle V. Lambert, Timothy D. Noakes, Steven C. Dennis and John A. Hawley

This study examined the effects of ingesting a glucose-polymer (GP) solution on the motor skill proficiencies of association football (soccer) players from two teams playing during two matches in a cool environment. Fifteen minutes before each match and at halftime, players from both teams ingested 5 ml/kg of either placebo or a 6.9% GP solution. GP ingestion did not improve tackling, heading, dribbling, or shooting ability. On the contrary, the mean of successful tackles was lower with GP ingestion than with placebo. The success rate for heading, dribbling, and shooting also tended to be lower in the GP than in the placebo condition. In contrast, success in passing and ball control was similar in the two conditions. Improvements in passing and ball control may have been related to a decrease in the intensity of play in the second half of the game. These data indicate that there are no measurable benefits of GP ingestion for the motor skill proficiencies of soccer players during games played in a cool environment.

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Julia H. Goedecke, Richard Elmer, Steven C. Dennis, Ingrid Schloss, Timothy D. Noakes and Estelle V. Lambert

The effects of ingesting different amounts of medium-chain triacylglycerol (MCT) and carbohydrate (CHO) on gastric symptoms, fuel metabolism, and exercise performance were measured in 9 endurance-trained cyclists. Participants, 2 hr after a standardized lunch, cycled for 2 hr at 63% of peak oxygen consumption and then performed a simulated 40-km time trial (T trial). During the rides, participants ingested either 10% 14C-glucose (GLU), 10% 14C-GLU + 1.72%MCT(LO-MCT), or 10% l4C-GLU + 3.44%MCT(HI-MCT) solutions: 400 ml at the start of exercise and then 100 ml every lOmin.MCTingestiondid not affect gastrointestinal symptoms. It only raised serum free fatty acid (FFA) and ß-hydroxybutyrate concentrations. Higher FFA and ß-hydroxybutyrate concentrations with MCT ingestion did not affect fuel oxidation or T-trial performance. The high CHO content of the pretrial lunch increased starting plasma insulin levels, which may have promoted CHO oxidation despite elevated circulating FFA concentrations with MCT ingestion.

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Estelle V. Lambert, Julia H. Goedecke, Charl van Zyl, Kim Murphy, John A. Hawley, Steven C. Dennis and Timothy D. Noakes

We examined the effects of a high-fat diet (HFD-CHO) versus a habitual diet, prior to carbohydrate (CHO)-loading on fuel metabolism and cycling time-trial (TT) performance. Five endurance-trained cyclists participated in two 14-day randomized cross-over trials during which subjects consumed either a HFD (>65% MJ from fat) or their habitual diet (CTL) (30 ± 5% MJ from fat) for 10 day, before ingesting a high-CHO diet (CHO-loading, CHO > 70% MJ) for 3 days. Trials consisted of a 150-min cycle at 70% of peak oxygen uptake (V̇O2peak), followed immediately by a 20-km TT. One hour before each trial, cyclists ingested 400 ml of a 3.44% medium-chain triacylglycerol (MCT) solution, and during the trial, ingested 600 ml/hour of a 10% 14C-glucose + 3.44% MCT solution. The dietary treatments did not alter the subjects’ weight, body fat, or lipid profile. There were also no changes in circulating glucose, lactate, free fatty acid (FFA), and β-hydroxybutyrate concentrations during exercise. However, mean serum glycerol concentrations were significantly higher (p < .01) in the HFD-CHO trial. The HFD-CHO diet increased total fat oxidation and reduced total CHO oxidation but did not alter plasma glucose oxidation during exercise. By contrast, the estimated rates of muscle glycogen and lactate oxidation were lower after the HFD-CHO diet. The HFD-CHO treatment was also associated with improved TT times (29.5 ± 2.9 min vs. 30.9 ± 3.4 min for HFD-CHO and CTL-CHO, p < .05). High-fat feeding for 10 days prior to CHO-loading was associated with an increased reliance on fat, a decreased reliance on muscle glycogen, and improved time trial performance after prolonged exercise.

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Laurie H.G. Rauch, Ian Rodger, Gary R. Wilson, Judy D. Belonje, Steven C. Dennis, Timothy D. Noakes and John A. Hawley

This study compared the effects of supplementing the normal diets of 8 endurance-trained cyclists with additional carbohydrate (CHO), in the form of potato starch, for 3 days on muscle glycogen utilization and performance during a 3-hr cycle ride. On two occasions prior to the trial, the subjects ingested in random order either their normal CHO intake of 6.15 ± 0.23 g/kg body mass/day or a high-CHO diet of 10.52 ± 0.57 g/kg body mass/day. The trial consisted of 2 hr of cycling at ~75% of VO2peak with five 60-s sprints at 100% VO2peak at 20-min intervals, followed by a 60-min performance ride. Increasing CHO intake by 72 ± 9% for 3 days prior to the trial elevated preexercise muscle glycogen contents, improved power output, and extended the distance covered in 1 hr. Muscle glycogen contents were similar at the end of the 3-hr trial, indicating a greater utilization of glycogen when subjects were CHO loaded, which may have been responsible for their improved cycling performance.