During Exercise

Fluid & Carbohydrate During Exercise

Ebert, T. R., D. T. Martin, N. Bullock, I. Mujika, M. J. Quod, L. A. Farthing, L. M. Burke, and R. T. Wither. Influence of hydration status on thermoregulation and cycling hill climbing. Medicine & Science in Sports & Exercise 39: 323-329, 2007.

Burke L. M. Nutrition strategies for the marathon: fuel for training and racing. Sports Medicine 37: 344-347, 2007.

Cox G. R., E. M. Broad, and L. M. Burke. Body mass changes and voluntary fluid intakes of elite level water polo players and swimmers. Journal of Science & Medicine in Sport 5: 183-193, 2002.

Minehan M. R., M. Riley, and L. M. Burke. Effect of flavour and awareness of kilojoule content of drinks on preference and fluid balance in team sports. International Journal Sport Nutrition Exercise Metabolism 12: 81-92, 2002.

Clark, V. R., W. G. Hopkins, J. A. Hawley, and L. M. Burke. Placebo effect of carbohydrate feedings during a 40-km cycling time trial. Medicine & Science in Sports & Exercise 32: 1642-1647, 2000.

Burke, L. M. Fluid and carbohydrate intake during team games: research and recommendations. Sportscience 3, 1999. http://www.sportsci.org/jour/9901/lmb.html

Burke, L. M., and J. A. Hawley. Fluid balance in team sports. Guidelines for optimal practices. Sports Medicine 24: 38-54, 1997.

Broad, E. M., L. M. Burke, G. R. Cox, P. Heeley, and M. Riley. Body weight changes and voluntary fluid intakes during training and competition sessions in team sports. International Journal of Sport Nutrition 6: 307-320, 1996.

Burke, L. M. Rehydration strategies before and after exercise. Australian Journal of Nutrition & Dietetics 53: S22-S26, 1996.

Rehrer, N. J., and L. M. Burke. Sweat losses during various sports. Australian Journal of Nutrition & Dietetics 53: S13-S16, 1996.

Hargeaves, M., D. Costill, L. Burke, G. McConnell, and M. Febbraio. Influence of sodium on glucose bioavailability during exercise. Medicine & Science in Sports & Exercise 26: 365-368, 1994.

PURPOSE: Although dehydration can impair endurance performance, a reduced body mass may benefit uphill cycling by increasing the power-to-mass ratio. This study examined the effects of a reduction in body mass attributable to unreplaced sweat losses on simulated cycling hill-climbing performance in the heat. METHODS: Eight well-trained male cyclists (mean +/- SD: 28.4 +/- 5.7 yr; 71.0 +/- 5.9 kg; 176.7 +/- 4.7 cm; VO2peak: 66.2 +/- 5.8 mL x kg(-1) x min(-1)) completed a maximal graded cycling test on a stationary ergometer to determine maximal aerobic power (MAP). In a randomized crossover design, cyclists performed a 2-h ride at 53% MAP on a stationary ergometer, immediately followed by a cycling hill-climb time-to-exhaustion trial (88% MAP) on their own bicycle on an inclined treadmill (8%) at approximately 30 degrees C. During the 2-h ride, they consumed either 2.4 L of a 7% carbohydrate (CHO) drink (HIGH) or 0.4 L of water (LOW) with sport gels to match for CHO content. RESULTS: After the 2-h ride and before the hill climb, drinking strategies influenced body mass (LOW -2.5 +/- 0.5% vs HIGH 0.3 +/- 0.4%; P < 0.001), HR (LOW 158 +/- 15 vs HIGH 146 +/- 15 bpm; P = 0.03), and rectal temperature (T(re): LOW 38.9 +/- 0.2 vs HIGH 38.3 +/- 0.2 degrees C; P = 0.001). Despite being approximately 1.9 kg lighter, time to exhaustion was significantly reduced by 28.6 +/- 13.8% in the LOW treatment (LOW 13.9 +/- 5.5 vs HIGH 19.5 +/- 6.0 min, P = 0.002), as was the power output for a fixed speed (LOW 308 +/- 28 vs HIGH 313 +/- 28 W, P = 0.003). At exhaustion, T(re) was higher in the LOW treatment (39.5 vs HIGH 39.1 degrees C; P < 0.001), yet peak HR, blood lactate, and glucose were similar. CONCLUSION: Exercise-induced dehydration in a warm environment is detrimental to laboratory cycling hill-climbing performance despite reducing the power output required for a given speed.

Order full article via the NSIC

Burke L. M. Nutrition strategies for the marathon: fuel for training and racing. Sports Medicine 37: 344-347, 2007.

Muscle glycogen provides a key fuel for training and racing a marathon. Carbohydrate 'loading' can enhance marathon performance by allowing the competitor to run at their optimal pace for a longer period before fatiguing. For the well trained runner, this may be achieved by tapering exercise over the final days before the marathon and ensuring carbohydrate intakes of 10-12 g/kg/day over the 36-48 hours prior to the race. Sports nutrition guidelines recommend that the runner consumes sufficient carbohydrate to promote restoration of muscle glycogen between training sessions. This strategy should allow the runner to 'train harder' and recover optimally between workouts. A recent hypothesis suggests that runners might 'train smarter' by training with low glycogen stores, since this might promote greater stimulation of the training response. However, there is no evidence that a low carbohydrate diet enhances the outcomes of training or provides benefits as a depletion phase prior to carbohydrate loading. In fact, a low carbohydrate diet may even impair performance if carried out for extended periods. If there are benefits to manipulating glycogen stores for some workouts, this is likely to happen as the natural outcome of the periodisation of the high-volume programme of an elite runner.

Order full article via the NSIC

Cox G. R., E. M. Broad and L. M. Burke. Body mass changes and voluntary fluid intakes of elite level water polo players and swimmers. Journal of Science & Medicine in Sport 5:183-192, 2002.

Calculated sweat rates (measured by body mass changes) and voluntary fluid intakes were monitored in elite level water polo players and swimmers during normal exercise sessions to determine fluid requirements to maintain fluid balance, and the degree of fluid replacement of these athletes. Data were collected from training and competition sessions for male water polo players (n=23) and training sessions only for swimmers (n=20 females; n=21 males). The calculated average sweat rate and fluid intake rate during training sessions for male water polo players was 287 ml/h and 142 ml/h, respectively, with a rate of 786 ml/h and 380 ml/h during matches. During training sessions for male swimmers, the calculated average sweat rate and fluid intake rate per kilometre was 138 ml/km and 155 ml/km, respectively; and for female swimmers, 107 ml/km and 95 ml/km. There was a wide individual variation in fluid intake and sweat loss of both water polo players and swimmers. Dehydration experienced by athletes in this study was less than typically reported for "land-based" athletes. Errors inherent in the technique used in this study are acknowledged and may be significant in the calculation of reported sweat losses and levels of fluid balance in aquatic athletes.

Order full article via the NSIC

Minehan M. R., M. D. Riley, and L. M. Burke. Effect of flavour and awareness of kilojoule content of drinks on preference and fluid balance in team sports. International Journal Sport Nutrition Exercise Metabolism 12: 81-92, 2002.

A palatable flavour is known to enhance fluid intake during exercise; however, a fear of excessive kilojoule intake may deter female athletes from consuming a sports drink during training sessions. In order to examine this issue, we monitored fluid balance during 9 separate training sessions undertaken by junior elite female netball players ( n =9), female basketball players ( n =7), and male basketball players ( n =8). The beverages tested were water, a regular carbohydrate-electrolyte beverage (6.8% CHO, 18.7 mmol/L Na, 3.0 mmol/L K, 1130kJ/L), and an identical tasting, low kilojoule electrolyte beverage (1% CHO, 18.7 mmol/L Na, 3.0 mmol/L K, 170 kJ/L). Each subject received each of the 3 drinks at 3 separate training sessions, in a randomized, balanced order. Subjects were aware of the beverage provided. Change in body mass over the training session was used to estimate body fluid change, while voluntary fluid intake was determined from the change in weight of drink bottles used in each session. The overall fluid balance on drinks classified as regular, low kilojoule and water was -11.3 ml/h (95%CI -99.6 to 77.0), -29.5 ml/h (95%CI -101.4 to 42.5) and -156.4 ml/h (95%CI - 215.1 to -97.6), respectively. The results indicate that, overall, better fluid balance was achieved using either of the flavoured drinks compared to water. These data confirm that flavoured drinks enhance fluid balance in a field situation, and suggest that the energy content of the drink is relatively unimportant in determining voluntary fluid intake.

Order full article via the NSIC

PURPOSE: The placebo effect, a favorable outcome from belief that one has received a beneficial treatment, may be an important phenomenon in athletic performance. We have therefore investigated the placebo effect of a carbohydrate supplement on endurance performance.

METHODS: Forty-three competitive endurance cyclists (2 female, 41 male) performed two simulated 40-km time trials on an air-braked ergometer. In the first trial they ingested water to establish baseline performance (mean power 265 +/- 46 W for 58 +/- 4 min, mean +/- SD). For the second trial 6-8 d later they were randomized to two groups: one group ingested 16 mL x kg(-1) of a drink containing 7.6 g x 100 mL(-1) carbohydrate; the other ingested an indistinguishable noncaloric placebo drink. Cyclists in each group were further randomized to three subgroups according to whether they were told the drink contained carbohydrate, placebo, or either (not told).

RESULTS: Changes in mean power in the second trial were: told carbohydrate, 4.3 +/- 4.8%; told placebo, 0.5 +/- 5.8%; and not told, -1.1 +/- 8.5%. The difference between the told-carbohydrate and told-placebo groups was 3.8% (95% likely range 7.9 to -0.2%). The change in performance in the not-told group was more variable than that of the told groups by a factor of 1.6 (2.6 to 1.0). The real effect of carbohydrate was a slight reduction in power of 0.3% (4.4 to -3.8%).

CONCLUSIONS: (a) The placebo effect of a potentially ergogenic treatment during unblinded laboratory time trials lasting approximately 1 h is probably a small but worthwhile increase in endurance power. (b) Blinding subjects to the treatment increases individual differences in endurance effort, which may reduce precision of performance outcomes in controlled trials.

Order full article via the NSIC

Burke, L. M. Fluid and carbohydrate intake during team games: research and recommendations. Sportscience 3, 1999.

http://www.sportsci.org/jour/9901/lmb.html

The effects of nutritional strategies on performance in team games are uncertain, because changes in performance are hard to measure accurately in these sports. A small number of researchers have used lab tests, simulated games, or real games to measure the effect of hydration status and carbohydrate feeding on simulated game performance. The lack of consistency in the outcomes of these studies may be due to lack of precision in the measurement of performance, differences between athletes or sports in the effects of the nutritional intervention, or differences in environmental conditions between studies. Until there are better research tools, those who work with team-sport athletes should continue to give nutritional advice based on research with endurance athletes. However, the fluid and energy requirements in a team game may differ considerably from those of an endurance event, so the advice should be tempered by common sense and experience.

Link to full article.

Burke, L. M., and J. A. Hawley. Fluid balance in team sports. Guidelines for optimal practices. Sports Medicine 24: 38-54, 1997.

Team sports require players to perform multiple work bouts at near maximal effort, punctuated with intervals of low intensity exercise or rest for the duration of a game. Such activity patterns are associated with a significant loss of body water which has a negative impact on physical and mental performance, as well as temperature regulation. There are a number of ways in which sweat losses incurred during team sports differ from those measured during prolonged, continuous exercise. Firstly, the work rate in team sports is intermittent, largely unpredictable and random in nature. Second, analyses of various team sports reveal that such games are characterised by a high degree of inter and intra-individual variability in work rates between players from the same sport. Finally, team players are less able to anticipate sweat losses than athletes competing in events which involve prolonged, continuous, moderate intensity exercise. Yet, compared with most endurance events, many team sports offer frequent opportunities to ingest adequate volumes of fluid and thus prevent exercise-induced hypohydration. The present review details the findings of modern studies which have determined body water losses and fluid intake practices of athletes from a variety of team sports. Special considerations which influence sweat loss and fluid intake that are unique to team sports are discussed, and guidelines for sound hydration strategies during training and competition are provided.

Order full article via the NSIC

Fluid losses (measured by body weight changes) and voluntary fluid intakes were measured in elite basketball, netball, and soccer teams during typical summer and winter exercise sessions to determine fluid requirements and the degree of fluid replacement. Each subject was weighed in minimal clothing before and immediately after training, weights, and competition sessions; fluid intake, duration of exercise, temperature and humidity, and opportunity to drink were recorded. Sweat rates were greatest during competition sessions and significantly lower during weights sessions for all sports. Seasonal variation in dehydration (%DH) was not as great as may have been expected, particularly in sports played indoors. Factors influencing fluid replacement during exercise included provision of an individual water bottle, proximity to water bottles during sessions, encouragement to drink, rules of the game, duration and number of breaks or substitutions, and awareness of personal sweat rates. Guidelines for optimizing fluid intakes in these three sports are provided.

Order full article via the NSIC

Burke, L. M. Rehydration strategies before and after exercise. Australian Journal of Nutrition & Dietetics 53: S22-S26, 1996.

Optimal hydration involves fluid intake strategies before, during and after exercise sessions. In most situations sweat losses during exercise exceed the opportunities for fluid replacement during the activity. Therefore, it is important that residual levels of hypohydration are corrected during post-exercise recovery, particularly in preparation for future exercise sessions. Athletes should aim to start each exercise session in fluid balance. At special risk are those athletes who dehydrate to 'make weight' in weight division sports. Hyperhydration before exercise may be useful where fluid losses during exercise are likely to greatly exceed the potential for fluid intake. However, more research is needed before special techniques such as hyperhydration with glycerol can be universally recommended. Rehydration after exercise requires a specific fluid intake plan since thirst and voluntary intake will not provide for full restoration of sweat losses in the acute phase (zero to six hours) of recovery. Steps should be taken to ensure that a supply of palatable drinks is available after exercise. Sweetened drinks are generally preferred and may help with carbohydrate intake goals. Replacement of sodium lost in sweat is important in maximising the retention of ingested fluids. A sodium content of 50 to 90mmol/L may be necessary for optimal rehydration. However commercial sports drinks are formulated with a more moderate sodium content (10-25mmol/L). It may be necessary to consume 150% of fluid losses to allow for complete fluid restoration. Beverages containing caffeine and alcohol are not ideal rehydration fluids since they promote an increased rate of diuresis.

Rehrer, N. J., and L. M. Burke. Sweat losses during various sports. Australian Journal of Nutrition & Dietetics 53: S13–S16, 1996.

A compilation of data with respect to sweat losses incurred by various sporting activities is contained in this article. The data vary widely in terms of type of exercise, intensity, environmental conditions and population sampled. Included in the table as well as the rate of fluid loss, is information with regards to these variables, when available. The purpose of this compilation is to give some indication of the fluid losses that may be expected with the practice of various sporting activities. However, these values only give mean values and, thus, for a given individual, measurements of body weight change are needed to accurately determine fluid needs.

Hargeaves, M., D. Costill, L. Burke, G. McConnell, and M. Febbraio. Influence of sodium on glucose bioavailability during exercise. Medicine & Science in Sports & Exercise 26: 365-368, 1994.

To examine the influence of beverage sodium content on glucose bioavailability during exercise, six trained men were studied during 30 min of cycle ergometer exercise at 65 percent VO2max. Immediately prior to exercise, subjects ingested 400 ml of a 10 percent glucose solution containing 100 muCi of D-(3-3H)-glucose, with a sodium concentration of either 0,25 or 50 mmol.l-1. Trials were conducted in the morning after an overnight fast and in randomised order at least 1 wk apart. Blood samples were obtained from a forearm vein before and during exercise and subsequently analysed for plasma glucose and 3H-glucose activity and blood lactate. No differences in oxygen uptake, heart rate, or blood lactate were observed between trials. Resting plasma glucose levels were not different between trials. The increases in plasma glucose and the plasma accumulation of 3H-glucose were similar in the three trials. These results indicate that alterations in beverage sodium content, from 0-50 mmol.l-1, have no effect on glucose bioavailability, as measured by increases in plasma glucose and 3H-glucose activity during moderate intensity exercise.

Order full article via the NSIC